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NEW YORK ACADEMY OF SCIENCES
SCIENTIFIC SURVEY
Porto Rico and the Virgin Isk
VOLUME II—Part
1
The Geology of the Lares District, Porto Rico— Be/a Hubbard
NEW VOKK;
k
A
!^*B^"''
NEW YORK ACADEMY OF SCIENCES
^'lfll^^
SCIENTIFIC SURVEY
OF
Porto Rico and the Virgin Islands
VOLUME II—Part
1
The Geology of the Lares District, Porto Rico— Bela Hubbard
i
-
Ml. %^
J'-
NEW YORK;
Published by the Academy
1923
I
THE GEOLOGY OP THE LARES DISTRICT,
PORTO RICO
By Bela Hubbard
CONTENTS
Page
Introduction
2
Nature and purpose of the work
,
2
Acknowledgments
3
Previous work
4
General
description
4
The Antilles
4
Porto Rico
5
The Lares District
6
The Cretaceous formations
6
Petrography
7
Igneous rocks
7
Sedimentary and
clastic rocks
15
Stratigraphy
25
The Rio
Culebrinas
Series
25
The Rio Blanco
Series
26
The Rio Yauco
Series
29
Structural
features
30
Age
34
Correlation
35
Llthogenesis
36
Geologic history
37
The Tertiary formations
38
Stratigraphy
39
The San
Sebastian shale
39
The Lares formation
^
42
The Cibao limestone
44
The Los Puertos limestone
45
The Quebradillas limestone
46
Summary of formations
49
Structure
49
Age and correlation
51
The San
Sebastian fauna
53
The Lares fauna
57
The Cibao fauna
58
The Los Puertos fauna
59
The Quebradillas fauna
60
Summary of the faunas
64
Correlation with the south coast
,
64
2
SCIENTIFIC SURVEY OF PORTO RICO
Page
Comi)arison with important
xVntillian
localities
|
71
Oligocene or Miocene
72
Tertiary
history
75
Magnitude
of
the
submergence
70
The
uplift
77
Physiography
77
The Complex Mountainous Oldland
78
The peneplane surface
79
Drainage features
,
81
The Elevated
Coastal
Plain
83
The
Coastal
Plain
belts
86
Drainage
88
Origin of the Pepino Hills
91
The Playa Plains
93
Features of the coast line
95
Terraces
95
The San Juan formation
98
The
consolidated dune sands
,
98
The
consolidated beach
deposits
99
Evidence of uplift
100
Age of the San Juan formation
100
Conclusions
103
The Playa
deposits
103
Summary
of Pleistocene—recent
events
104
Mineral resources
105
Iron
105
Copper
105
Kaolin
108
Brick clay
108
Lime
100
Building
stone
109
Road metal
110
Lignite
110
Guano
Ill
Oil
Ill
Summary
Ill
Bibliography
Ill
INTEODUCTIOJST
]^ATURE AND PURPOSE OF THE WORK
The present paper gives the results of a survey of the geology of the
Lares District, Porto Eico, made during the summer of 1916 under the
auspices of the ISTew York Academy of Sciences and the Insular Govern-
ment of Porto Eico.
It
is one of a series of reports, each covering the
f
I
HUBBARD, GEOLOGY OF THE LARES DISTRICT
3
geology of a portion
of the
island.^
The Lares
District comprises the
northwest corner of Porto lUco and Desecheo Island, a total area of about
500 square miles.
The survey of this district involved the study of the
rock fomiations, mineral resources and the making
of a geologic map.
As the field work was done in three months time, methods of the recon-
naissance type had
to
be used.
Before entering the
field,
a base map
(3 inches= 1 mile)
was prepared from available sources and this was
divided into quadrangles of 5 x 2 miles.
The field work consisted chiefly
in running traverses across the general strike of the rock formations.
All
traverses were made by pacing or by the time elapse method.
Elevations
were obtained by aneroid.
In recording the data, two methods were used
1.
Cavalry Sketching
Case.
The blue print quadrangles
of the base
map were used on the sketching case.
Contours were sketched in for a
distance of one-half mile or more on either side of the line of traverse.
2.
N'ote Book Traverse, using Brunton compass and protractor.
This
method was found best for the more detailed work in limited areas, and
during rainy weather.
Photographs were used repeatedly as a means of
gathering data
for the topographic map.
In portions
of
the area
not
covered by photographs or traverse,
the topographic features shown on
tlie map have been generalized.
The first two months of the
field work were spent among the Tertiary
rocks of the district, while the final month was spent in the area of Cre-
taceous rocks.
Subdivision and
correlation
of
the
Tertiary formations
was the most important problem to be worked out, and hence required
the expenditure
of more time than a well balanced survey would
ordi-
narily warrant.
A large number of fossils were collected in the Tertiary
area, which have been d.escribed in Volume III, Part 2, of these reports.
Acknowledgments
Acknowledgment
is made particularly to Dr. C.
P. Berkey for assist-
ance and advice in almost every phase of the work.
Dr. A. W. Grabau,
Dr. D. W. Johnson and other members of the Columbia University Geol-
ogy Department have given valued
suggestions and
aid.
In
the
field
work, every courtesy and assistance was given by the government officials
at San Juan, and acknowledgment
is made especially to Colonel George
K. Shanton, Chief of Insular Police, Major Basil H. Dutcher, U.
S. A.,
and Judge Bonner, Auditor.
In Mayaguez, Mr. D. W. May, Director of
the Agricultural Experiment Station, took very great interest in the work
1 For an account
of
the
plans and
progress
of
the
geological
survey
of
Porto
Rico,
see
Scientific Survey
of Porto
Ilico and the Virgin
Islands, New York Academy
of
Sci-
ences,
vol.
I, part
1,
pp.
1-10,
24-20,
1919.
4
SCIENTIFIC SURVEY OF PORTO RICO
and assisted in every way.
SefLor Narciso Rabell^
of San
Sebastian,
a
student
of the
geology
of the
island,
aided
in
collecting many
of the
fossils, in giving valuable information, and in showing a lively interest in
the progress of the work.
The aid of the Insular Police in every town
in the Lares District was a big factor in carrying on the survey.
Eesi-
dents
of
the
district not only
displayed
the warmest
hospitality,
but
showed a keen and intelligent interest in the purposes and results of the
field work.
Phevious Work
The
list of
articles dealing with Porto Kico
is given in the
Bibliog-
raphy.
Prior to
1916,
little
geological work
of
a
detailed nature had
been done in northwestern Porto Kico.
E. T. Hill made a reconnaissance
in 1898, and published several
articles.
He visited the Lares
District,
and made observations along the Lares Road, where he
collected many
fossil corals, later described by T. W. Vaughan.
In 1914, in the course
of their reconnaissance
of the
island,
C.
P. Berkey and
C.
R.
Fenner
covered the main routes of travel in the
district, and noted many of the
geologic
features characteristic of
this part of the
island.
During the
same year, certain
fossil
localities
in the
district were visited by
C.
A.
Reeds, where a large amount of material was collected.
In 1915, E. T.
Hodge made a brief reconnaissance of certain
localities in the southern
half of the district, to investigate the reported presence of
oil shales.
GENERAL DESCRIPTION
The Antilles
Porto Rico, forming the easternmost member of the Greater Antilles,
is an almost completely submerged complex mountain
chain having
a
maximum elevation of about 28,000 feet above the adjacent lowlands or
submarine valleys of the Atlantic and Caribbean.
This mountain chain
is more or less continuous in an
east-west direction,
its highest portions
forming the islands of Cuba, Haiti-Santo Domingo, Jamaica, and others.
Lateral branches of this chain form the Lesser Antilles and minor groups
of
islands.^
The rocks
of this
x\ntillean chain
are
chiefly of Mesozoic
age, and comprise volcanic flows, intrusives, and sediments of marine and
continental
origin.
These
rocks,
as a whole, show evidence
of having
been formed during a more or less continuous period of volcanic activity,
which reached a climax in extensive orogenic movements at the close of
'^ An excellent illustration of the sub-sea relief of the Antillean region
is given in Bull
lOa, U.
S. Nat, Mus., Plate
73, 1919.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
5
Mesozoic time.
With the Tertiary, began a differential submergence, not
affecting some portions of the region until Oligocene time, and resulting
in a deposition of reef and shell limestones.
Subsequent uplift in Mio-
cene time, has exposed the fringing beds of Tertiary limestones on nearly
all the islands of the Antillean group.
Finally, there have been minor
crustal movements and fluctuations
of
sea
level during the
Pleistocene
Epoch.
The crustal movements have continued to the present.
Porto Eico
As a result of the major geologic
events
just
outlined,
modified by
minor events peculiar to the vicinity of Porto Eico, the following are the
chief geologic and physiographic elements or units found on the island
:
1. Complex mountainous oldland area, which makes up the core of the
island.
The rocks so far as known are
all of Cretaceous age and com-
prise
intrusives,
flow^s,
tuffs,
ash,
shales and
other
sedimentary
rocks.
The igneous rocks are predominantly andesitic, though other types occur.
The structure is highly complex, marked by many thrust faults and over-
turned, folds.
The predominant
strike
of
the
structure
is
northAvest-
southeast, as
it
is in most places throughout the older rocks
of the An-
tilles.
This oldland
area
is
characterized by
its mountainous
aspect,
steep soil-covered slopes, with marked absence of cliffs or other exposures
of unweathered rock.
The present cycle of erosion has reached maturity.
The existence of at least two previous cycles of erosion has been shown
by Lobeck (1922).
2.
Elevated
Coastal
Plain.
Marked by nearly horizontal deposits
of
reef and shell limestones of Oligocene age, deposited along the north and
south coasts
of the island
(see Berkey,
1915).
These limestones with
basal
shale members
rest unconformably upon
the Upper
Cretaceous
rocks, the time interval covering the Eocene period and the early Oligo-
cene.
The total vertical uplift reached a maximum of 1500
feet.
The
uplift on the north coast was differential, accompanied by gentle warping.
On the south coast it was accompanied by local faulting and considerable
tilting of the beds to seaward.
Thus the north coast beds dip at angles
averaging less than 5°
in a seaward direction, while the average for the
south coast
is considerably greater.
Erosion of the present cycle
(post-Oligocene)
has affected these Ter-
tiary limestones,
chiefly by surface and subterranean
solution, and has
stripped the oldland area of a portion of its Tertiary covering.
3.
Pleistocene
to
Eecent
Coastal
Deposits.
These
include
alluvial
plains or playas at mouths of the large streams, elevated beach gravels.
6
SCIENTIFIC SURVEY OF PORTO RICO
and
indurated dune
sands.
They
rejaresent
the
effects
of
Pleistocene
variations of sea
level, accompanied and followed by local crustal move-
ments, which, over most of the island, have resulted in a series of uplifts
totalling at least 200
feet.
The Lares District
This
district contains
a portion of almost every major geologic and
physiographic
unit
to be found on
the
island.
The
Older
Series,
or
Upper Cretaceous rocks, occupy the mountainous area south of the Lares
Eoad.
In
this area there are remnants
of the
earliest post-Cretaceous
or
late Cretaceous peneplane,
the
first erosion
cycle.
Evidence
of
the
second cycle, however,
is not found within the district.
The entire area
north
of the Lares highway
is occupied by the
overlapping,
relatively
undisturbed limestones, marls and shales of the Younger
Series or
Oli-
gocene, resting upon an irregular surface carved in the highly disturbed
Cretaceous rocks, and dipping gently seaward.
This is the most complete
development of these rocks
to be found on the
island.
Post-Oligocene
uplift has
raised these massive
reef limestones
to
a present maximum
elevation of 1300
feet.
Solution with extensive development
of under-
ground drainage has produced in this limestone belt a peculiar type of
Karst topography with its pepino hills.
The Pleistocene and more recent
uplifts are well shown along the extensive coastline of the district.
They
are marked by marine terraces and elevated
fossil beaches.
The typical
playas are developed at the mouths of the largest streams,
particularly
on the west coast.
THE CRETACEOUS FORMATIONS
As a whole, the rocks are more nearly similar to those of the Ponce
District to the south than to the rocks elsewhere on the
island.
When
compared with eastern Porto Rico, the most striking feature in the Lares
District
is the predominance of shales and volcanic
elastics, with an ab-
sence of intrusives of the large batholithic type.
The
strike of the beds
is northwest-southeast,
as
is the case throughout the rest of the
island.
In the Lares District, the average strike is north 50° west, although there
are variations from this over considerable
areas.
The prevailing dip
is
to the
southwest, and the northernmost beds
are
therefore
the
oldest,
except locally in cases of repetition by folds and faults.
In the eastern
portion of the area, volcanic tuffs predominate, while in the western half,
shales predominate.
Of igneous
rocks, none of the deep seated, batho-
HUBBARD, GEOLOGY OF THE LARES DISTRICT
7
lithic
types
occur,
although some
coarse
diorite was found
in
stream
gravels
near
Aiiasco.
The
prevailing
igneous
rock
in
the
district
is
andesite porphyry, occurring as
sills,
laccoliths, and irregular intrusive
bodies.
Petrography
igneous rocks
The igneous rocks
are
characterized by
a
small petrographic
range.
Andesite porphyry makes up about nine-tenths of all the igneous material
in
the
district.
The
textural
range
is
from
felsites
to
porphyries.
Glasses are rare or not easily recognizable because of the
devitrification
and weathering.
The following types have been determined, and their
distribution shown on the geologic map
Table 1
Name of Rock
Usual Occurrence
Quartz
diorite porphyry
Irregular intrusives.
Andesite
Surface
flows,
usually amygdaloidal.
Andesite porphyry
Surface flows,
sills, dikes, and lacco-
lithic or
elongate intrusive
bodies.
Augite andesite
Surface
flows, amygdaloidal.
Augite andesite porphyry
Surface flows,
sills,
dikes, and irreg-
ular
intrusive
bodies grading
into
Gabbro porphyry.
Gabbro porphyry
Irregular
intrusives,
grading
into
Augite andesite porphyry.
Serpentine
Dikes
and
irregular
intrusives
of
small
size.
Quartz Diorite Porphyry.
The
only
locality
in which
this
rock
is
found
is in the Eio Blanco valley south of Lares.
It occurs as a boss of
considerable
size, and
is
associated with
one
of
the most conspicuous
centers of former volcanic activity in the district.
The rock
is compara-
tively resistant to erosion, and forms the prominent peak of Mt. Torre-
cillo
(Fig. 1).
8
fiCIENTlPIC SURVEY OP FONTO RWO
Tile principal minerals are pkgioclase
(aodesino-oligoflase,
aTitlcsiiic,
labradorito),
h()Tiil)l(mclo, and quartz.
Tliese
iiiincirals form
the
plieiio-
CTvsts, many of tliem large and in some cases occupying much more space
than
the gromidmiiss.
The plagioclase occurs abundantly
as
niicrolites
in the? fclsitie gromidmass.
The qnartz crystals are not idioniorpliic, hut
occur as sub-romid grains, in some cases quite conspicuous in hand speci-
mens.
The accessory minerals are magnetite,
apatite,
ruiile, and
prob-
ably some orthoclase.
The apatite crystals occur characteristically in the
feldspar grains, and the ratile
is confined to the quartz.
Quartz
is ordy
m^M
Fig.
1.--r/cM-
Inoki'i
he
l)Hirkgr(Himl.
wItJi
K. P,
LarcR-IHo
rH(tu<
'
left.
Cordlllei
cntral
of the Ponce
•alley
of
tlie Rio
Bl
ail awM'ssory eoiiHtituent
in some portions of the rock.
Tlie elrief altera-
tions
aj-e, hornblenjJe
to
chlorite and
serpeutijuy;
plagioclase
to
kaolin,
sanssnrjte, and carboiuite: and magnetite to limonite.
Th<' rock grarles into audesite porphyry, and locally has a higli content
of.
triffaceuus
material, and Tesembles
a
true
tuff.
Portions which
are
(!X{!eedijig]y high
in tuifaeeous material are generally Jdgldy kaolinized.
Ojie of the pockets of white kaolin
is shown
in^ figure
2.
The
typi(:al phases
of the rock are
readily recognized
iji
tlie
field by
the freshness, resistance
to erosion,
light gray color mottled
with
black
liornbleiide
plienocrvsts,
and
the
conspicuous
grains
of
quartz.
It
is
<pdte
differeul.
in
general
appearance
from
the
mon,otouous
audesite
porphyries found throughout
tlie
district.
HUBBARD, (iEOLOflY OF THE LARES DISTRICT
<}
Andesite.
Andesites of the type foiincl in the Ponce District are rather
rare.
They omnr in limit<Kl areas along the Rio Blanco east of Ailasco.
The most typicaJ occurrence is on the north side of t)ie Bio Bhrneo valley,
one mile east of the teriniiiiiB of the sugar railroad.
This ro(;k is locally
amygdaloid al, and luidoubtcdly a surface flow.
It is one of the few large
exposures of andesite in tho Lares District which
is not porphyritie.
In
tfiiu section, it shows miero-diabasic structure, produced hy inicrolites or
minute latli4ikc crystals of plagioelasc
(apparently oligoclase), with an
interstitial groundmass
of
ferromagnesiaii
mineral,
probably
entirely
hornl)len(le, though some
of
this may be secondary
frcini
auy-itc.
The
rock has undergone! considerable alteration
frojn^ weathering.
Andesite Porphyry.
This is
l»y far tlic most al)uudant type; of
ig.iH^njs
rock found in the district.
Its chief occurrences as
)utrusi\'c bodies are
sliown on the geologic map.
It occurs
as: surface
flows at other
points,
notably south of-Aguada,, and at
Iv.
;10.7 on
tho Miiyaguez^Las
.\Iii,rias
Koad.
Its (>xtnisive origin in these instances
is showji
t>y
tiwi
al)ijinhiii(-'e
of amygdaloidal
ca.vities.
In a few
localities
typical
palisade structure
is develoijcd,
as,
for example, uu
the Kio JManco,
Ih^
miles east of
tlie
end of the Lares-Rio Blanco Road
(Fig.
:]).
Dikes of andesite ptyrphyry
intruded
in massive
tuffs
are found
at
various
points
along
tlie
Rio
10
SVIEXTIFIV SUMl'HY OF PORTO RICO
Blaiieo sugar
jiiilway, cast uf Ailaseo.
This rock also
or-ciirs \m
sills
iii
the tuffs and ash beds, but many oC tlieso are small and not shown ou the
geologic
inia.|j,
Plagioclftse is nlwavs the
cliiol* mijieral and most persistent p1ieoo(n-\'st-
forming constituent.
Of
tlie plagioclases, andesine and
andesiue-oli«-o-
clas(3 are bj far the most common.
In many cases, labradoritc
is present
as a principal mineral with the audesine.
Oligoclase rarely occurs
as
a
pririci|)al mineral.
Hornblende
is the ever present ferrojiiagnesian
coii-
stitiieiit.
Where
it
is not abundant,
th(»
ro(!k
is
|)ink
or
light grgy
in
HUBBARD, GEOLOGY OF THE LARE.S DWTR/CT
H
color due to the preponderance of feldspars, but where the ferromagne-
sian element is present in abundance, as
is more commonly the case, the
rock
is gray or dark gray
if labradorite
is present.
The accessory min-
erals are angite, magnetite,
apatite,
pyrite, and
rarely
ilmenite,
albite^
and
quartz.
The common
alterations
are, hornblende
to
chlorite,
ser-
pentine,
epidote,
limonite,
calcite,
and
quartz;
plagioclase
to
calcite^
kaolin, quartz and saussurite.
The plagioclases occur also as microlites
in the groundmass, and
ex-
hibit a marked parallelism in their arrangement
as
a result of flowage
of the magma during crystallization.
Fractured
or fragmental pheno-
crysts are very common,
also indicating flowage.
Strain
effects can
in
many cases be observed in the phenocry^sts,
as might be expected.
The
zonal
effects
of the
plagioclase
phenocrysts
is one
of the most typical
features of nearly all of the andesite porphyries.
These zones of growth
are of variable composition,
as shown by their slightly different extinc-
tion angles and
differential
alteration.
In some crystals the interior
is
completely altered, while the border
is fresh.
In other cases the altera-
tion has affected alternate zones,
so that the concentric structure
is
vis-
ible in hand specimens of the rock.
The abundance of carbonate as an
alteration product of the plagioclases
is illustrated by the fact that many
of the weathered andesites effervesce with acid.
From a study of
slides of the andesite porphyries certain associations
of minerals are evident.
Either andesine or andesine-oligoclase
is always
present as a principal mineral, and
in some
cases both
varieties
occur.
With these either oligoclase or labradorite may be present, but oligoclase
apparently does not occur with the labradorite.
Where augite
is present
as an accessory mineral, labradorite usually occurs as a principal mineral.
Ilmenite
is
associated with magnetite but
does not occur
unless mag-
netite
is present.
It
is not nearly
as common
in
the
andesites
of the
Lares District as in similar rocks of the Ponce
District.
Quartz and calcite are frequently found filling joints and small cracks.
In a few localities pyrite and chalcopyrite occur in the quartz veins.
The
best example of this
is found on the Eio Blanco 3iear Alto Sano.
The
quartz
veins
occur
in
andesite
porphyry
dikes
cutting
massive
tuffs.
With the chalcopyrite
are minor
quantities
of
bornite.
The
veins
are
nowhere more than 3 inches thick, and most of them much
less.
In the
amygdaloidal andesite porphyry in the area south of Aguada, and on
tlie
Rio Blanco sugar railroad,
3 miles east of Ahasco, introduced matter
is
found in the form of amygdules, partially or wholly
filling the
cavities.
The amygdules
are
cluefly
of
amorplious
silica
and
zeolites.
In
the
12
SCIENTIFIC SURVEY OF PORTO RICO
locality south
of Aguada, massive veins
of banded
agate with
core
of
crystalline quartz, occur abundantly, cutting the andesite porphyry, and
apparently associated with nearby
intrusive bodies
of
serpentine.
The
amygdaloidal
cavities
are elongate
parallel with the
ilowage alignment
of the plagioclase phenocrysts, and are lined with banded agate or other
amorphous silica, the interior being filled with crystalline quartz.
Where
they are
only
partially
filled,
the
quartz pyramids form
small geodes,
with incrustations of prehnite and other zeolites.
More or less tuffaceous material
is present in practically* all the ande-
site porphyries.
In thin sections, this included matter
is seen to consist
mainly
of broken
crystals
of feldspar, hornblende, and
other minerals.
It
is often
difficult
to determine whether these
are merely phenocrysts
broken up by flowage during crystallization,
or whether they are
tuffa-
ceous materials of extraneous derivation.
Not infrequently fragments of
devitrified glass, or lithlic fragments of more than one crystal are encoun-
tered, and these indicate the true nature of the broken crystals associated
with them.
In some cases the rock
is so crowded with fragmental ma-
terials that
it
is impossible to
tell whether
it
is of igneous or of clastic
origin.
In examining thin sections, one may be considerably surprised to
find fragmental foraminiferal
shells
in what otherwise appears
to be
a
typical andesite porphyry.
The
difficulty
is increased by the
fact that
many
of the tuffaceous crystalline fragments,
especially the
plagioclase,
are so regular in form that they are not distinguishable from phenocrysts.
It may truthfully be said that there are
all gradations between a tuffa-
ceous andesite and an andesite tuff.
Augite Andesite,
The only known occurrence of
this rock
is
in the
southern half of the area of lava flows shown on the geologic map south
of Aguada.
It
is
a dark
massive
amygdaloidal
to
vesicular
lava,
in
some places very ropy and scoriaceous.
The amygdaloidal cavities have
no regularity of occurrence which might
indicate
successive
flows.
In
some places, the
cavities
are
filled with amorphous
silica,
zeolites, and
native copper with associated copper minerals.
Elsewhere the amygda-
loidal cavities are empty, and the rock
is light in weight due to
its ex-
treme porosity.
In some exposures
it resembles
a coarse pumice.
The
mineralized areas are associated with points and small fault crush zones.
The latter are
filled with
calcite,
stilbite, native copper, malachite, and
other minerals associated with the copper.
This mineralization
fills both
the crush zones and adjacent
cavities in the rock.
This
locality
is de-
scribed more fully in the chapter on mineral resources.
In
thin
section, this
lava shows
a
dark ferromagnesian groundmass
HUBBARD, GEOLOGY OF THE LARES DISTRICT
13
with fine lathes or microlites of plagioclase
(chiefly andesine)
exhibiting
perfect flow structure.
Occasional small grains of ferromagnesian min-
eral show the presence of augite, but whether hornblende
is present also
was not determined.
Augite Andesite Porphyry.
'Next to andesite
porphyry,
this
is
the
most abundant type
of igneous rock
in
the
Lares
District.
Like
the
former,
it occurs
characteristically
as elongate intrusive
bodies,
appar-
ently
sills or laccoliths.
It
is also associated with the larger and more
massive bodies *of gabbro porphyry, of which it seems to form lateral off-
shoots.
Examples
of
this relationship are found on the Mayaguez-Las
Marias Eoad between K. 8 and K. 9, and south of the Lares Road near
K. 37.
In some other localities it
is associated with, and seems to grade
into andesite porphyry, as in the Rio Blanco valley south of Lares, and
in the same valley farther west, near east terminus of the sugar railroad
from Anasco.
This rock
is distinguished in the
field from andesite porphyry by
its
darker
gray
to
greenish
color,
the
latter
being
due
to
alteration
of
augite.
The large phenocrysts of augite are another distinctive charac-
teristic.
There are
all gradations between augite andesite porphyry and
gabbro
porphyries, depending merely upon
the
relative prominence
of
phenocrysts and groundmass.
Both varieties of rock may frequently be
found in the same outcrop.
The principal minerals are augite, andesine-
oligoclase, labradorite, andesine, and oligoclase.
It
is seldom that more
than two of these
are found occurring together
as .principal minerals.
The
commonest
association
is
labradorite
with
andesine.
The
usual
accessory minerals are magnetite, hornblende, one or more of the above
named plagioclase
varieties,
apatite, and pyrite.
Hornblende
is present
as a principal constituent in a few of the specimens.
Occasionally what
appears to be primary quartz
is found in small quantity.
The common
alterations
are,
augite
to
chlorite,
uralite,
serpentine,
and
carbonate;
plagioclases
to
saussurite,
carbonate,
kaolin and
quartz;
magnetite
to
limonite.
Of introduced material, calcite occurs in minor quantities in
some specimens.
Texturally, the rock
is a typical augite porphyry, with the plagioclase
phenocrysts generally
smaller than
the
augite, though
considerable
of
the
latter
is
disseminated
as
small
grains
in
the
groundmass.
The
plagioclases are characterized by strain effects, zonal growth, and usually
by
parallel arrangement due
to
flowage.
They
are
in many
instances
fractured, with the
cracks
filled by groundmass.
A
poikilitic habit
is
occasionally
seen,
with
inclusions
of
ferromagnesian
minerals
in
the
plagioclase phenocrysts.
14
SCIENTIFIC SURVEY OF PORTO RICO
The outcrops of this rock are, as a rule, fairly fresh, but thin sections
show considerable alterations.
Secondary carbonate is often so abundant
that the rock
will
effervesce with
acid.
This
is
especially true where
there
is a considerable quantity of included tuffaceous material.
Gabhro Porphyry.
This rock
is found on
the Mayaguez-Las Marias
Koad between K. 8 and K.
9, and south of the Lares Eoad near K.
37.
As
previously
noted,
it
is
associated
with
the
less
porphyritic
augite
andesite porphyry.
It forms rather large intrusive bodies which should
be classed as bosses or volcanic necks.
They do not exhibit the elongate
form characterizing the andesite porphyries.
In the case
of the occur-
rence on the Mayaguez-Las Marias Eoad,
the
intrusive
relationship
to
the adjacent shale beds is evident.
Mineralogically,
the
rock
is
about
the same
as
the
augite
andesite
porphyry, the
chief difference being in
texture.
The commonest prin-
cipal
minerals
are
augite,
andesine,
and
labradorite.
As
accessories,
magnetite
is present in every specimen, with usually some hornblende,
apatite, and andesine-oligoclase.
The characteristic alteration products,
as in the augite andesite porphyries, are chlorite, secondary hornblende,
serpentine,
epidote,
kaolin,
carbonate,
saussurite,
quartz, and
limonite.
Strain
effects, and evidences
of fracturing and flowage during
crystal-
lization are characteristic.
Likewise,
the usual zone
effects are seen
in
the plagioclases, with apparently the more acid variations making up the
outer zones of the crystals.
The groundmass
is characteristically dark,
due to the predominance of ferromagnesian constituents, and
is usually
marked by microlites and
plagioclase.
Quite
often very
little ground-
mass
is
present.
In some
specimens,
the
augite phenocrysts
are
very
large, and the augite is greatly in excess over the feldspar.
These phases
are
typical
augite
porphyrites,
like
those found near
Maricao,
in
the
Ponce District.
Serpentine.
No outcrops of this rock were found in the Lares District,
but
its presence
is shown by
its surface weathering product, the typical
red limonite
soil with limonite concretions, like that covering the Mesa^
southeast of Mayaguez.
It is presumably derived from an olivine-bearing
rock.
The distribution of this material
is shown on the geologic map in
the volcanic area south of iVguada.
It occurs in small irregular patches,
lying wholly within the area of andesite lava flows.
The serpentine prob-
ably
occurs
as
dikes
in
the
andesite.
The
limonite
ore
is
similar
in
general character and
origin
to the Mayaguez
ore, which has been
de-
scribed by Fettke
(1918)
and by Mitchell
(1922)
and shows that
the
Mayaguez
serpentine body
is undoubtedly
intrusive
into
the
adjacent
HUBBARD, GEOLOGY OF THE LAREIS DWTRICT
15
shales, and hence represents one of the later phases of volcanic
activity
affecting Porto Eico.
The Aguada
serpentine
is doubtless the contem-
porary of the Mayaguez intrusive.
SEDIMENTARY AND CLASTIC ROCKS
The sediments are almost entirely of clastic make-up, and derived
in
large part from volcanic sources.
There
is a total absence of sandstones
and conglomerates, and very few of the fragments in the tuffs show even
the slightest suggestion of roundness.
Limestones are also lacking except
in
a few
isolated
instances
of no importance, and
even
these
contain
minor quantities
of tuffaceous material.
The following types of
clastic
or sedimentary rock seem worthy of separate
description
:
Table 2
Name of Rock
Occurrence
Massive
tuffs and agglomerate
Chiefly
in
the
eastern
part
of
the
district,
and
in
the
central
zone
called
the Rio Blanco
series.
Bedded
tuffs
Interbedded with shales and ash.
Ash
Fnterbedded
with
shales
and
tuff",
beds
usually
not
as
thick
as
the
tuffs.
Ashv shale
Do
Black
shale
Southwestern portion of the
district.
Tjime
shale
Wide
spread,
but
characteristic
of
Pt. Jiguero region and the Atalaya
Range north of Aiiasco.
Tilnipstoiip
M
•••
South of Las Marias and on Desecheo
Island.
As lenses
of small extent
in
shale
or
tuff.
Ohert
North of Mayaguez and on Desecheo
Island, as lenses
in shale.
Massive Tuffs and Agglomerate.
These rocks are so deeply weathered
that very little can be found out about their structure and composition.
1(5
SeiKN'JlFIV miRfEY OF PORTO RICO
In many places wt^athcrod exposures
will show the
lithic fragments ap-
parently
well
preserved,
\mi with the entire rock
so badlv cleeiwed
thai,
it can
be cut
into
sliees with
a knife
as
easily
as
clay.
In
the more
material.
Thiis rork
)
time.
ma-^isive
CX| Khsure.'
fragm(!ii:tsc11 Ji
l)e
up of
fla.ginent^;
<
<:;h,ief w(athering
labs of
li
c o£ the
, IHn
Biftnvn
iiear m
w shale imbedded
In
uth
of mo PrieAo
I. matrix of finer pyrocln
Ivity rluflog late Cretac<
(it the
slightei^t
trace
of bedding or assortment
of
1.
Iti eoinpo8ition, they are for the most part made
tmhiHite
p<)rj)hyry.
Kiiolin and iron oxides
are- the
thieliS.
The more reddish
areas of clay
soil on
the
HUBnARD, (IKOLOOY OF THE LARKt( Dlf^TUICT
l'^
mountainous
^^lopes
in the southeastern
pr)rtioii of
tlie Larci^
iJistrict are
derived chiefly from
tliej^(> iiiaKsive
tuffs.
The coarse
a,iJ;gioinera.t;ic phases
are of iiMjre limited
oe<:-urrctie(\ usually
lieiug fount!
elose to the centers
where volcauie
activity was
particularly
stTong.
One
of
the
best
ex-
posures of this
tvp(! of rock
is fourn]
in
the Kio Blanco valley soiitliwest
of Lar(!s, elose to the mouth of the Rio
J'l-ieto.
I.arge slahs of lime
slnile
are foirnd ijnbedded in a massive, ecnnpaet agglomerate, mad<>
\ip largely
of antlusite fragments
(Fig. 4).
The lime shale oecur.s nearby
in
a
rela-
ti\'ely
undisturbed
condition,
and
alternates
with
lliin
lavers
ol'
tuff.
^#*.wli'^*^-*^--
^X'^-^-
*'
-*W*-.iS*"s^^
:*.
-
^»-'
: V-
^?*:^'^^'^-^.,£.;-. - .^
*
-
^'L
Ajiparently
tlie lime shale eousolidatetl soon aftef deposition, leaving the
inierbedded
tuff: bods
still uneonsoLid,ated, so that vvlieu the voleauie dis-
ruptive action
tt)ok phice,
la.rgc
slabs of
tlie
slialc were thrown up and
ind)ed<led
in
tln^
I'esulting agglomerate.
'J'ho
tulf
layers,
being uneoo-
s(didated, were completely broken up and distributed
as groundmasH
in
the agglomerate.
This grouiulmass
is composed of fragments of felds))ar.
(juartz,
ealeite,
magnetite,
I'erromagues ian
minerals,
pieces
of
andosite
por|ihyry, and
lime
shale.
Another type
of agglomerate,
in
which
the
fragments are chiefly large blocks of an,desite
porpliyry
is
well
e.vptJSed
south
of K.
8, Lares
Itoad
(Fig.
5}.
This
unit<,'ria,l
lies
innnediately
adjacent to
a
s^mall
Ikj.-s or
volcrtisie
ncek
of
im,<,ntx'
aiulcHito
|)(>rphyry,
and in turn k j^urruuiided by thin \mh uf totl* and Fhale
B-hii'h have been
dilfererifially
indurateil and
alt.ored
in
proximity
to
tlie intrusive body
and aggiomerate
(Fig.
0).
A
sihort distance from the area of volcanic
activity, they are rehitively undistniiiod and are raialtered.
Bedded Tuffs.
TJiese are, for the mo.«t
part,
fine grained, and
gradt^
into
typical ash
beds.
They
occur
characteristically
interhcchh;^!
with
shales and ash, and are distrilnited
tltnniglioof^ the
district, thongJi more
ahumjaiit
in the
western,
part.
Whih;'
tlic coarseness of the
njatcrial
is
tilnilcH ami
tlihi-bedunl
tulf,
Hhowirijj
ulteratiun
i'ffff<i.<i
pm<lm-r<l
!>»
a
m-tn-hii
qnite
variable, even
in
short
distances, few
of the
fragments
are
hirger
tlian
one-half
incli
in
diameter.
They
are composed
of
broken, pbeno-
crysts or erystalfin,e ;particles derived from andesite porpliyries, and lienco
contain
tlic
nsual
nn'nerals,
sncli
a$
plagioclasc,
t^rromagiicsians, ami
magnetite.
Wlicre finely intcrhedtk;d with
sliaJcs, they contain
a
(ronsid-
erablc percentage of ealcite and ipiartz graiiis.
Some of the cpiartz grains
show slight traces of rtninded edges, as thoiigli subjected to current action.
Wherever the tnt! is iiiterl)edded
witli bbick
shale, the black sliale forms
a
matrix
for
the angular grains
of the
tulL
Where
interbedded
with
linie shale, iJmw have a Jiigh content of ealcite and fragments of the lime
HfHUfAh'TK i!B(UA)(JY OF T!IK LM{Et< DISTllKJT
HJ
sliiile, and
.have many
veins
of
r-ahMte
in
Irx-alities
of
close
folding
or
faiiltin^n
The ImUUHl tuffs r-an usually bo n'cogiiizfMl
in
tlio field by their
cliaracteristic
Hiilun-oidal
m'i'atlieriii_g
(Fig.
T)
whieh, distinguishes them
from
tile
oecai^ional
thin^
si Lis
<jf
andosite.
This type
of
weathorijig
is
also
to
be
found
in^
the more massive
tiilfs
(Fig.
8).
It
is sometimes
ditlieiilt to distinguish
soeli oeeurreiiees
fron.i congJomerate.
.\s has
been^ pointed out
by Berkey.
Aliiehell, antl
others, the
tutfs are
much more suhjei't to surface weatliering than igneous
nn'ks of the same
mineral compositinn.
It
is only where the tuff
lias been
iiKhirnted.
rlue
1#^...-'
y.
",.
..:.:--
. Asiuitla-Uinwn
llima.
to the action of a nearlyv intrusive body, that
it
is more resistant to ero-
sion than the shale
witli which
it
is interhiMhhid
( Fig. 5).
The tut! beds
are
(iuiractcrisiically
thicker
tliiui
the
adjacent
beds
of
shale.
Tliey
range from
6
inches
to
several
feet, and have no minor
stratification.
alt.h.ough some beds sh,ovv a slight assortment of coarse and
fine material
Aj<h.
Beds of ash occur
witli
the shales and
Ixnhled
tuffs.
Th.rv
re-
semble the
shales, and
where; the outcrop
is weathered
tht; two
are
not
distinguishable.
Typical ash beds are not abundant.
The most <'omm,ou
<K!eurrein:!e
is shale
high,
in ashy niaterial, and
there
an>
all
gradatiruis
between a typical sliale and a typical ash.
Where the roc-k
is exceedingly
iine grained,
it
is sometimes
impossible
to
deterrnine
the
eoint»osition.
20
i^C/KNTlFJC iyURVEY OF POUTO RICO
Anj
fine
partj'<'le?
of
volcttiiie
glai?B
wliieli may have been present are
usually
not
rceogJikiible
in
tliiii
s(X;tioiis, owing
to
devitrifioatiou and
alteraf.io;ii.
Jn some
ease-s,
iniiiutc
rod-like
particles
are
present,
siia;-
g(»sliv(» of fibrous glass or pele's; hair, but tbese are invariably found badly
altered
l)y
weatliering.
Tlie
ash
beds
grade,
in
texture,
to
fine
grain
tnifs
whit;!i do
not.
<liffer otherwise froui
typical
ash.
In, thin sections,
ash may usually be distingirished from
sluile by
its lack of assortment of
nm
Expo
.
pla:
paTticb?« and absesice or >eareity
ol;
b)raniinil'eral
shells.
In baud
speci-
mens
it
i.s usually
distingiiishal)le by
its fracture.
Berkey has pointed
out the
fact that the shales gen(*rally break with a bloeky fracture, espe-
cially
in
th(!
case
cd*
\veather(;d
bbick
sfiale.
Tlie lime
shales
liave
a
emooti), curved
or almost coneboidal
fractur(i.
Typical
aslies,
liowever,
havi' a ronghi'r. more
irregular
fraetirre,
dn.<^
to
tlie uuevenness
of the
weathering which tliey have undergone.
As a
rule, the typical ashes arc
ol slightly coarser texture
tliau most of the
sbales_, but this dilTereuce is
not evident in band specimens.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
21
The ashes, and ashy beds
in general,
are
divisible into two
types,
calcareous and
non-calcareous.
The former
are
chiefly
shales high
in
ash
content, and carrying
a few
foraminiferal remains and
grains
of
calcite.
The calcite grains are in some cases slightly rounded and pitted
as though by solution.
The non-calcareous variety
is made up chiefly of
andesitic material, minute angular crystalline particles, and an indeter-
minate
groundmass,
containing
possibly
devitrified
glass
and
altered
crystalline fragments.
They contain no foraminiferal shells and but very
little calcite, except as introduced matter filling veins.
Of the two types
of ash, the calcareous
is the more common in the southwestern part of
the
district, while
in the northwest portion
of the Cretaceous
area, the
non-calcareous variety predominates except in the area south of Moca.
Ashy Shales.
Shales high in the ash content can usually be classified
only by the use of thin sections.
The depth to which they are weathered
increases the
difficulty.
The weathered ashy
shales are generally non-
calcareous, and resemble the weathered black
shale, but are higher
in
kaolinized material.
They are typically reddish brown to gray in
color,
and at some places contain white patches or streaks of kaolin
(Fig. 9).
Black Shales.
These have been described by Berkey and Mitchell, and
similar shales have been described by Semmes.
Mitchell shows that the
red, blocky shales exposed around Mayaguez are the weathering product
of a black shale, found exposed as such, only in quarries or stream chan-
nels.
Fresh exposures of
this shale occur south and northeast of Con-
sumo and at K.
3.7, Mayaguez-Consumo Eoad.
There are all gradations
between a pyritiferous black shale and a very dark lime shale.
The black
shale
exposed
near
Consumo
shows
in
thin
sections,
a
dark
opaque
groundmass, apparently in part carbonaceous matter,
in which
are im-
bedded many foraminiferal
shells,
a few minute angular fragments
of
quartz and feldspar, and comparatively large well formed cubes of pyrite.
The
pyrite
in
crystallizing
out has crowded the surrounding carbona-
ceous groundmass.
Carbonaceous films, apparently small plant remains,
are seen in some hand specimens of the rock.
The only calcareous ma-
terial
visible
in
thin
sections
consists
of
foraminifera,
but
these
are
sufficient to cause the rock to efl'ervesce with acid.
K microscopic strati-
fication or assortment
is
visible in thin
sections, but
is not evident
in
hand specimens.
A more
calcareous phase
of
this
shale
occurs
at K.
3.7, Mayaguez-
Consumo Eoad.
The rock in this locality is an alternation of thin bands
of the dark foraminiferal shale with bands of fine grained andesitic
tuff*.
The contact between the two
is very sharp.
In thin
sections, the shale
22
fif'll-]NTIFlC ^^URVI'jY of PORTO RICO
sliows a (lark carbonaceous gr-oinuliiias.s witlt
foraniijiiferal
nliolls in
aljuii-
(lanec.
The adjacent
tiilT
layers are cuinpused of angular
fragiiients of
andesite and
fragmeiital
crystals, all of
whieli are imlieddcd
in
tlie j^ame
dark
groiiiidmass
wliieh
forms
the
l)ulk
of
the
shalt;.
f^'oramiiiiferal
shells are
ft)uiHl
in
the
tuff
layers
with
the
carl>onac:eoiis
matrix.
In
addition
to foraminifera, the shale coiitains an abundaiiee of radiolariau
Hl'BBAlilf, OEOLOflY OF TIIK LAMES lUsTHIVT
23
reiuaiiis, markccj
by
touihIcm]
Ixxlien
(»f amorphouK
or
?;j)lioriilitii'
<;ili(ii.
Tlowevor, a few
iiiialtereil
ra^ijiolariaii
sbclls
(teeiir.
lAme
Slialen.
Tliese arc miicli commotH'r
tluiii the
hliK-k
shale.
Tvf)-
k-al. eAposiires of litne slialo are shown
in ii^ure^
10 uud U.
Many of
ilic
layi'i's are
liigiily
ealcareoiiH, and have the eharaeter of a
tliiii-beflih^e!
or
handed, hard,
conrp?i('t limestone, dark
hliiish gray
in
erdor.
Expo.<iire.^
of this type arc to be soon in fjuarries along the autrjinobilo road
erosf^ing
Ihe Alabiya Kange northwest of Afuiseo.
In thin section, thbs nsek proves to be made up eldetly
<d'
c'ak'"-ito grains
with
a variable amomit of voh-anio
nuittcn-.
sricli as amndiir i'rao-ments of
^£^.
-^^H^.
M-.^-^y^^'
foldnpar,
hornblemlo,
and
magnetite,
('onsiderahle
reerystallizatiuji
of
the caleite
is evideirt, and this gives
tlie rock
its
liigii density and
eojn-
piietness.
Iroramini feral sliells occur, oficn^
in considerable (juantitj, hut
in
some
specimens they
arc
absent.
Tlriis
this
roek
difl'er.s
from
the
])yritiferons black shale in that
its calcareous content
is not dcpetident
upon
the presence of foraniinifera.
Tlie foraminiferal
shells are
all of
microscopic size, and include indeterminate forms rescnd)ling (jlohi<jm'imi
and Rotalia.
In some cases their structure has
liecn entirely destroyed
by subsequent crystallization
to aragonitc.
Some of the shells are frag-
24
SCIENTIFIC SURVEY OF PORTO RICO
mental,
as though having been
transported before
deposition, but
this
feature is not common.
Owing to its high lime content, lime shale
is more resistant to erosion
and weathering than any other rock occurring abundantly in the Lares
District.
Along the Atalaya Kange, north and northwest of Aiiasco, their
influence upon the topography
is rather marked.
Other,
less extensive,
developments of the same type of shale occur on Desecheo Island, in the
Eio Blanco valley near the mouth of the Rio Prieto, at certain points in
the
hills
south
of Moca, and
along
the
automobile
road
northeast
of
Rincon.
Limestone.
True limestone is almost unknown in the Cretaceous rocks
of the Lares District, although one might
class certain portions of the
lime shale as a limestone.
A
small but prominent
inlier
of
limestone
occurs
southeast
of
the
Mayaguez-Las Marias Road opposite K. 22.
This rock is a hard, compact
fine grained crystalline limestone, gray to pink
in
color, and white on
weathered surfaces.
The structure is massive, with no bedding visible in
the outcrop.
Bedding, however, may have been obscured, since the out-
crop shows faulting and fracturing to an extreme degree, with numerous
calcite veins.
In thin section, the rock appears as a fairly pure limestone,
made up almost entirely of recrystallized calcite grains with a very small
admixture of tuffaceous material.
It contains many foraminiferal shells
(chiefly Nummulites
and
Glohigerina)
and
fragments
of
RadioUtes.
Lithologically, the rock resembles the San German limestone of the Ponce
District.
A few thin beds of similar rock occur with the lime shales on
Desecheo Island.
These exhibit distinct stratification, and are banded by
zones of tuifaceous material.
The fossils are chiefly foraminiferal shells
(Glohigerina and others),
in great abundance, and a few fragments of
Badiolites.
Chert.
Chert beds are of very local extent.
They occur as lenses in
the shales north of Mayaguez.
Mitchell found an outcrop on the north
side of Mayaguez Bay.
Another exposure
is to be seen just west of K. 2,
Mayaguez-Aiiasco Road.
Chert
is
also found in small quantity,
inter-
bedded with shales on Desecheo Island.
In its field occurrence, this rock
is
conspicuous
because
of
its
relatively
fresh,
unweathered
condition.
The color varies from gray to pink, or reddish brown where stained by
limonite.
It
is
extremely hard throughout, but
extensively
fractured,
and breaks into sharp, very irregular blocks, not having the conchoidal
or curved fracture surfaces characteristic of most cherts.
In thin section,
under crossed nicols,
it appears as a fine aggregate of amorphous
silica.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
25
crowded with microspherulitic portions^ which are very probably the re-
mains of radiolarian
shells.
Tuffaceous material is present in very small
amounts.
Stratigraphy
Owing to
difficulties of field work and the limited time spent in the
field,
it is impossible to correlate rocks of similar lithologic habit occur-
ring in various portions of the district.
It is possible, however, to make
larger
subdivisions, each doubtless containing several formations.
The
geologic map shows two belts of shales, with a central belt of tuffs, intru-
sives, and
extrusives.
The northern
or
oldest belt
of rocks comprises
tuffs and intrusives in the southeast, which grade into shales to the north-
west.
This belt is called the Eio Culebrinas Series because of its develop-
ment along the Rio Culebrinas valley.
The central belt, predominantly
tuffs and andesitic
rocks,
is
called the Rio Blanco
Series, from
its ex-
posure along the Rio Blanco.
The Rio Yauco
Series,
in
the southern
portion of the district,
is named from the Rio Yauco
Shale, a term ap-
plied by Mitchell to the black shales which form a prominent part of this
series between Mayaguez and Consumo.
The relationship of these three
series, as well as the complexity of the structure,
is shown in the cross-
sections
(Plate
I,
Figs.
7-10).
It should be noted that the boundaries
of these subdivisions are marked more or less by gradational change from
one prevailing type of rock to another, hence the boundary lines as drawn
on the geologic map are somewhat arbitrary.
On this map no attempt
has been made to show all the occurrences of each type of rock.
Thus
where shale
is the predominant rock, the entire area
is mapped as shale,
and only the larger tuJf beds or intrusives are shown.
THE RIO CULEBRI^^lS SERIES
In the region south of Lares, this series
is represented by a wide area
of tuffs.
Exposures of unweathered rock are so few that the true char-
acter of the underlying formation cannot be determined in most places.
At least one small area of shale occurs east of Lares, and there are doubt-
•
less many others.
Intrusive bodies of andesite porphyry, augite andesite
porphyry, and gabbro porphyry are found, chiefly in the vicinity of Lares.
To the northwest, the
tuffs grade
into
shales.
This
transition
is
well
shown by the alternation
(interfingering) found in the section south of
San Sebastian (Plate
I, Fig. 7).
Still farther to the northwest, shale
is the predominant material, but
is interbedded throughout with ash and
tuff.
A small area of intrusive,
niniHAIlD, IIFAHAKIY OF THH L.t/?/vK DIHTIflVT
the prevailing dip
in to the s<)lltllW('^i.
Most of the old
voleaiiic vmr< are
in
this
1)elr.
Aiuoiig these, the two
luoHt important ones oeenr
f^outh of
A^ruHda
and
smitli
of Lares
in
tlio
I{io Blanco
valUn-.
The former
is
marked hv the
largest area of surface
flows
in the
district, eosisisting of
amygchdoidal
rocks
(an(h^site and
aiiu-ite andcsitc).
It
is
in
the angite
aiidesite that native copper occurs.
Jn
this
hirgv area
of surface
flows
are
siiiidl
intTOsive^
of serpentine, marked on
the
snrfacte hx
patclies of
iron ore like that rsecurring
oji
the Mesa
at
Mnyagucz.
An<itlier srirface
flow of an,gile andesite
is found in tlie llio Anasco valley al)oiit tlirec miles
ea.st of Ana^co.
Tlicre
is no niitivc co[»per,
liowevcr,
in
tliis roek.
w«>st.
Tho wnsKive
Uii»-k-l«'d(l<.r:i
(nlTs are seen on tlw
rljrht.
dijipiiig
s^twiil.v
t<»
tlic rast
Tills
dlscor(lan<>o
in
dip
tin«1
strike was
louud upon
eXiiHilnatton
to
l.e
the
result
<ȣ
a
tliriwt fann
uf
considerable
iiiiisijltude.
Tlw
area, south of Lares
in
th(>
l{io
.Blanco
valley k
an.
old
volcanic
complex and
;|.n.'obahl.y represents the location of several former
volc.-uioes.
Only one
area
of
iindouhted.
surface
flows
is
.found
in
the
R.io Blanco
Series,
l)ut andesite
ptvrphyry showing eohimnar structure
occu.rs along
the river
al)ou.t
I.14 miles east of the 'end
of the Larcs-lio
Bla..nco Hoad
(see
I/i.g.
3).
Tlie
(.'ohimns
exhibit
radial
arrangemeiit and
prol)ably
mark an old volcanic throat.
The size of the plienoerysts seems to show
that this roek was not a surface How.
.Northwest of this point
is a rattier
large in.eluded area of sliales,
tull's, a.nd lime shales, lithologieally similar
2S
SCIENTIFIC SUR'VEY OF PORTO RICO
to those of the Eio Taiico Series.
Tliis iiilier of acKiimeiits lias bcori badly
iiuiltx'd and
exit up by
tlic surroiuuling
iiitrusives.
In
the Eio
Bbiiieo
valle)'
lit
tht! nioiith oC
tiie Kio
Prioto,
tht;
liirie
sliaJos are
fauJteil
(Vig.
12} and
l;)rokeJi up,
IVirmiiiff in places a vidcaiiic a«'giumeratt; of cUKlositic
cojiteiit in wliidi large slabs of the
sliale are imbedded
(Fig. 4).
^'he
ijitriLsioii
of quartz diorite porphyry, sliown on the map in
this volcaruc
area,
is best exposed
in
a higfi peak known
as
M;t.
Torrecillo
(Fig.
I),
located
at K, 48-^9 on the Lan>s-Eio illaneo Eoad.
Tlw
b(\st: exposures of the liio Blaiieu Series are foioid along
tlie sugar
railroad
east of Ailaseo
in
liie Rio Bhmco valley,
(loiug upstream, one
passes from tlie h(4t of Eio Yaueo Series shalcis into a su(;eession of
tuJfs,
andesite thiws and
andesite porphyry
intrusivos,
throiiglioiit
whicli
verv
litth'i uf the stru('tTire ean be seen.
C)<;(-'asionalIy ohsenre hediling may be
fouud in
tlie tuffs, or
tlie dip of a fh.w or
sill of andesite may he
dislj'n-
guished.
Ileri; ami
there
are
small included
patetses
of shale
of verv
limited
extent.
'Fhe most
curions
au.d
njiexptK^ded
feature
cd'
ilw
Rio
J^hineo Heries
is
tlie pivsonee of an
isidated ouierop of limestone, rising
from
a monotonous
aJ-ea of deeply
weallicred
roek,
all a'pparcntly
tuff.
As shown on
ilie
jiiap,
this
liinestojie
lies 1^,
mihiS southwest
of Las
ifarias, and may be seen from a point near K. 23 on
tJit^ Mavaguez-T^as
Marias Road.
It
is similar
lithologieally to the San Oerman
limestone
HUBBARD, GEOLOGY OF THE LARES DISTRICT
29
of the Ponce District and contains Radiolites sp. and many foraminifera,
including
indeterminate
species
of
Glohigerina,
Nummulites
and
an
Orhitoides or similar form belonging to the Orhitoides group.
THP] RIO YAUCO SERIES
This
series,
as previously noted,
is predominantly an area
of
shales,
often interbedded with ash and tuif, and with occasional andesitic flows
and intrusives.
Most of the shale
is much
less weathered than that of
the Eio Culebrinas
Series.
This is due to the relative abundance of the
hard gray-blue lime shales which are much more resistant to erosion and
weathering than are the more ashy, easily kaolinized shales of the series
to the north.
It would seem that the Rio Yauco beds have undergone
greater folding and faulting than the underlying rocks of the other two
series, but this difference is probably only apparent, not real.
The details
of structure in the other two series are usually obscured by weathering.
The northern,
or
stratigraphically
lowest,
portion
of
the Rio Yauco
belt contains much shale of the ashy type alternating witli
tuffs.
Excel-
lent exposures of these are found along the automobile road northeast of
Rincon.
A very
characteristic
feature
of
these
beds
is
the
spheroidal
weathering of the tuffs
(Figs.
7 and 13).
Southeast from Rincon, and
forming the range
of
hills known
as Atalaya
Peak,
are
developed the
blue-gray, hard lime
shales, often banded, and resembling the
Peiiuelas
shale of the Ponce
District.
In the almost total absence
of true lime-
stone in the Lares District these lime shales are among the ridge-form-
ing types of rocks to be found.
Southeast from Ahasco these lime shales
grade
into the
pyritic black
shale
of
the
vicinity
of Consumo, and
its
weathered product,
the
red
kaolinized
shales,
are
well exposed
in
the
vicinity of Mayaguez.
Tuffs of the more massive type are more abundant southeast of Ahasco
Playa.
An
intrusive
body
of
considerable
size
composed
of
gabbro
porphyry, grading into augite andesite porphyry,
is well exposed at vari-
ous points from K. 8 to K. 10 on the Mayaguez-Las Marias Road.
It
is
clearly intrusive into the shales.
North of Mayaguez, near K. 2 (Maya-
guez-Ahasco Road), are several small areas of chert, occurring as
lenses
in the surrounding shale.
They are apparently of radiolarian origin.
Desecheo
Island,
lying
15
miles west
of
Pt.
Jiguero,
contains more
features of geologic interest than any area of equal size on the mainland.
It
is a continuation of the belt of lime shales of the Rio Yauco
Series
which form the back-bone of the Cordillera Central, known on the west
coast of Porto Rico as Atalava Peak.
The arid climate of Desecheo has
•io
sciKxiine sritvEr of i'orto meo
preserved the ntck.- in a rclat.ivcly iiu weathered state, heuee tlie striicf.urc
can
be well
.^eeri.
The
I'liormoiis
amoiiiit
ol
faulting and
t'raeturiiig m
w(dl
i^howri
in
tigiire.< VI and
15.
Amtojg
tli<» special fcatiir(!S should be
mentioned the presence of eliert beds like those north of Majaguez, and
i)t
A
{(nv
thill
beds
of
limestone
of the San German
type,
containing
RndiolUeh- and an alnindauee of furaminifera.
STinTf:Ti:«AL Features
The
strike
of
the beds
is,
as
prtn'iously
stated,
north west-stnithcast.
ih\
tlie map
tiu;
lines desii^natina- Hhah» have
lieen drawn
so
as to
iiidi-
p^
^k-^' t
eate the sti'ike wherever known.
Tlic dip
is to tlie southwest, at variabh',
Irut
almost nniversally Idgh
angles,
Varial>lc
or northeast
dips denote
folding or fanlting, or both.
The ebaracteristic structure of the series
is sh,own
in
tlie four sections
(Plate
J, Figs.
T,
H,
<), 10).
Figures
!• ami
10 are generalized
or
i<h>al
sections, and illustrate the dilTerence between, the east and
tiie west parts
of the Lares
District..
Figures T and 8 are
ba.se<l on traverses nuide acrijss
the
series.
Wherever the course
of
the
traverse
has deviated
Crcmi
the
line of
tli(^' section,
tlie data has bficn projected on the section.
Tlie
l<K:a-
tions of the sections are not shown on the map, but may
readily be
se:Mi
from
the desigiuitions of towns througli wJdch they
pass.
Structurally,
the rocks of
tlie
district are of two kinds,—massive and
bedded.
Tlu; massive rocks include
t,lie andesite intrusives, of laecolithic
Scientific Survey of Porto Rico and the Virgin Islands
Volume
II, Part
1, Plate
I
FIG. 3
FIG4-
FIG.5
FIG. 4
HA TO ARKI8A- ISABELA
SECTION
DiRCCTtON
- NORTH 5* EAST
TOTAL TillUHtSi
3250 rZET
ft
T~~~
1
MltfS
C0UA20- Ql/EBRADILLAS
SECTION
JDiRtCflON
- NORTH- SOUTH
TOTAL THICKNESS
iSlSft.U
LARES-CAMUY
SECTION
DIRECTION- NORTH
|0° EAST
TOTAL THICKNESS
3850 TEET
sr
FIO CAMUY-HATILLO
SCCTION
DIRECTION - NORTH
t-OUTK
TOTAL
THlCKrJESS
3700fEET
SAN SteMWM
RIO Cl/UEBftHM
^
ILAIRIES MSTEECT
THE NUMBERS
ON
Pl^OflLES
REFER TO
FIELD STATIONS AND DIP OF THE BEDS
JPILATM
RIO BLANCO
SERfCS
HUBBARD, GEOLOGY OF THE LARES DISTRICT
31
or irregular form, some of the thicker surface flows, such as those south
of Aguada, and the massive tuifs.
The latter cover large areas, and are
almost
universally
too
deeply weathered
to show
their
real
structure.
The
few unweathered
exposures show
either
very
obscure
bedding
of
massive proportions or else none whatever.
The bedded rocks comprise
chiefly the shales, with interbedded
tuffs, the smaller andesite
sills and
flows.
It
is chiefly in these bedded rocks that the highly disturbed con-
dition of the rocks can be seen.
Intrusives.
These are characteristically of the laccolithic type, or oc-
curring as
sills in the bedded formations or massive tuffs.
Their origin
is indicated by their occurrence as elongate bodies following the general
strike of the surrounding beds.
Many of them may be surface flows, but
their porphyritic habit is so nearly universal, and the phenocrysts of such
considerable
size that
it seems safer
to interpret them as intrusives.
It
is nearly everywhere impossible to solve this problem from the nature of
the contact with adjacent sediments and volcanic
elastics.
The reasons
for this are chiefly because of the deep surface weathering and absence
of extensive exposures of fresh rock.
Another factor is the similarity in
mineral make-up of the igneous and
clastic
rocks, and the consequent
absence of contact metamorphic effects.
Some of the large and irregular
igneous
bodies, such
as the gabbro
porphyries,
are
obviously
intrusive
into the sediments.
They cut across the beds and are not elongate parallel
to the strike.
Folds.
At various localities in the shale areas are zones of exception-
ally
strong folding and
faulting, marked by
crush
zones and
extreme
variability of dips and
strikes.
Wherever the folds are exposed to view,
they
are
seen
to be
overturned
folds, usually pitching
at
considerably
high
angles.
A typical example
is shown
in
figure
11.
These minor
folds
are superposed on major
folds which form
the largest and most
continuous structural units of the shale areas.
The best example
is the
Anasco Synclinorium , shown in the section on Plate
I, figure
10.
This
great syncline in the Eio Yauco shales underlies the Anasco Playa.
Its
northeastern limb and
a
portion
of
its southwestern limb
are exposed
along
the
south
side
of Atalaya
Range,
northwest
of
Aiiasco.
From
Aiiasco Playa
it extends southeast, across the Mayaguez-Consume Road
between K.
6 and K.
12.
Other
large
synclines and
anticlines
occur
northeast and
southeast of
Rincon.
The
axial
planes
of most
of
the
minor folds dip with the adjacent and less disturbed beds, hence to the
southwest.
The pitch of the folds
is
to the northwest, though in some
instances in the opposite direction.
32
SCIENTIFIC f^Umi'JY OF PORTO RICO
In
aclditioij
to
this
northwest-southeast
systoni
ol' major and minor
folds, there is evidence ot another, and probatdy hiter systom. more of
tlie
nature of broad, ext.onsive flexures, whose ax(>s seem to extend in approxi-
mately a north-south
direction,
rieajiy n,ormal
to
tlie
strike.
Evidence
of this ma\' he seen
in the very gra<hial and progressive (-liange of strike
of
the
shales
over
eerlaiii
areas
of
considerahle
size.
This
feature
i?*
shown on the uiai) in nunierous plaeen, notahlv
in the area
(d' liio Yauco
sliales lying south of Aiiaseo and north of Mayagnez.
Tlie same move-
ments wliich produced
these
north-soiitli tlexures
fn-otiahly were respon-
sible, at
least to some extent, for the plunging of
the overturned
anti-
clines and syneliues a!>o¥e noted.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
33
Faults.
Both
normal
and
thrust
faults
occur
in
great
number.
Usually^ evidence of faulting
is to be found only in the presence of crush
zones.
In such cases very
little can be determined as to the nature or
extent
of the movement.
They
are probably
for the most part thrust
faults of considerable extent.
Most of the normal faults seem to be
of
relatively
slight movements,
tliough
very numerous, and in
aggregate,
doubtless
represent displacement
to
be measured
in thousands
of
feet.
While the data are insufficient to make a positive statement, observations
seem to show that the normal faults more commonly cut across the gen-
eral
strike
of
the formations, and hence are
to be
associated with the
north-south
flexures
previously
described.
The
thrust
faults
also
cut
across the
strike
of
the beds, but on
the average
at lower
angles, and
sw
/
/
/
////
^77
'
'
r*"^
NE
Fig.
16.
Thrust
-fault
in Rio Yauco shale
K.
13.85, Mayaguez-Consumo Road.
Strike of beds, N.
ment,
several
feet.
75°
W., dip 45°
S. W.
Displace-
hence are to be associated with the earlier, or mountain-building move-
ments, which produced the intensive folding.
Except in the cases of minor faults, the amount of displacement could
not be determined
in any
of
the
faults
observed.
Figure
12 shows a
thrust fault which probably indicates a movement of some hundreds
of
feet.
The view shows massive tuff beds almost normal to the adjacent
shales, against which they have been faulted.
'No marked folding occurs
in the vicinity, thus the discordance shown in the illustration could not
have been brought about by any minor displacement.
Thrust faults are
well exposed on Desecheo Island, as shown in figure
14.
The faults are
accompanied by extensive fracturing, and the numerous joints thus pro-
duced have been
filled with quartz or calcite.
The extent to which the
fracturing has gone
is well shown in figure
15, illustrating an exposure
on the northwest coast of Desecheo Island.
34
SCIENTIFIC SURVEY OF PORTO RICO
A minor thrust fault of special significance
is exposed at K. 13.85 on
the Mayaguez-Consumo Eoad.
iVs shown in the
illustration^
figure
16,
the fault plane dips at a very low angle and shows a displacement of only
a few feet.
It
is located on the north limb of a large syncline, and
is
apparently a break thrust, produced during the folding.
'That
it
is not
a normal fault
is sliowm by the very slight dip of the plane.
It, there-
fore, serves to emphasize the point previously made, that the thrust fault-
ing was probably contemporaneous with the intensive northw^est-southeast
folding.
Thickness.
The complexity
of
structure, absence
of continuous
out-
crops, and limited time spent in the field make accurate determinations
of thickness out of the question.
In the section of Eio Culebrinas Series
south of Moca the shales and tuffs measure over 3000 feet in thickness.
This probably
is near the maximum figure for
this
series.
In the Rio
Yauco
Series, about 6000 feet of shales and tuffs are exposed along the
Mayaguez-Las Marias Eoad from K. 13 to K.
19.
k traverse across the
series northeast from Eincon shows a
total thickness of over 8000
feet.
This measurement makes considerable allowance for duplication of strata
by folding and faulting, and probably represents the maximum thickness
of the entire Eio Yauco Series.
The thickness of the Eio Blanco Series
cannot be determined, owing to the massive character of the tuffs and the
many intrusive bodies.
These estimates of thickness include the interbedded flows and intru-
sives, which, although individually small in the sections measured, doubt-
less make up a large portion of the total.
The measurements take account
of the larger folds and allow for many
of the minor
folds.
The
effect
that faulting may have had in duplicating the succession of beds
is, how-
ever, impossible to estimate, since the amount of movement along fault
planes could in no
case be determined.
It should be pointed out that
many faults, both thrust and normal, occur along the sections measured,
and their effect has doubtless been to greatly increase the apparent thick-
ness in each case.
Nevertheless,
it may be stated that the total thickness
of clastic rocks exposed in the Cretaceous series of the Lares District
is
to be measured in thousands
of
feet.
A conservative
estimate
for the
entire group would be about 10,000 feet.
Age
The age of the Eio Yauco Series is Upper Cretaceous, as shown by the
presence of Radiolites, and
the same foraminifera
characteristic
of the
Upper Certaceous beds in the Ponce District.
The Eio Blanco Series
is
HUBBARD, GJEJOLOGY OF THE LARES DISTRICT
35
also probably Upper Cretaceous, though the presence in
it of RadiolUes
is not as reliable evidence as in the case of the Eio Yauco
Series, since
the limestone which contains
this
fossil may be an in-faulted remnant
from another
series of
rocks.
The Eio Culebrinas
Series
is the
oldest
group in the Lares
District, but there
is no evidence
to show that
it
is
older than Upper Cretaceous in
age.
It has beds of lime
shale which
contain the same foraminifera
as the Eio Yauco lime
shales, but these
minute organisms are hardly determinable specifically.
There
is no evi-
dence of unconformity between or within the three series of the
district.
Mitchell has found the same to be probably true in the southward con-
tinuation of these beds in the Ponce
District.
There
is no evidence of
Eocene sediments in the Older Series of the Lares District, and likewise
none in the Ponce District, as Mitchell has shown.
Correlation^
The Eio Yauco Series
is probably the equivalent of most of the shales
in the Ponce District.
Mitchell states that the Pefiuelas shale is probably
.
equivalent to the upper portion of the Eio Yauco shales.
The lime shales
of the Eio Yauco
Series of the Lares District are
lithologically similar
to the Pehuelas shale, and may be equivalent.
The presence of RadiolUes
cannot be used to correlate different members, since
all the limestones as
yet found in both the Ponce and Lares Districts carry
this
fossil, and
have similar foraminifera.
From stratigraphic and structural consider-
ations, the following statements seem probable
:
1. The Eio Yauco Series is equivalent to most of the sediments occur-
ring in the Ponce District.
2.
Other formations in the Ponce District of doubtful correlation will
very likely prove to be younger than the Eio Yauco shales.
3. The Eio Culebrinas
shales
are
older than anything
in
the Ponce
District, but are probably not older than L^pper Cretaceous in age.
4. The entire range of sediments from the lowest members of the Eio
Culebrinas to the uppermost members of the Eio Yauco Series represent
a continuous succession, unbroken by any unconformities or disconformi-
ties of appreciable magnitude.
Further correlation with areas of Porto Eico to the east
is practically
impossible at the present stage of investigation.
The strike of the forma-
tions in the Coamo District,
if projected, would seem to show that they
pass northwestward across the northern half of the Lares
District, cov-
ered by Tertiary formations.
The presence of Radiolites in the Coamo
SCIENTIFIC SURVEY OF PORTO RICO
36
tuff limestone
is
significant
in showing that at least a portion
of the
formations exposed in the Coamo District are of Upper Cretaceons age.
LiTHOGEXESIS
The origin of the various types of igneous rocks has been pointed out
in the discussion of the petrography of these rocks.
The origin of the
clastic rocks has likewise been referred
to, but deserves further elabora-
tion before attempting to outline the Cretaceous history of the
district.
It has been noted that practically all of the
clastic rocks contain ma-
terial derived from volcanic sources.
From the tuffs, which were entirely
derived from such sources, to shales and limestone, which contain minor
amounts of
this material, the entire succession
of beds show that they
were deposited during an almost uninterrupted period of vulcanism.
As
Mitchell has pointed out, the massive tuffs, which show no bedding, were
doubtless accumulated on land
surfaces, while the
shales and
stratified
tuffs and
ashes were accumulated
u.nder
water.
The
lime
shales and
black
pyritic
shales
contain
so many
foraminifera
that
their marine
origin seems highly probable.
In the case of the limestones, interbedded
with
these
shales, and carrying RadioHtes and abundant
foraminifera,
their marine origin cannot be questioned.
The
alternation
of massive
unfossiliferous tuffs with shales and limestones containing marine organ-
isms thus shows a frequent oscillation of level during deposition, in which
arms of the sea frequently encroached on the land area.
The sparsity of
marine
life, and the
singularly small number
of types making up
the
fauna,
illustrate the very hazardous and unfavorable conditions
for the
development of living organisms.
A glance
at the geologic map will at
once suggest, in view of the above interpretations, the significance of the
predominance of massive tuffs in the eastern portion of the district.
The
land mass apparently lay to the east, and the invasions of the sea came
from the west and southwest.
The predominance of calcareous beds on
Desecheo Island, and their relatively greater foraminiferal content, sup-
port this view.
The greater abundance of limestone in the Ponce District
to the south
is
also
significant.
The
lens-like habit of the
tuffs
inter-
bedded with the shales seems to indicate that rivers played an important
part in the deposition of both the shales and bedded tuffs.
In regions subjected to periodic violent volcanic eruptions, we should
expect the animal organisms inhabiting the adjacent sea to be killed
off
periodically in great numbers.
Consequently, we should look for occa-
sional strata crowded with fossil remains, with intervening beds contain-
ing exceedingly few fossils due to the rapid accumulation of clastic sedi-
HUBBARD, OEOLOGY OF THE LARES DISTRICT
37
ment and the
scarcity of marine fonns living under such continuously
unfavorably conditions.
This is precisely what is found in the sediments
both in the Lares and Ponce Districts, as the following evidence shows
1. The limestones of the San German type are in places crowded with
the remains of RadioUtes.
2.
Certain beds of the lime shale show an abundance of foraminifera,
while others show very few or none.
3. The black pyritic shale
is indicative of the rapid accumulation of
organic remains, chiefly foraminifera, and possibly plant matter.
4. The occasional chert beds seem to be derived from sudden and rapid
accumulations of Eadiolaria, and possibly other silicious organisms.
Foraminifera do not thrive in impure, sediment-laden waters.
Their
abundance in some of the layers of calcareous ash and lime
shale
con-
taining ashy material, and the evenly banded character
of
these
rocks,
points to the following special conditions of deposition
:
1.
Quiet waters,
free from strong currents, and presumably not very
shallow.
2. Pure water, except during showers of volcanic
ash, which resulted
in
a sudden pollution
of the water, killing
off of the foraminifera and
other planktonic forms, and the rapid settling of both ash and shells on
the quiet bottoms.
Geologic Histoky
From the above
data, including the descriptions of the various types
of rock wliich occur in the Older
Series, the conclusions regarding their
age, and the interpretations of conditions under which they were formed,
the following steps are given in the Cretaceous and early Tertiary history
of the Lares District
1.
Pre-Cretaceous.—Unknown.
The
basement
of
the Upper
Creta-
ceous volcanic flows and elastics has not been found in the Lares District,
and probably will not be found.
2. Upper Cretaceous.
Period of almost continuous volcanic eruptions
of the explosive type, marked by the ejection of enormous quantities of
lithic fragments and frequent showers of ash, resulting in the rapid ac-
cumulation
of
great
thicknesses
of
clastic
deposits.
This
action
was
accompanied throughout by lava
flows
of andesite and
augite-andesite,
and intrusives of the same material into the surrounding
clastic rocks
during their formation.
There were' frequent oscillations of level with
invasions of arms of the sea covering portions of the western part of the
district, and on one occasion, at least, extending as far east as Lares.
38
SCIENTIFIC SURVEY OF PORTO RICO
3. Late Upper
Cretaceous.
Final
phases
of
vulcaiiism^ marked
by
minor intrusions of magma of extreme composition^ such
as
peridotite,
now altered to serpentine.
These
final intrusives were accompanied by
the beginnings of great orogenic movements.
4.
Close of the Cretaceous.
Orogenic movements^ comprising extreme
folding and thrust faulting, with the maximum forces applied in a north-
east-southwest
direction.
This was
probably
the
local phase
of
wide-
spread crustal movements which outlined the present sub-sea mountain
chain, the basement upon which the Antillean islands rest.
5.
Close of the Cretaceous or Beginning of Eocene.
Further crustal
movements,
involving north-south warping, normal
faulting, and
pos-
sibly accompanied by further uplift.
6.
Eocene.
Continuous
erosion, and reduction
to oldland surface
of
comparatively slight relief.
7. Early
Oligocene.
Kenewed
uplift, with
dissection
of
the oldland
surface to a region of considerable relief.
Deposition of coarse gravels in
valleys of torrential streams
(seen southeast of Moca).
8. Middle Oligocene.
The beginning of a partial submergence of the
island with deposition of the San Sebastian shales in embayments along
the north coast of that time.
Further events are given in the discussion
of the Tertiary and Pleistocene.
THE TEKTIARY FORMATIONS
The most complete development
of the Tertiary formations in Porto
Rico
is in the Lares
District, and occupies the area between the Lares
Road
and
the
north
coast.
These
formations
are
a
structural
unit
(Berkey, 1915, p. 12), resting unconformably upon the highly disturbed
Cretaceous rocks, and overlain unconformably by Pleistocene and Recent
consolidated dune sands and beach gravels, limited to the coast line.
The
Tertiary beds are a
series
of limestones,
in small part of
coral
origin,
underlain nearly everywhere by basal
shales,
clays,
marls,
or conglom-
erates.
The maximum thickness of the entire group
is about 3800
feet,
with the strata having an average dip of about 4° seaward.
The range
in age is from Middle Oligocene to Upper Oligocene.
Table 3 shows the subdivisions and names applied to the Tertiary for-
mations of the north coast by previous writers and in the present paper.
Hill's term Pepino Formation, although having priority over the term
Arecibo of Berkey, will not be used in the present paper, because (a)
the
corals
collected by
Hill from
his Pepino formation
occur
also^ in
the
R. T. Hill
(1899 c, p. 15)
Pepino
formation
Miocene
"At least 100
feet thick"
Greensand
marl
Eocene or
Oligocene
Lignitic shale
and
clay
Eocene
Cretaceous
Table 3
SUBDIVISIONS OF THE TERTIARY OF THE NORTH COAST, PORTO RICO
C. P. Berkey
(1915, p. 17)
San Juan formation
Pleistocene to Recent
Unconformity.
Arecibo
formation
Eocene?
Oligocene
Miocene?
"At least 500-
600 feet
thick"
Quebradillas
reef
limestone
San Sebastian
shale
Probably
Eocene
.Unconformity.
Older
Series
Cretaceous
C. A. Reeds
(1916, 1917)
Arecibo
formation
Upper
Eocene-
Oligocene
Collazo
shale
Upper
Eocene
D.
R. Semmes
(1916, p. 434)
San Juan
limesand
Recent
Tertiary
formations
B. Hubbard
*
(1917)
San Juan formation
Pleistocene to Recent
.Unconformity.
Arecibo
Group
Quebradillas
limestone
Upper Oligocene
(Bowden)
Los Puertos
limestone
Cibao limestone
Lower Oligocene
Lares limestone
Lower
Oligocene
San Sebastian shale
Lower Oligocene
or
Upper Eocene
T. W. Vaughan
(1919, p. 260)
"Pepino
formation
of Hill"
Middle and
upper
Oligocene,
possibly
some
Miocene"
Older
Series
Unconformity.
Older Series
C. J. Maury
(1919, p. 214)
Arecibo
formation
Quebradillas
limestone
Lower Miocene
(Bowden)
Aguadilla
limestone
Upper Oligocene
Lares limestone
Upper Oligocene
Rio Collazo shales
Middle Oligocene
D.
R. Semmes
(1919,
p.
55)
San Juan
consolidated
dune sand
Pleistocene
Unconformity.
Arecibo formation
Upper Oligocene
(Bowden)
"At least 500 feet
thick in San Juan
District"
San Sebastian shale
Lower Oligocene
Subdivisions used in the
Present Report t
San Juan formation
Pleistocene to Recent
Disconformity.
Quebradillas limestone
Upper Oligocene (Bowden)
700-875 feet
Los Puertos limestone
Upper Oligocene
550-1000 feet
Cibao limestone
Middle Oligocene
250-1000 feet
Lares formation
Middle Oligocene
350-1275 feet
Arecibo
Group
Unconformity.
Older
Series
San Sebastian shale
Middle Oligocene
Max. 700 feet
.Unconformity.
Older Series
Upper Cretaceous
Proposed
subdivisions,
based
on
field work
in
1916,
and
presented
before
the
Geology Section, N. Y. Academy Sciences,
in 1917.
t Same as were made by the writer in
1917.
The ages assigned to the formations
have been modified, as the
result of a study
of the
fossils made during the summer of
1919.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
39
Upper San Sebastian shale^ and
(h) Pepino refers to the litliologie and
topographic character of the formation, and
is therefore an undesirable
term.
Stratigraphy
the san sebastian shale
The existence of a basal shale was
first pointed out by Hill
(1899 c),
who did not apply a formational name to
it, but merely mentioned
its
position relative to the Pepino Formation.
Berkey (1915) was the first
to suggest
a formational name
(San Sebastian
shale).
Subsequently,
Eeeds gave the name
Collazo
shale, because
of
the
excellent exposures
and numerous fossils found at Callazo Falls, east of San Sebastian.
Fol-
lowing Eeeds, Miss Maury has used
the name Rio
Collazo
Shales.
In
the present paper the name suggested by Berkey
will be used, because
{a)
Berkey's name has priority, and
{h)
it
is a more appropriate name
than Collazo.
A glance at the cross-section
(Plate I)
or columnar sec-
tions
(Plate II)
will show that the maximum development of this basal
shale is near San Sebastian.
The columnar sections
(Plate II) show the various types of material
of which the formation
is made.
Although
it
is predominantly a shale,
it consists in large part of dark bluish clay carrying seams of lignite with
pyrite and
marcasite,
conglomerate
and
pebble
beds
(in most
places
unconsolidated), red calcareous sand or lime sand, green marl, and im-
pure
limestone.
Many
of
the
lignitic
clays
contain
fossil
leaves and
fresh
or brackish water
molluscs.
Sharks'
teeth and
vertebrate bones
occur in many of the beds.
The marls and red lime sands contain marine
fossils, and as shown in the Collazo section
(Plate II)
alternate with the
hrackish
or fresh water
beds.
The marls
at
the
top
of the formation
carry an abundance
of large
coral heads.
Limestone
(argillaceous)
is
the only material in the San Sebastian formation which
is firmly con-
.solidated.
In other strata, the lower the content of lime the
less consoli-
'dated the material.
Most of the ^^conglomerates," which predominate
in
the basal part of the formation, are merely loose gravels or pebble beds.
•Ground water has easy access in all strata except the dark blue clays, and
it has leached out large
quantities of calcium carbonate.
Most
of the
fossil molluscs are found
as molds with but small portions of the
shell
Temaining.
As previously noted, the San Sebastian shale is thickest in the vicinity
-of San Sebastian.
From this point
it pinches out on the west and thins
to ]00
feet at the east end of the Lares
District.
Farther
east,
in the
SCIEXTIFia SUirVEY OF I'ORTO RICO
fiio Arecibo cajiYou,
it
is absent
(Borlicy,
1!>15,
p. IG).
Tbiis, as com-
pared with overlying format ions, the SaJi
Sr^bastiiiii is of local occ.iirrciiec.
Tliat
its
relatioiisliip
wltli
tlie
Crctaeeons
ronk^
h
iinccail'ormMble
bas
been
sh(jwii by Berkey, and
is too evident
to
require
furtber demonstra-
tion.
Tbe actual
tiasal contact cannot
be
ncen
in any
of
tlic
loealities
visite<l, but exposures close to the contact ran
be seen
in
several
places,
the most
significant of
wliicli
are
in
tbe
immetliate
vicinity
(>t
141 res.
From a survey of tbese exposures,
tJic Atdowing- features are to
be; noted
:
.•
.
,:*;.j';s*
^/"v;;J':.
:
•':.„''.,;!*
"
•%,.#•:-
%
is™
-
«
',
'
'%/".
'i
c-
.i
'''.:;''"-:\';°-i'' -'"''..'-
-
'^
'^
--.Ai
y. *""
',.
:;%
J.
''"':'
•*
"
-
'
.,.'
i-^
'>s#ir
W&'
»* w
,
.
!.
r.
,
•
;
,'=. -*
!-•;.•
'
„
"rf^.::
^'-^
-
•K^.:-.*.
*,,
m./.K^i,;-^^:;^^.f::''":^
'm
Fi«.
;17.
fSmiMrr Hini uwl
<
I
fit
biifie
ttf
thr
Terliarj.i
K.
S.ft.
Lares Koad,
near Mocr
roeksi,
wlJicii
in
lliis
le»caltty
This
deposit
lies
nne«nr<.rmMliIy
on
!'
nmiat ut slmles,
a!«h, and angih-nnKk«it
J.
Tlie surface upon wliicb the San Hebastian
sbiile was deposited was
one of considerable
relixd' (probably two or
tJirec^ liiindred
feet).
2.
Tlie
Ban, Sebastian
rests not u|ron
fresh, cleanly
lu-CKled ('rctaeeons
rock, Init ii])on, badly decayed rock or residual
soil.
J.'ortions
o'f tbe basal
Han Scbastiaji at Lares
art; made up of reworked residual
soil, difteriug
in, no respecrts from modern residual soil of tlie interior mountains, exce])t
tliat
it contains, here and there, tbe molds of Tertiary fossils.
At K.
8.9, LartiS Koad, there
is an
ijit<:'rcsting deposit of boulder clay
and coarse gravel (Fig.
11') resembling glacial
till in its general appear-
ance.
This
material
is
unconsolidat(>d, and
composed
of
boulders
of
Cretaceous rocks imbedded in a kaoliriized matrix.
It contains absolutxdv
HUBEAR IK fiKOLOOY OF THE LAKK& PJSTRICT
41
,110 material wJiieli coiiLl }iavc been derix-ed from any of thci Tertisiry for-
matiojis.
To the iiortliwest thin deposit piiiehes out and
is
r<>;plaeed by
bayal
Tertiar}-
pebble
beds
and
shale
(Lares
formation)
near
^foca.
Thene large boulders probably mark
tlie cbamiel of an Eoeeiu! early
()1i-
ir(teene
river.
Sneh
eoarse
gravel
would
probably not
be
conunetii
in
ebannels of old streams Jlowingoii a peiieplaue.
On the other hand, sueb
deposits sliuuld be eonimon
in
a region of considerable
relief, sueh
as
is
sliowri by the rehitionsliip of
tlu^- basal 'rtTtlary at Lares.
At the top of
the San
Sebastian
sliab\, the eontaet with
tbc overlying- Lares limestone
is a
a'radatioiial
one.
The
ehaiiy-e from marl
to limesbme by an
alter-
Kridge
at
Co:
mition
(inter
figure
IS.
'^From
tlie
sections
(Plate
II)
it
will
l,e
noted
that
pebble beds
liguJiie elays predominate
in, the lower part of the San
Sebastian
s'
Tliese
irulividual
beds,
liowever,
carrnot
Im
eorrelated
in
the
diffe
sections.
In the section at Ijares, the upper mendier
is a
rcrl lime
,'
eontaidng
f'eclcn
laresense.
Tfiis
bed
is
tracealile
as
far
oast
as
(Jamuy Kiver. but <-annot
be correhited
ivitli
certainty with
anytliiii
sections to the west.
The most
coid.iniious ineml)er
in
tlie
fonnatio
tlie zone of grcKjn marls witli abumlaiit eorals. which forms
tlie
t'op of
San
Sebastian
formation
at
C'ollazo.
This same
zone,
with
the
s
the
;iiiie
42
SCIENTIFIC SURVEY OF PORTO RICO
corals,
is found at the top of the San Sebastian in the Hato arriba sec-
tion, west of San Sebastian.
Some of the same corals occur in the basal
portion of the Lares limestone in the cnesta just north of Lares, hence
this portion of the Lares may possibly represent a limestone facies equiv-
alent to the upper San Sebastian marl beds.
In the Collazo section, im-
mediately below this coral zone
(zone C of Vaughan, 1919),
is a zone of
marl containing Clemeniia dariena and
Turritella
to7mata.
This zone
likewise underlies the coral zone in the Hato arriba section, but the rock
is much more calcareous than the same horizon
at
Collazo.
To sum-
marize, there are only three zones, of any considerable continuity in the
San Sebastian shale
1. Upper Zone
(zone C
of Vaughan).
Marls with abundant
corals,
grading eastward into limestone and red fossiliferous lime sand.
2. The Middle Zone.
Marls and other types
of material marked by
abundance
of dementia dariena and Turritella tornata
var.
portorico-
ensis.
3. Lower Zone.
Predominantly
lignitic
clays and gravel
beds, with
some marine marl and limestone, the latter containing numerous tests of
a small Scutella
(like 8. mississippiensis)
.
The extremely local nature
of most of the beds and fossil occurrences makes it inadvisable to attempt
a finer subdivision of the formation.
THE LARES FOEMATION
The term Lares Formation was first proposed by the writer
(see Table
3)
for the massive limestone overlying the San Sebastian shale, and most
conspicuous north of the town of Lares, where
it forms the cuesta and an
extensive area of haystack or pepino
hills.
The distribution and bound-
aries of the formation were definitely shown in a paper presented before
the Few York Academy of Sciences in 1917.
In a paper on the Porto
Eican
Tertiary,
Dr. Maury
(1919)
uses the term Lares
limestone for
beds
overlying the San
Sebastian
shale, but no
distribution
or
strati-
graphic limits are given.
The
chief horizon marker
of the formation,
according
to Maury,
is
a
large
cerite
shell
{Campanile
(Portoricia)
larica Maury).
This
shell was not found by the writer in any of the
numerous localities studied, but a very similar shell {Cerithium {Campa-
nile)
collazum Hubbard) was found in abundance in the San Sebastian
shale, and sparingly in the lower part of the Lares formation.
It should
be stated in this connection that these
shells are poorly preserved, and
that the above two designations may represent the same
species.
In the type locality, north of Lares, the Lares formation
is a massive,
SciEJ^TiFic Survey of Porto Rico and the Virgin Islands
Volume
II, Part
1, Plate
II
EZI
Ezr
EZT
EZI
rzi
E^g
fmmental wMe kt
Ostrea
aniiguensis
mra 1st fragments in soft
*
i ^
,
.
,,
Tsetr/x
Upicat QuebradiUas fcum-
Fqranosa, Metis trivitaria
Qfthaulat fiortericoensis
'
Cjittierea berkt^,(cmi5 catemtis,
m/ctTiofi/ij'lia sp.
soft^ cklkt
with ht CBmloimnk ~^
ht.covqhn. witiMAoukx
corals
"^corafs
alternate Juni aJid soft 6tds
hard, red -pi/ik,semi<nist(injiie.
Tmsshe red
"^^"^"^
lo<:a]ij fragmntal
red ht Hith vmsroui
,
smil shell frvgwents
mtftm Mtcateriteus retf ht
*•!?/ wrrridf/r catvrwus M.
with mtnf stalactites. » fossi/s
mmrous ttkmids ^ n-cien sp.
•^te
• ^
'•
•
vW;;^ti,f^%$t"tVd
red
horizon at Agitadilh -massite
carerncus. Ixall] fraqmental,
witA sM/ fmqmevts,
hard,
phik^ utith Tnuih vein cakite
-
Bttknofihjllia and comIs of
trinitaria and it
-Met
massive pink
- red caterncus,
Se nhCf-ptcll- "f - nrthnu Tn i
avd Tnanf
si
red ir^staliine ht
soft,»,h:tc,chcn.f 1st -ccrah
of ynassne cofonia] type
soft, triable, podularyellow
^, U
v6 fossils
'
safl , cream -colored 1st nn
neten sp and Teredo ixnssa,
rpn(E n
uft,jelhn- tc white weotAerf*
Ist^iocaHf hard, sem-cry^alii^
pink to redJ and with
occasional coTKlomeritic beds —^
Teredo incras'sata and Lam
fauna
mined mith some
I
upper San Sthasiien Species
pebble bed
- pebbles of
Older Series rocks
base
not exposed
j
creciTihcohrvdM -fkctev s^H
f- ^(.frmUe^rrvam-coiorfdl^
•- wftfj Smaster sp and Cstrea
.
antiguensis
yelloK ht
with anjular
1st.
f-crystallirre /onJk^^f'
mh foramir.ifera
mkite to mottled red
^<t/i
smIl fragments of mdluscm
hard,pink endn^ite kt.
semicrystallirKj cciernous
pink am wme reef
1st -no
hard iMtk arJ%hi\e*W'»itb
^
corah
HE?-
-i^
red reef ht
- semi -crystalline
with much seccndari reJcak'lt
and ara^mte in MrjeidaJmssrs
red crjstalline ht
white chalky
conglomeritic lit
with molluscs and corals
moliuscan launa
soft chilly reef M. with
byanchina corals ixkmathcn rat
soft white shaly
1st
-neathei
nitttkd red
- miluscan fau
pebble bed- fra<imental
oyster shflis -Cstrfa spfjoU
bed^nlih pSisfiedpc-oiM-^ 'Ihitk
•fij:3-'><vi^ sandii calcareous shale
vtith
S>S&£A
fra-jments of marl
rB^S'lSl red sandy c/au
red sand If cky
.San Sebastian
shale
not
exposed
MG
iLMim
T
FIGURES
1
TO
6
CORRESPOND
WITH
FIGURES
I
TO
6
ON PLATE
I
^
S^H^P
EIIZI
red ht and red
1/ mesand
g thm,irre<jukrbeds cf
g Ksicuhr, crfstaUine red ht
n^lternate redend -.Mte
1st
'^ x»K/Jf irreaukr
and Hmesm
]j nassive, pin/i reef At.
-
ceierrtous , rvfossih
CjycijTi white
co/or, medium
Ksra, Jmcnstailim
treat/wrs
teabuffccicr
aifernaie hardarfdsoft beds
soft
w/th kcal hard beds
with abundant Pecten she/Is
ft'irdoini', and wliite-von
ccIoTe abundant, - /ock
weathers reddish -contaifis
seme
ccrcls
'
Salanopofl/ia
beds xto 10 feet tfiick
Chione urcodwcrdi
s/i^iitiy
caifernous
rznzT
5^Q
rZT
izr
I~E
-
- ,
- -
-S, pinfi afid
w/iite 1st.- small molluscs
red cri'stallirie 1st itith tein,^
iakit.e apd ncdu/a,r coral 'heads
'^rtliTlT>&saLu,iSni
fihite chalky ht
narl
with Corns, N.^/ica and
Tnarf nith knsos
or pockets
of lit
r- f'ecton.Cardium,
TUrritelia
, Butla ria
arqillaceous Ist-Cpterkds
marl with beds of white
marl vnith Oyster lieds
white chalk with small mdiuscs
'' %'^cn^?^iis
weathered sh -friable calcareous
bbcky fracture, -smallpekcypods
coral zove - marl mth large
Twssive corals - Ostrea Ftcten
zone ofpementia and mrritella
cysltrb-jd
ioft w/iite, meothered ht
cgkarecus sh. and marl
Tcssiliferous fmy sh and maff
>lap sh- sandy le calcareous mth
thin seams of lignite
blue-qray
to qrten sh
^ra<i
sh
red
, sandy
sh
pebble beds, each about
1 foot
thick
24
'k'^-liA'^
hrotrn sandy sh mth ptitL
fj-ifijfai
ccngkmeralt -13 ft
exposed
I—- f
j
^ji^j,
„^,^ cipescd
beds
WllG 3
JPILAT
m
^^s
CO
rza
53
^EJ=^
S^Z^
ClZl
:S^;
zzr
izr
riE
rrr
S^
rTTT^
rzE
Ezniii
g^
s^
rzzizrr
eES:
r*&sWM
hmesand and chalk
- femmiwfera-
massive
reef ht
1st. conglomerate
^itropods and corals
hard, pink, cotennis,- veimttcaldte
*""§^» noHuican skelts
Metis trintar,a, dilaruphyriia sp.
plnli to nkte ht.-unfoiSiliferoai
same (osnk an at sta. SZ(a)
bmnchiiiftjipe
corals, reiemUnt
those in the Lares 1st
'
caferrtom reef
Iit.
bed,
/ to ,0 feet thick
Fiden sp
, Strcmbui
sp.
soft, nhite, nodular 1st
- fossils
viarl
with
bedi ot Ostrea
ip-
7r:olhscttn fauna -chiefIf
pekci)pods(<tOf>l
soft. ^Iiite, chaihj, nedylar
Osti^a. fx ipcc.fS),iini>,
iP
, and numeniis
n.t tK
[© 4
Cpiif coral zffnt - skihlit m/h
-- bridge -Lares
read
,
ara'd-qrecii marl- mamve coiui
'top it
is!
fdHs
jra^-qrccn marl
«- top led. lolh^
- 10 ft
marl bed
,_
CTener.tia darieriazone
ca^wonacec Hi lmh
mitn
i» •;.,.'
seems, fcr.v kaves, f-rcsli titer
carbonaceous shale
gra^ marl nith Ostrea
^
'
colfai,
btse net exposed
E^
K^
ra
m
JZL
rnH
_.xi:
X-i
ir^
=c^
XZI
tathtred
lit un^hmeratt
pink
to wiite ht
tilth large
Orthaulat
stielh
mmsin white chalk
a.'terfiittiiif
Kith tttrdfink be<i% -unhiUliletcia
except for sniill pekcypoi
thin bedded ht
hard, cavernous
beds I foot thick (average)
thin bedded ht
chalkf
fviite ht
with
niimiltnt
Echinclatnpas sp
ariii
nolhscan skells, cfi.'eriv
fictei
and inlerntl mads of ^strepods
Orkitoid foraminilera hafi
SoH,m4iiive,m/iite *eds
aiteriutii^
with thin
beds of cciiutiia
made
up chiefIf of Often and Ostrea
caarneus, iitii piiik cakite
fiilinf
picten and Strqmtus abundtnt
ka'i, covernous wMe or pmk iicds
alternatino mth salt
ee/fta/ish ist
untiur.lnii
feiji-like misses cf v^kite
corals
- massive colonial tipe-
2 feel in diameter
nodular white Ist
wm
numerous branckinf type corals
'''o/heroir""*
Teredo incrassata
- crtistcceans
red hmesand
sendst, carbonaceous sfi seems
sh. wdh intercalated
1st
fossiliferous
sanist
5S
Ballaria ^favosa
rTicHkd jelloM and vtMe
Tfidish Hfllon - «!!<(«
- MiwrnoKi
»i,t>i vnmscin
fauna
nd U conqkm. - nulluKan ftunt
altfrnate -Imd^fink anj ult.feHc*
WHiJiVf
tt irrfplttr hedS i'tDW'
c:ruls - tetf buUirt^
t,pi:'$
hard cawrmu! -
viitis
larae
Orthaulat
shills
ye/Zow uverncus
Ist
- mlliscs
,ilV'S- I'ihijrlhw caternous
utlhu calartou,
'e^creaa-cohrei
reef Ist
f;hiKidloxe-ib;;tidant [durnhmpal
icds Kuh ctuvdant
ficten
tktili
wA/f»
chclk^
Ist
mth Ostreain
•xtremely cavernous reef Ist
i
!— zone of k/oklt weathered ht
marMinqpelMof submergence
of Rio camuj
- massive ht
with brancHiru type corals
wm
(B
white
Ist -weatners ycllowhcm
wry
fossiliferous
-
Corals,
Notica, Cstrea, shell froipnefts
fcraminifera
qrmi-blue-ma^'^'^'''"^'
red undgray chj/s
HUBBARD, GEOLOGY OF THE LARES DISTRICT
43
cavernous^ white limestone^ with scattered portions of thin bedded, hard,
or chalky limestone.
As a whole,
the Lares limestone
is
of
shell and
foraminiferal origin, although many massive, reef-like portions are made
np
in large part of
corals.
The coral limestone
facies
is
so intimately
mixed with the well bedded portions that it is not possible to estimate in
what proportion the formation is of coral origin.
However, the limestone
should be
classed
as a
reef formation.
It shows
the
lateral gradation
from stratified to massive structure, characteristic of reef formations in
general.
The massive portions are either largely of coral origin or
else
made up chiefly of foraminifera
{Lepidocyclina and others) and micro-
scopic
shell fragments.
In most
localities,
this massive rock
is
hard,
compact, and partially crystalline, and extremely cavernous.
The thin
bedded portions are
chiefly chalky and
soft,
or alternate hard and
soft
layers, many of which are yellowish from limonite
stains.
These strata
are not true chalk, but finely ground limestone, practically a rock
flour,
derived probably from the adjacent
reef
structures.
Most
of the mol-
luscan shells occur in these well stratified portions of the formation.
So far the description applies to the eastern part of the district.
From
the Eio Guajataca westward there
is a change in facies.
The white lime-
stone, with
its massive exposures and pepino
hills, grades into
a
softer,
more argillaceous limestone.
The topographic expression of this
differ-
ence in character of the rock is brought out in the geologic map.
In the
Hato
arriba
section, west of San
Sebastian,
the formation
consists
of
alternating beds of white chalky limestone,
argillaceous
limestone, and
marls with abundant oyster shells,
(0. virginica).
Still farther west, in
the vicinity of Moca, the San Sebastian shale
is missing, and the Lares
formation is the basal member.
It might easily be mistaken for the San
Sebastian
shale
in
this
locality, because
of
its
lithologic
similarity,
as
shown in the columnar section (Plate II).
The presence of typical Lares
fossils, such as Cardium
cinderellce alternatum and Pecten grabauh and
the absence of nearly all of the most characteristic San Sebastian species,
is sufficient evidence to show that these basal beds are equivalent to the
Lares limestone of the eastern part of the
district.
South of Aguadilla,
the formation
is largely buried by the recent playa deposits of the Eio
Culebrinas.
However, the Lares formation occurs southwest of the Eio
Culebrinas in a narrow coastal belt extending to Pt. Jiguero.
Through-
out this area the rock is a limestone, grading from soft white or yellowish
chalky
material
to
a
hard,
semi-crystalline
limestone,
predominantly
white, but locally red in
color.
At the base of the formation,
a small
thickness of shale or gravel beds
is present in most of the localities.
At
44
SCIENTIFIC SURVEY OF PORTO RICO
Pt. Jiguero, however,
the limestone
. apparently
rests
directly upon
the
upturned
Cretaceous beds, though
the actual contact
is not exposed
to
view.
The contact with the San Sebastian shale has already been described
as conformable.
Similarly the
contact with the overlying
Cibao
lime-
stone
is a conformable, somewhat gradational contact, though very
defi-
nitely marked topographically.
There
is no evidence of a discordance of
dip, of an erosion interval, or of a faunal hiatus between the Lares
for-
mation and Ciboa limestone.
The same was shown
to
be
true
of
the
contact with the San Sebastian shale.
The position of the Lares as basal
formation in the west shows a progressive overlap during a gradual sub-
mergence of the oldland.
N"o other interpretation of the above data
is
possible.
THE CIBAO LIMESTONE
This name is taken from the barrio of Cibao, north of Lares, where the
formation is best developed.
In the type locality
(barrio of Cibao), the
Cibao is essentially a soft, white, chalky limestone with an abundant but
poorly preserved molluscan fauna.
Interbedded with this white chalky
limestone are
1. Beds
of marl with abundant
oyster
shells
(0.
sella'formis
porto-
ricoensis)
2. Beds
of
hard
pink
or
white
limestone
with
LepidocycUna
and
Orbit olites.
The predominant softness of the material explains the rolling prairie
lowland developed on this
belt.
The high content
of
argillaceous ma-
terial in the rock is well shown by the extensive covering of red and black
residual clay soils.
From
its maximum
thickness in
the
barrio
of
Cibao,
the
formation
thins westward, and south of Aguadilla
it
is only 300
feet
thick.
The
character of the rock
is different in this locality, the white chalky facies
containing a greater abundance of hard intercalated beds, many of which
are red in
color.
E"o evidence was found of an unconformity,
discon-
formity or faunal hiatus between the Cibao and the overlying formation.
It
is not possible to distinguish zones w^ithin the Cibao limestone which
continue laterally throughout the formation.
A white chalky zone, strati-
graphically near the top, contains an abundance of an echinoid (Echino-
lanipus resembling E. aldrichi).
This echinoid zone can be traced from
the Rio Camuy westward for at least six miles.
IIVBBARIK (JFJyUKiY OF THE LAKHS DISTKICT
tii:e
H).s
ih:eri:os limkstoxe
The
uaiiie
is taken
IVrtm
tiie barrio
of Loh
Puertos,
jiortli
of
Li:
vliere the >-\ iii-:;^"'
I
'^^
-
:—
is
m
^^'.>*^h:'M^%
Fio.
Irt.^^^^^^L-/.^-
P((«-f'>«,-
f/wes^-ne.
wfl.s.s/re
reef fmw
I'Isposed
in
rei'ilcal
clitf, tn-uiiy 150
feet
IUkIi, and in
wliieli
tliere
Is
i
The rock
is
prerioiiiiiuiiitlv a series of massive, reef-hke hoih, alternat-
ing with
thill
betldecl
etialky
st^rata.
It
is a
reef formation
essentially
46
SCIEXTIFW SUBVEY OF PORTO RICO
like the Lares limestone, but with pepino
hills developed on
a greater
scale with greater
relief.
Excellent exposures of this formation may be
seen in the canyons
of the Eio Camuy and Eio Gnajataea
(Fig.
19).
The massive
reef-like portions are hard, semi-crystalline, and very
cav-
ernous.
The prevailing color
is pink.
Fossils are not well preserved and
are
chiefly small
forms,
including foraminifera
{OrhitoUtes and Lepi-
docyclina).
The thin bedded portions of the formation are
soft chalky
strata^ alternating with harder strata.
The soft layers are in many places
stained
with
limonite,
giving
a
varied
coloring
to
the
canyon
walls.
These thin strata are in some localities intercalated with other soft chalky
beds
containing angular fragments
of hard
reef-type
limestone.
The
chalky beds
are
iisnally deeply weathered and contain very few
fossils
preserved well enough to recognize.
Corals occur in the massive zones,
but apparently for only a fraction of one percent of the entire formation.
Foraminifera and molluscs are the chief contributors.
The Los Puertos limestone
is entirely conformable with the overlying
Quebradillas
limestone, and shows no faunal
hiatus.
JSTo
zones,
either
fossil or lithologic, can be estabHshed in the Los Puertos limestone.
Ex-
cept for the variations above noted, the formation is lithologically a unit,
just as it is a physiographic unit, with its distinctive belt of pepino hills.
The columnar sections
(Plate II)
illustrate the
differences found.
In
the west coast section, the formation contains a great deal of the hard,
red, semi-crystalline limestone which
is characteristic
of portions of
all
the limestone formations of the Arecibo group in this part of the district.
It will be noted
(Plates I and II)
that the Los Puertos limestone, while
variable
in
thickness,
does not thin
to the
west,
as do
the underlying
formations.
THE QUEBEADILLAS
LIMESTOiN^E
As shown
in Table
3, the name was
first proposed by Berkey.
It
is
named after the town of Quebradillas on iha north coast, where the best
exposures of the formation are to be found.
The type
fossil locality
is
near Quebradillas at the mouth of the Eio Quajataca
(Fig. 20).
This
formation consists for the most part of a hard cavernous reef limestone
which has
a
flinty appearance.
It
is made up
chiefly
of minute
shell
fragments (molluscan and foraminiferal) and may be in part chemically
deposted lime carbonate.
A few corals
occur, but these are not of the
reef building type.
In places, the formation
is well bedded, with
the
hard
flinty limestone
alternating with
soft white
chalky limestone
or
with layers of limestone
breccia.
Fossils occur in
all
of
the beds
but
HPHBAHn, UfJOLOdY OF
Till': LARES DJi^TRICT
47
are
iicvor
foiiiid
abiiiulaiitlv
(!XC(»pt
in Hmall areas or
"|}(>ckeiH."^
These
"por'kot.s*' are uiimoriraK, njid do jiot all ecmtaiii the mmc
S|)(!ei(>s.
.Some
of liiem are extremely crowded willi iiiolhiseaii shells.
Almost invariably
the
shell rtnietnres have heeu dissolved away, leaving only
the exfeiMial
and
internal
molds.
Tliis
eliaraeteristie,
together
witli
the
general
aspect of
the
rock,
is
so
tyijieal
of
tlie
Qnebradillas
liiuestoiie
that
it
affords
a
relialile
naean^s
oi recognising
the
furiiiation
in
tlie
field.
A
tyjdeal exposure of
the hard
fossilifei'oiis
beds
is shown
in
figure
20,
which also ilhistrates the massive character.
Other phases of tlie fomia-
'.
QuebraiUllm Imv.rtMie at
the
li/j.e
fon-sll
Inealitp
d
chM:
entrance
of Ameiicai)
rainHnid
tunnel.
Month
Oiiajntaca.
ivest of OuebmdiJlas.
tioii of le»eal extent o<'eiir at Isabela and on
Pt. Borincpien.
The
sea, cliffs
at
IsiilMda are made up
in
large part
(jf
a
thin
l>ed(led
red
limestone,
ali.ernating wdtli thin he<ls of indiirjitfid red lime sand, the entires exj»osiirc
totalling iriu fed thick.
The rock
is largely fragmeiital
in origin,
rela-
tively
free from argilhiccons ma,tter,
like
all
the
rest
of the formation,
and abnost inifossiliferoiis.
On
Pt.
Pnvrinfjiien,
i.n the sea cliffs norl beast
of
the
Uglithonse,
is
a
UO-fiiot
exposiirc;
(d* a
w^hite
to yellowish
elialky
limestone
in
which, are interbedded
vast iimnhers of
large oyster
shelLs
(0.
antiffiienKis).
well shown
in
iigiire 21.
This zone of 0. antignetms
can be traced
a,s
far east
as
l<af)ela. and
apjiarently occurs
in
the
San,
Jiain, District.
The
Qiiehrailillns
limostuiu'
is
cojiroriiiiilile upon
tiii^'
nndtMiyii),!
PiiertOH
liiin'stoiie
aiul the buiUH.hiry hetwccu
the twd
i.^ somewhat
trary.
Thf
QiU'bradilla.s
is
iinieh more
iinit'orm
in
tiiifkiic.ss
t\\i\
l<nvor
fomiatiuiis
(see
sociioiis,
I'latos
\ and
11).
It
is
{\xv
ii,o>t
^mM\(] Tertiary formfrtion
in
Porto Kico, and
it jnarks the
final
dt
tion of a iransgreBsiiig sea at
th.e imviml of maxinniin
Hiibinergeiift'.
Qiiel)radiJla.« lime.stoiie rests
uj:h)11 Oetaeeous roe-kn in the San diian
trJet
find on Yieques
Islarnl and shows
elear evi<lenee
of a
prouTf
HUBBARD, GEOLOGY OF THE LARE^ DISTRICT
49
Like the lower formations, the
Quel)ra(li]las limestone
is not readily
divisible into zones or horizons, as sm inspection of the columnar sections
(Plate II)
will show.
Lithologically, tliere can be no sulidivision.
From
a study of the distribution
of the
fossils, howe\'er, two zones
oE
ratlier
doubtful validit}^ may be distinguished
:
1. Upper Zone, including uppermost
2()() to 300
feet, characterized by
extremely abundant molluscan
sliells (Bowden fauna) and 0. antiguensis.
2. Lower Zone, marked by relative scarcity
of
fossils and presence
of
Oi^J)ifolites mixed with Bowden
fossils.
>SrMMAirY OF FOHMATIONS
Table 4
(page 50)
gives
briefly the general make-up
of the Tertiary
of the Lares District, with a few of the important features
l)y which the
different formations are recognized.
STRrCTUKE
It has already been
pointed
out that
the
Tertiary formations
are
a
structural unit.
There are
lU) breaks anywliere
in
the
series.
Altliough
there
is no discordance of dip denoting an unconformity,
it will
l)e no-
ticed from the dips recorded on
the
cross-sec^tions
(Plate
I,
Figs.
3-6)
that those of the San Sebastian shale are somewhat erratic and
in most
cases at higlier angles than tliose of the overlying formations.
The sec-
tions further show that there
is, on the whole, a gradual decrease
in dip
as one passes from the base
to the top
of the
series.
Thus, while
the
average dip of
all formations
is 4°, the San
Sebastian averages over
()°
and the Quebradillas between 2° and
3°.
The erratic dips recorded
in
the San
Sebastian
shale
are due
in most cases
to
cross-bed diiig
wliich
cannot always be distinguished as such in the field.
Frequently abnormal
dips
are noted
in the limestone
formations,
l)ut
these were
almost
in-
variably found to be due to slumping resulting from underground solu-
tion.
Such abnormal dips are in every case very local in extent.
From
the San Juan District west to Lares, the normal dip of the beds is to
tlie
north, but from Lares
to
the west
coast
it
is
to
tlie
northwest.
The
nearer the west coast the more westerly the direction of dip.
The signifi-
cance of this will be takn up later in discussing the Tertiary In'story.
Except
for the
gentle warping and
local slumping ahove
noted,
no
faults, folds or other structural features
w^ere observed anywhere
in the
Tertiary fonnations
of the
district.
There
are no
dikes
or
other
evi-
dences of igneous action in any of
tlie Tertiary rocks, hence
all volcanic
activity had died out before middle Oligocene time.
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HUBBARD, GEOLOGY OF THE LARES DISTRICT
51
Age and Correlation
The
various
speculations and
conclusions
regarding
the
age
of
the
north coast Tertiary formations has been shown in Table
3.
The diver-
sity of opinion
is explained by the
fact that not
until the summer
of
1919 has a detailed study of the fossils been made.
A large collection of
fossils made by Dr. C. A. Eeeds in 1914 was described by Maury (1919),
and her conclusions regarding the age
of the formations
are given
in
Table
3.
During the survey of the Lares
District in 1916, the writer
made a very large collection of Tertiary fossils, all of which were recorded
as accurately as possible as to locality and stratigraphic position.
These
fossils were studied during the summer of 1919, and the final conclusions
regarding
. the age of the formations based on
this study
are shown in
Table
3.
It will be noted that the writer's conclusions agree with those
of Dr. Maury with the exception of the Miocene or Oligocene age of the
Quebradillas limestone.
Dr. Semmes and the writer recognized in 1917
that the Quebradillas limestone
is approximately equivalent in age to the
Bowden marl of Jamaica.
The remaining point at issue
is whether the
Bowden
is Upper Oligocene
or Lower Miocene.
Evidence
will be
pre-
sented to show that
it is Upper Oligocene.
About nine-tenths of
all the fossils collected are molluscs.^
Of these,
140
species or
varieties have been described or recorded by the writer
(Hubbard, 1921).
A study of these fossils by horizons brings out some
interesting results.
In figure 22, the stratigraphic range of the fauna of
each formation is shown graphically, the ordinate, or height of the curve
at each formational horizon from left to right, denoting the total number
of species of the fauna appearing in that horizon.
Thus the curve of the
Los Puertos fauna shows that a total of 17 species were recorded from the
Los Puertos limestone.
Of these, only 1 was present in the San Sebastian
shale, and about 13 survived in the Upper Quebradillas limestone.
These
faunal curves illustrate the following facts
1.
All the faunas are transitional.
There is no hiatus anywhere in the
series.
This corroborates the stratigraphic evidence.
2. The faunas of the San Sebastian shale. Lares formation, and Que-
bradillas limestone are somewhat distinctive.
The faunas
of the
other
two formations are mixed or transitional.
3. The San Sebastian fauna
is almost wholly
distinct from the Que-
bradillas fauna, and had almost died out in
this region
at
the
close
of
Middle Oligocene (Cibao) time.
3 Wherever the term "fauna"
is used
in
the present paper
it applies to the moUuscan
element, except where noted otherwise.
52
^ClEyTIFlC t^URVEY OF I'ORTO RICO
4. The Lares fauna
eoiitaiiis three elements:
{a)
Speeies
limited
to
the Lares formation, about 40 pereeiit of the total,
{h)
Speeies left over
from the San
KSebastian.
(c) A few speeies typieal of the Quebradillas,
markiiior the be^i^imiino^ of the invasion of the Bowden fauna.
Pig.
22.
Graphic
representation
of the range
of the
Tertiary faunas
The figures at the
left show the number of species.
5. The Cibao fauna has more in eommon with
tlie Lares than with the
Los Puertos, and hence the Cibao limestone should be grouped with the
two lower formations in the Middle Oligocene.
6. The Los
Puertos fauna
is most
like the
Quebradillas
fauna, and
properly belongs
in the Upper Oligocene.
It marks a further increase
of the invading Bowden species.
HUBBARD, aKOLOGY OF THE LARE^ DISTRICT
53
7.
Ill
general,
tlie invasion of
tlie region by new
species was a more
sudden process than the decline of the species already
established.
The
only exception
to
this
is the
early appearance and gradual
increase
in
number of a portion of the Bowden
(or Quebradillas)
fauna.
The ap-
pearance of the bulk of the Bowden fauna, however, was relatively sud-
den.
The term Bowden fauna, as used here, includes that characteristic
group
of molluscan types common to the Bowden marl of Jamaica and
the Aphera-Sconsia formations of Santo Domingo.
8.
It should be pointed out that a further collection and determination
of species from these formations would undoubtedly alter the curves given
above.
(3wing to
the poor
state of preservation
of
fossils
in
the Cibao
and Los Puertos limestones, only a small percentage of their faunas could
be determined and recorded.
Further work in the Los Puertos limestone
will probably increase the number of known
Quebradillas
(or Bowden)
species in that formation, and show that the appearance of the Quebra-
dillas fauna was not so sudden as the curve indicates.
9.
Tlie decline of the San
Sebastian fauna was due to the change
in
bionomic
conditions.
]\Iany
of
the San
Sebastian
species
arc
brackish
water forms, none of which survived the (^hauge to pure marine condi-
tions, but some of which survived through Lares time.
The
latter are
found chiefly in
tlie western part of the
district, where the Lares
is to a
large extent a brackish water or near shore deposit.
TRE SAX SEBABTTAX FAUNA
The following is a list of the molluscs found in the San Sebastian
sliale
and described by the writer
(Hubbard, 1921).
Those forms
wliich do
not occur above the San Sebastian shale are indicated by an asterisk:
"^ Pecten (Pccten)
larcseusc
ii.
sp.
* Feeten
(Chlamijs)
coJlazoenms
ii.
sp.
^ Peeten
{Chlamys) porioricoohns
n.
sp.
Pc'ctcii
(Chlaniifs)
portoricoensiH
var.
retivulatu.
Pecten
{(Jhlamifs)
portorlafcnms var. grand i"^.
* Pccten
{Plagiocteniirni)
ra belli
n.
sp.
* Amusium papyraccum Gabb.
* Amusium mauryi
n.
sp.
* AmuKium
(Propcamusium)
hoUicki Maury.
^^po 1idy I u s
b os tryehi t es G u pj )y
l^pondyJus giimanomacon B. and P.
Ostrea collazica Maury.
"^ Area dariensis B. and P.
* Area
sp.
iiidet.
(u.
sp.).
Area
{^eapliarea)
eollaziea Maury.
54
SCIENTIFIC SURVEY OF PORTO RICO
* Glyeimeris collnsocnsis
n. sp.
* Lucina collazoensis
n. sp.
* Phacoidcs
(Pseudomiltha)
laresensis
ii.
si).
* dementia dariena Conrad.
Pitana {Hyphantosoma) carhasea Guppy.
* Solen
(Plectosolen)
collazoensis.
Corhula collazica Maury.
Teredo incrassaia Gabb.
Dentalium
sp.
indet.
"^ Neritina
{chipolana var.?)
collazoensis
ii.
sp.
* Crucibulum
{Dispotaea)
collazum
n.
sp.
* Natica (Ampulinaf)
collazoensis
n.
sp.
* Epitonium
(Cirsotrema)
collazoensis
n.
sp.
Turritella tornata var. portoricoensis
ii.
var.
* TurHtella planigyrata Guppy.
* Turritella initchelli
n.
sp.
* Petaloco^whusf collazoensis
n.
sp.
Cerithium {Campanile) collazum
n.
sp.
* Cypraca sancti-sehastiani Maury.
* Cassis sp. indet.
* TurMnella
cliipolana
var. precursor
n.
var.
Bullaria paupercula Sowerby
This
list brings out the following important facts
:
1. A large part of the fauna consists of new species or new varieties.
2. There are many brackish water and near shore forms.
Among these
are Neritina, Ampulina, Cerithium (Campanile) and a large number of
pelecypods of fresh or brackish water type, most of which are specifically
indeterminate and not listed above.
The San Sebastian fauna
as a whole
is unique among the Antillean
Tertiary faunas so far described.
It has, however, certain elements show-
ing relationship with other faunas.
The fauna has a very strong resemblance to the Gatun (Upper Oligo-
cene).
In most cases, however, the species are similar, but not identical.
It seems probable that a portion of the Gatun fauna was derived from
the San Sebastian fauna.
The following
is a
list of the San Sebastian
species with their Gatun analogues
:
San
Sebastian.
Gatun.
1. Olycimens
collazoensis
n.
sp.
Glycimesis gatunensis Toula.
2. Lucina collazoensis
n.
sp.
Lucina
sp.,
an
undescribed
shell
in
the Kemp
collection,
at
Columbia
University.
'i. Natica
{Ampulina)
collazoensis
Natica
sp.
indet. B. and P.
n.
sp.
4.
Turritella planigyrata Guppy.
Turritella gatunensis Conr.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
55
5.
Turritella mitchelU n.
sp.
Tiirritella
altilira Conr.
6. Spondylus gumanomocon B. and P.
Spondylus
scotti
B. and
P.
7. Afnusiuni mauryi n. sp.
Amusium luna B. and P.
8. Area daricnsis B. and
P.
Area dariensis B. and P.
9. dementia dariena Conrad.
Clementia dariena Conr.
There are many forms showing a general Flint Eiver aspect.
Many
of the indeterminate fossils found in the San Sebastian^ and not
listed
or
described in the present
report^ show that
the fauna resembles
the
Flint
Eiver
Oligocene
fauna more
closely
than
the
species
described
would indicate.
The two most notable Flint Eiver types are
San
Sebastian.
Flint River, Ga.
1.
Turritella mitehelU n.
sp.
Turitella halensis Dall.
2. Cerithium {Campanile)
eolla^^um
Cerithimn halensis Dall.
n.
sp.
There
is a fairly large representation of Bowden and Aphera-Sconsia
species, which migrated from Porto Eico before Bowden time, since they
do not occur in the Quebradillas fauna.
These are
San Sebastian Shale.
Other Occurrences.
1. Amusium papyraeeum Gabb.
St. Domingo, Bowden.
2. Spondylus hostryehites Guppy.
St.
Domingo,
Bowden,
Tampa,
Chl-
pola,
Anguilla.
3. Spondylus gumanomoeon B. and P.
St. Domingo.
4.
Pitaria earbasea Guppy.
St. Domingo, Bowden.
5.
Turritella tornata var.
St. Domingo
(type), Cumana.
0.
Turritella planigyrata Guppy.
St. Domingo, Caroni
Ser., Gatun.
Chipolan.
1. Turhinella
ehipolana
var.
proeursor
n.
var.
2. Neritina
{Chipolana var.?)
eollazoensis
n.
sp.
The variety of T. chipolana
is more primitive than the Chipolan
shell,
and hence supports the evidence that
tlie San Sebastian
shale
is
older
than the Chipola formation.
The Eocene aspect is only superficial, and there are actually few fossils
closely related to American Eocene
species.
Solen
(Plectosohn)
colla-
zoensis
is strongly suggestive
of Eocene
species, such
as
S.
plagiaulax
Cossmann, 8. laversinensis Lef. and Wat., and S. obliquus Sowerby,
all
from
the
Paris
Basin.
Corhula
coUazica Maury and
Ostrea
collazica
Maury
(especially the small variety with regular, divaricate ribs) might
be considered to have an Eocene aspect.
Ceritliium
{Campanile)
colla-
zum resembles some of the large Cerite shells from the Paris Basin Eo-
56
SCIENTIFIC SURVEY OF PORTO RICO
eeiie, but
is really niucli
closer
to the Flint
Iliver Oligoeene
forms,
as
|)reviously pointed out.
The possible European connections of Solen col/azoensis and Cerifhuun
coUaziim have been referred to.
One of the most interesting shells found
in the San Sebastian shale is a Cirsotrema, described as Epiioniuin
{Cir-
sotrema)
coUazoensis n.
sp.
Apparently,
its nearest relatives are found
in the Paris Basin.
It
is closest to C. ^iil)spinosum
(Grat.)
of the Aqui-
tanian and ('. bourgeoisi de Boury of the Helvetian.
In Europe, the genus
Cirsoirema
ranges from
the Eocene
to
the
Pliocene, but
those
of
the
peculiar type represented by the above-named species are limited
to the
later Tertiary, and are particularly characteristic of the Aquitanian and
Helvetian of the Paris Basin region.
In the discussion of the stratigraphy,
it was noted that the San Sebas-
tian shale
is divisible into three zones
1. Upper or Coral Zone (Zone C of Vaughan), 150'-200\
2. Middle or Clementia dariena Zone,
50'.
3. Lower Zone, lignitic clays and gravels.
The
corals are confined to the Upper Zone, and many
of these same
corals
are found
in
the lower
part
of
the Lares
limestone.
Vaughan
(1919)
lists the following species from the Pepino Formation, giving the
other occurrences of each species
1.
Afitrocoenia pcirioriccnMS Vaughan, Antigua, and Canal Zone.
2. OrhiceUa costata (Duncan), Antigua, Anguilla, Canal Zone.
o. AntiguaHttea celiulosa (Duncan), xVntigua, Florida, Georgia,
etc.
4. Meandra portoru crisis Vaughan.
5. Lcptoscris portoriccnsis Vaughan.
n.
I'ironastrca anguiJlensis Vaughan,
Anguilla.
7.
Side rastrca c<mferta
(Duncan), Antigua, Canal Zone, Anguilla.
8. Cyathomurpha antigiiensis
(Duncan), Antigua, Cuha, Mexico.
0.
Cj/dtJiomorpha
tenuis
(Duncan),
xVntigua, Cuba.
10.
Diploastrca vrassolamellata
(Duncan), Antigua, Cuba,
(jJeorgia.
11.
.1 strcopora portoricensis Vaughan.
12. Goniopota portoricensis Vaughan, Antigua.
These corals are from the collections made by K. T.
Hill, and
a few
collected by the writer.
They are probably
all from Zone C
or Upper
San Sebastian shale, with possibly a few from the basal Lares limestone.
Vaughan points out that 8 of the 12 species occur in the Middle Oligo-
eene of Antigua, and he concludes that this portion of the Pepino For-
mation
is of Middle Oligoeene age.
At Collazo, near the base of the San
Sebastian
shale,
occurs a small
ecliinoid Scutella resembling
S.
fnwsissippjiensis Twitchell
of
the
Clai-
HUBBARD, GEOLOGY OF THE LARES DISTRICT
57
borne, but apparently a new species.
AVhile
it resembles the Eocene spe-
cies superficially,
it also resembles some of the small ScuteUas from the
Pacific coast Miocene, and hence does not indicate an early Tertiary age
for the basal portions of the San Sebastian shale.
The following foraminifera were found in the San Sebastian shale
:
1.
OrJdtolifcs
cf. americana Cushman, Zones 2 and
8.
2. Polystomclla
sp., Zone
3.
3. Leindocyclma
(Heveral species), Zones 2 and
3.
Of these, the first
is by far the most abundant, and
is found in
all of
the Tertiary formations of tlie Lares District.
It
is very similar in
size
and
structure
to
0.
americana
Cushman, from
tha Culebra and Em-
perador formations of the Canal Zone.
Lepidocyclina
is
also abundant.
On the whole, these foraminifera indicate
tlie Oligocene age of the San
Sebastian shale, but they will require further study before anything more
definite can be said.
Conclusion.
The corals listed by Vaughan are undoubtedly conclusive
evidence of the Antiguan, or Middle Oligocene age of the San Sebastian
shale.
The evidence of the other kinds of fossils
is not so conclusive, but
does not disagree with the evidence of
tlie corals.
Tlie molluscan fauna,
while somewhat mixed in
its aspect, shows that the San Sebastian shale
cannot be as old as Eocene.
It
is more nearly similar to the recognized
Upper Oligocene than to the Lower Oligocene faunas.
THE LARES FAUNA
The
following
is
a
list
of the molluscan
fossils found
in
the Lares
formation :*
Pecten
(Chlamjjs)
portoricoensis
n.
sp.
Peetcii
(Ckkwii/s)
portoricoensis
var.
reticulatis.
Pecten
(Chlamijs)
portoricoensis
var. grandis.
"^Pecten
(Chlami/s)
grahaui
n.
sp.
'^Pecten
{Chlamijs)
gralxiui yhy. aguddensis.
Pecten {Chlamys)
grahaui
var.
haioensis.
Pecten
{ChlcDriys)
grahaui
var. guagahensis.
* Pecten (Aequipeeten)
lohecki n.
sp.
* Pecten
{Plagiocteniuni)
cercadica Maury.
Ostrea haitensis Sowerby.
Ostrea seUaeformis var. portoncoensis
n. var.
Ostrea
collazica Maury.
* Ostrea
virginica,
Gnielin.
Ostrea cahohasensis var. portoricana
n. var.
^ Index of the Lares formation
is indicated by an asterisk.
58
SCIENTIFIC SURVEY OF PORTO RICO
* Lithophaga nigra d'Orbigny.
Area yaquensisf Maury.
Area
(Scapharca)
collasica Maury.
* Venericardia scabricostataf Guppy.
. Phacoides {Miliha)
riocancnsis Maury.
Cardium (Laevicardiiim)
cf. serratum Linne.
Curdium {Trachycardiiim) muricoides n. sp.
* Cardium {Trachycardium)
cvyiderelUe var. alternatum n. var.
Pitaria {Hyphantosoma)
carbasea Guppy.
Chione woodwardi Guppy.
CJiione hendersoni Dall.
* Semele
sp. indet.
Cyathodonta reedsi Maury.
Corhula coUazica Maury.
Teredo incrassata Gabb.
Xetiophora conehyliophora Born.
Turritella
tornata
var. portoricoensis
n.
var.
Cerithium
{Campanile) collazum
n.
sp.
* Xancus n.
sp. ?
* Mitra symmetrica Gabb.
.
Bullaria granosa Sowerby.
Ill addition to these species^ many indeterminate fossil?^ chiefly gastro-
pods, were collected.
Some of these are apparently limited to the Lares
formation.
The presence of some
of the Antiguan
corals in
the
basal
Lares limestone indicated that the formation is Middle Oligocene in age.
Other fossils indicating Oligocene age are
1.
Agassizia sp., a small species resembling A, conradi (Bouve),
Oli-
gocene
of Bainbridge,
Georgia.
It
is
apparently
limited
to
the
basal
portion of the Lares limestone.
2.
Orbitoid foraminifera, including OrUtolites cf. americana (Culebra
and Emperador), and Lepidocyclina cf. mantelli, a large flat form.
The
same species occurs abundantly in the Juana Diaz shale and basal Ponce
formation
of the south
coast
of Porto
Eico, and
is
listed by
Mitchell
(1922)
as Orhitoides maritelli.
THE CIBAO FAUNA
The following
list represents probably not more than a third of the
molluscan species in this formation.
JSTearly all of the material is poorly
preserved and largely indeterminate
Pecten (Envola)
reliquus
var. portoricoensis
n.
var.
Peeten
(Chlamys)
portoricoensis var. grandis.
Pecten (Chamys) grahaui var. hatoensis.
Pecten
iChlamys)
grahaui
var. guayahensis.
Spondylus hostrychites Guppy.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
59
Spondylus gumanomocon Brown and Pilsbry.
Ostrea haitensis Sowerby.
Ostrea haitensis^ var.?
Ostrea sellaeformis
var. portoricoensis
n.
var.
Area
sp. Maury.
Cardium
cf. serratum Lirme.
Pitaria
(Ilyphantosoma)
Carhasea Guppy.
Ghione Jiendersoni Dall.
Teredo incrassata Gabb.
Hipponyx poriorieoensis n.
sp.
This
list illustrates clearly the mixed or transitional character of the
molluscan fauna.
The only distinctive index fossil of the Cibao limestone
is an
echinoid, Echinolampas
sp., which
is very
similar
to E.
aldrichi
Twitchell
of the Vicksburg
Oligocene.
This
fossil
is
found! in
great
abundance in the upper portion of the formation, and the zone in which
it occurs can be traced from the Kio Camuy westward for a distance of
5 or 6 miles.
The basis for including the Cibao limestone in the Middle Oligocene
has already been given in discussing the faunal curve
(Fig. 22).
THE LOS PUERTOS FAUNA
A comparison of the following list of the Los Puertos
fossils with the
list of the Quebradillas species will show the close relationship of the two
faunas
Ostrea sellaeformis
var. portoricoensis
n.
var.
Glyeimeris portoricoensis n.
sp.
Lucina
cf. clirysostoma
(Meuschen)
I*hilippi.
Phacoides
{Miltha)
riocanensis Maury.
Cardium muricoides n.
sp.
Chione woodA^ardi Guppy.
Chione hendersoni Dall.
Metis trinitaria Dall.
Teredo incrassata Gabb.
Xenophora conchyliophora Born.
Hipponyx portoricoensis n.
sp.
Orthaulax portoricoensis n.
sp.
The poor state of preservation,
as previously noted,
is the reason for
the
small number
of
species
listed.
Future work should
increase
the
total of Bowden types in this formation, but the above list is sufficient to
determine the age.
Orhitolites cf. americana is the most abiindant fossil
throughout the formation and in the lower part of the Quebradillas lime-
stone.
^ Index of the Cibao Hmestone.
60
SCIENTIFIC SURVEY OF PORTO RICO
THE QUEBRADILLAS FAUNA
The following list includes all the species identified and described, but
probably represents less than half of the entire molluscan fauna of this
formation.
Those forms which are an index
of the
Quebradillas lime-
stone are designated by an asterisk
* Atrina rabelU
ii.
sp.
Pectcn
{Euvola)
reliqtius var. porforicoensls
ii.
var.
* Feeten
{Nodipccten) nodosus
IJiine.
* Feeten
iChlamys)
hodgii
n.
sp.
* Fecten
{Flagiocteniumf)
honnquenense
li.
sp.
* Ostrea
antiguefisis Brown.
* Leda peUella
Dall.
Ai'ca yaquensiH Maury.
* Area {Seapharea)
ef. donaeia Dall.
* Barbatia retieulata (imelin.
* Barbatia
ef. bonaezyi Gabl).
Crlyeimeris portorieoensis
n,
sp.
* Chama involuta Guppy.
* Chama portoricana u. sp.
Lueina
ef. chrysostoma
(Meuscb.)
Pliilii)pi.
Fhaeoides
(MiliJia) rioeanensis Maury.
* Fhaeoides (Miltha)
sp. indet.
* Fhaeoides
{Miltha)
sp.
iudet.
* Fhaeoides
(Lueinisea)
ealhounensis
Dall.
* Divarieella prevarieata Guppy.
* Codakia
magnoliana
var.
borinquenense
ii.
var.
Cardium
ef. serratiim Liune.
* Cardium
(Trigonoeardia)
sambaieuni
var. portorieoensis
n.
var.
"^Cardium
{Trigonoeardia)
n. sp.?
aff.
C. sambaieuni Maury.
* Cardium
{Trigonoeardia)
haitense var. eereadieum Maury.
* Cardium
(Trigonoeardia)
haitense
var.
areeiboense,
ii.
var.
* Cytherea {Cytherea)
berkeyi n. sp.
Chione tvoodwardi Guppy.
* Tellina strophoidea n.
sp.
* Tellina portorieoensis
ii.
sp.
* Tellina
(Seissnla)
grabaui
n. sp.
* Tellina aff. T. (Angulns)
atossa
Dall.
Metis
trinitaria
Dall.
* Fsammosolen
saneti-dominiei Maury.
Cyathodonta reedsi Maury.
Teredo inerassata Gabb.
* CalUostoma
portorieoen^ns
u.
sp.
* Turbo fettkii
ii.
sp.
* Liotia (Arene)
coronata var. portorieoenis
ii. var.
Xenophora conehyliophora Born.
Hipponyx portorieoensis
ii.
sp.
HUBBARD, GEOLOGY OF THE LAREH Dim'RlCT
61
* Calj/pfniea
cf. ceniraUH Conrad.
* CruciJnilum aurivnla
var. portoriroensis
n.
var.
* Crucihiihim
(luricnlaf var.
'^ Natica canrena?
(Liiiiie)
Moercli.
* Turritclla portoricoensis
ii.
sp.
* TurritcJld
bcrJrCj/i n.
.sp.
* J*etalo('onchus
dofnim/cnsis
Sowerby.
* PprdtnideUa portoricocnsis
ii.
s\).
* Bittimn
sp.
iiidet.
* Cerithiirm portoricocnsis
ii.
s]).
* Ccrithium qnebradilk thsis
ii.
sp.
* Modulus 7nodulus var. hasilcns Giippy.
* l:<1romJ)U8 proxunns? Sowerhy.
* t^tromhuH Mfronsf Sowerhy.
* Orthaular gahhif Dall.
Orthanhix portoricocnsis
ii.
sp.
* Cf/praea spurcoides?
Gab)).
* Malea ('(innira (iuppy.
* Stromhina portaricana
n.
sp.
"^ J'Jios costat us
(Jahh.
* Phos elvgans var. portoricocnsis
ii.
^'ar.
^ Murcx
{PhjiUonotns)
cornnrcctns
(iii])i)y.
"^ Alcctrion guraJnnsis var. portoricocnsis
n.
var.
^ Alcctrlon gurahcnsis var. varicuni
ii. var.
* Fusus hcnclcni Sowerby.
* TurJyinclla chipoJana
var.
arccihocnsc
n.
var.
* Xancus valid us Sower) )y.
* Mitra hcnckcni
Sowerl)y.
* OlivclJa
nufticoidcs
var.
poiioricociisis
ii.
var.
* Olivclla porioricocnsis
n.
sp.
* Canccllaria lacvcsccns Guppy.
* Turris alhida var. haitcnsis Sowerby.
* Turris alhida var.
ef. virgo Lamarck.
* Drillia consors var. portoricocnsis
ii. var.
* Drillia grahaui
ii.
sp.
* Drillia portoricocnsis n.
sp.
* Drillia scmmcsi
ii.
sp.
* Cf/thara
ef. donga ta
(labl).
* Tcrchra que brad illcnsis
ii.
sp.
* Conus catena tus Sowerby.
* Conus
ef. warginatus Sowerby.
Bullaria paupcrcula Sowerby.
* Bullaria portoricocnsis
n.
sp.
Bullaria granosa Sowerby.
The most
striking feature
of
this
list
is
the
large
representation
of
Bowden and Aphera-Seonsia
species.
The approximate age equivalence
of the Quebradillas limestone with Bowden horizons in the West Indies
is too evident to require further discussion.
62
SCIENTIFIC SURVEY OF PORTO RICO
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HUBBARD, GEOLOGY OF THE LARES DISTRICT
63
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64
SCIENTIFIC SURVEY OF PORTO RICO
Foraminifera are abundant and include Orbitolites cf. americana Cush-
man
(lower Quebradillas) and Polystomellaf
sp.
(upper Quebradillas).
In the uppermost horizon and fragments of Scutella sp. and Cidaris sp.
and a Schizasier resembling 8. scherzeri Gabb, a species from the Sapote,
Costa Eica and the Emperador limestone, Canal Zone.
It also resembles
S, floridanus Clark, of the Vicksburg Oligocene.
Corals are not common
in
the
Quebradillas, and none
of
the
chief
reef-building
types
occur.
Semmes
(1919)
illustrates
a
Fungid
coral,
indeterminate,
from
the
Quebradillas limestone of the San Juan
District.
This
fossil occurs in
the Lares
District, but
is not abundant.
The commonest coral
in the
Quebradillas limestone
(and equivalent horizons on the south coast and
on Vieques Island)
is a Balanophyllia.
There seem
to be two
species
(or varieties), each resembling species found in the Bowden or equivalent
formations.
Another
abundant
coral
is
a
Stylophora
like
S.
affinis
Duncan.
SUMMARY OF THE FAUNAS
The
molluscs, which make up about nine-tenths
of the fauna, show
that the uppermost Tertiary formation of the Lares District is of Bowden
age.
The basal member is shown by the corals, and less certainly by the
molluscs,
to be of Antiguan or Middle Oligocene
age.
There are only
two distinct faunas, the San Sebastian-Lares and the Quebradillas.
The
others are mixtures of San Sebastian-Lares and Quebradillas (Bowden)
types.
Table 5
(pages 62-63)
is based upon the molluscan species above de-
scribed^ and shows the relative numerical distribution of the Lares Dis-
trict fossils in other localities and horizons of the West Indies and else-
where.
It shows the
close relationship
of the Quebradillas fauna with
the Aphera-Sconsia faunas of Santo Domingo.
Another feature brought
out
is
the
effect
of
geographical
position upon
the
similarity
of
the
faunas.
Thus formations of the same age as the Quebradillas limestone,
such as the Gatun formation and the Chipola-Oak Grove series, but situ-
ated at considerable distance from Porto Eico, have few identical species
in common, but a considerable number of similar species or varieties.
CORRELATION WITH THE SOUTH COAST
The question of the correlation of the north and south coast Tertiary
formations has been awaiting the
collection and comparison
of
fossils.
« In
the
case
of the two south
coast formations
(Juana
Diaz and
Ponce)
the data
include other types of fossils besides molluscs.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
65
Berkey made no attempt at a correlation because the fossils then collected
(1914)
did not warrant
it.
Semmes (1919, p. 59)
pointed out that the
Ponce formation is ^^in part practically equivalent to the Arecibo forma-
tion of the northern coast/^ but he did not make any subdivisions of the
south coast
series.
Maury
(1919,
p. 214),
after a study of
fossils
col-
lected by Eeeds, made a correlation of the north and south coast Tertiary
(Table 6).
This correlation was advanced by Maury as a tentative one
and
is the first attempt of a detailed character.
However, the evidence
at hand seems
to warrant
certain
departures from
this
correlation,
as
shown by the following considerations
Table 6
North Coast
South
CoavSt
1
Middle Miocene
.2
I
o
1<
o
.2X
'p.
02
Quebradillas limestone
with Bowden fauna
and Metis
trinitaria.
Lower Miocene
Upper Oligocene
Aguadilla limestone
with Orthaulax
aguadillensis.
Ponce chalky beds with
Ostrea
cahobasensis.
Lares
limestone
with
Campanile
{Portori-
cia)
larlca.
Middle Oligocene
0)
MS
o
Rio
Collazo
shales
with
dementia
ra-
helli.
Guanica
shaly
lime-
stone
with
Ostrea
antiguensis.
1.
Mitchell (1922) shows that the beds at Guanica are stratigraphic-
ally ^t or near the top of the Ponce formation.
In speaking of the Ponce
chalky beds, Berkey
(1915,
p. 14)
says:
^'It
is judged that the portion
of the formation seen at Guanica
is a
still higher horizon, but the exact
age values have not been worked out/^
From this,
it seems evident that
66
SCIENTIFIC SURVEY OF PORTO RICO
the Guanica beds overlie the Ponce chalky heds^ and are at the top of the
column, not at the bottom as Maury stated.
2. The Guanica beds
(together with the upper part of the Ponce beds
to the
east)
carry a typical
Quebradillas
(Bowden)
faima^
as
will
be
shown
later.
Osirea
antigiiensis which Maury
lists from the
Guanica
beds
is one of the best index fossils of the Quebradillas limestone.
Eegarding the Juana Diaz, the basal shale member of the south coast
group^ Maury
(1919,
p. 215)
says:
^'The Juana Diaz
shales furnished
very few molluscan
shells.
.
.
.
The evidence
at hand
is too scanty
for
any
definite
stratigrapliic
conclusion
regarding
these
beds.^^
As
shown by Table
o, and as will appear later, the Juana Diaz shale carries
several of the best index fossils of the San Sebastian shale, among which
dementia dariena (= C rabelli of Maury)
is the most significant.
From a careful comparison of south coast fossils collected by Berkey,
Lobeck, and Mitchell, with material from the Lares District, the correla-
tion
table
(7)
is proposed and
is believed to be
essentially
correct.
A
more detailed comparison
is impossible because the south coast series has
not been definitely subdivided in the
field.
Table 7
North Coast
South Coast
Pleistocene
San Juan formation
_
__
- Diseonformity.
San Juan formation
_
_
Disconformitv
Quebradillas limestone
Guanica beds and Upper
Upper Oligoeene
Los Puertos limestone
Ponce limestone.
Cibao limestone
Lower Ponce limestone
Middle Oligoeene
Lares formation
___?_
San Sebastian sliale
Unconformity
Juana Diaz sbale
_
_
_Unconformitv
Upper Cretaceous
Older Series
Older Series
HUBBARD, GEOLOGY OF THE LARES DISTRICT
67
The essential points in the basis of correlation have been referred
to,
but
the following
lists
of
fossils from
the south
coast formations
are
offered as further evidence.
The localities recorded are taken from notes
bj Berkey, Lobeck, and Mitchell
:
FOSSILS FROM THE JUANA DIAZ SHALE
Name of Fossil
TuriteUa
halensis Dall
Tiiritclla
halensis
var.
alpha Mitchell
Scalaf
sp.
Localities
Remarks
Juana Diaz, Yauco
Juana
Diaz
Closely
related
to
T. coUa-
zocnsis of the San Sebas-
tian
shale.
Juana Diaz
An
internal mold.
A
simi-
lar
or
identical
fossil
occurs
in
the upper San
Sebastian
shale.
Pccten
(Chlamys)
por-
toricoensis
n.
sp.
Near Juaua Diaz
Index of San Sebastian and
Lares
formations.
Clementia
d a r i en a
Conrad
{=C.
rabelli
Maury)
Juana Diaz
Index
of
San
Sebastian
shale.
Solen
{Plectosolen coll-
azoensis n.
sp.
Juana
Diaz, Yauco.
Index
of
San
Sebastian
i
shale.
•
Natica
sp.
indet.
Juana Diaz
internal
mold.
Similar
molds
are
found
in
the
San
Sebastian
shale and
range through
the
Cibao
limestone.
Cypraxi
sancti-se'bas-
tiani Maury
Juana Diaz
Index
of
San
Sebastian
shale.
Pecten rahelli
n.
sp.
Juana
Diaz,
Yauco
and
K.
25,
Ponce-
xVdjuntas
road.
Index
of
San
Sebastian
shale.
Stromhus
sp. indet.
Yauco
Resembles
some
internal
molds
from
the San
Se-
bastian shale.
Vencricardia
cf. scahri-
costata Guppy
Juana Diaz, Yauco
Occurs
in Lares formation.
68
SCIENTIFIC SURVEY OF PORTO RICO
FOSSILS FROM THE JUANA DIAZ SHALE—Continued
Name of Fossil
Localities
Remarks
Teredo incrassata Gabb.
Ranges
throughout
the
series,
north
and
south
coasts
and
Vieques
Is-
land.
Of no value as an
index fossil.
LepidocycUnaf sp.
Juana Diaz, Yauco.
A
large
foraminifer,
ap-
parently the same as the
one occurring abundantly
in the Upper San
Sebas-
tian
shale,
Lares
forma-
tion, and Cibao limestone.
FOSSILS OF THE PONCE LIMESTONE
Name of Fossil
Localities
Remarks
OrMlolites
cf.
ameri-
ccma Cushman
Ponce, Guanica
Quebradillas limestone.
Balanophyllia
sp.
Widespread, along
coast.
Extremely abundant in both
the
Quebradillas and the
Upper Ponce limestones.
Stylophora
sp.
(like
^.
afflnis Duncan)
Culebrinas
Pt.,
and along coast.
Characteristic
of
Quebra-
dillas
and
Upper
Ponce
limestones.
Btrombus
proximus
Sowerby
Mona Island
Quebradillas
limestone.
BuUaria
cf. paupercula
Sowerby
Mona Island
Quebradillas limestone.
Turritella
cf.
gatunen-
sis Conrad
Mona Island
Apparently
related
to
T.
portorieoensis of the Que-
bradillas limestone.
Cardium
cf. muricoides
n. sp.
Near Guanica
Ranges from
Lares forma-
tion to Los Puertos lime-
stone.
Cardium
cf. lingualeonis
Guppy
Near
(iuanica
Bowden, Jamaica.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
69
FOSSILS OF THE PONCE LIMESTONE—Continued
Name of Fossil
Localities
Remarks
Cardium
haitense
var.
cercadicum Maury
Ponce
Quebradillas limestone.
Cardium
haitense
var.
areeihoense n. var.
Ponce, Culebrinas Pt.
Quebradillas limestone.
Corhula sp. indet.
A
small
species, not found
in any of the north coast
formations.
CMone woodwardi
Guppy
Culebrinas
Pt., Ponce,
Guanica, Rio Yauco
2y2
miles
S.
E.
of
Yauco.
Abundant
in
Quebradillas
limestone.
Olycimeris
cf.
portori-
coensis
n.
sp.
Guanica
May
be
the
Quebradillas
species.
Lucina
cf.
chryostoma
(Meusch.)
Phil.
Vieques Is., Culebrinas
Pt., Ponce, N. W. of
Ponce,
near
Guan-
ica,
K.
2.4,
Ponce-
Adjuntas road.
The most characteristic fos-
sil
of
the
Ponce
forma-
tion.
It
occurs
in
the
Quebradillas
and
Los
Puertos limestones.
Phacoidcs
{Lucinisva
)
calho unensis Dall
Near
Ponce
Quebradillas limestone.
Cytherca
( Gytherea )
herkcyi
n.
sj).
Ponce, West of Ponce,
N. E. of Ponce.
Quebradillas limestone.
Teredo incrassata
Gabb.
Vieques, Ponce
Quebradillas limestone.
No
index
value.
Ostrea antigucnsis
Brown
Guanica?
Quebradillas limestone.
Re-
ported by Maury from the
Guanica
shaly
limestone.
Terehra cirrus Dall
Near Guanica
Found in the lower Alphera
formation
of
Santo
Do-
mingo.
Tcrchra
cf.
quehradil-
lensis n.
sp.
Ponce
Resembles the Quebradillas
species
and may
be
the
same.
70
SCIENTIFIC SURVEY OF PORTO RICO
FOSSILS OF THE PONCE LIMESTONE—Continued
Name of Fossil
Localities
Remarks
A rca
( Scap liarca )
cf
riocancnsis Maury
I*once
Upper
Aphera
of
Santo
Domingo.
Not
found
on
the
north
coast
of Porto
Rico.
Bullaria granosa?
Sowerby
Cuiebrinas
Pt.
Quel)radillas
limestone.
Olivella muticoides
var.
portoricoensis
n.
var.
K.
75.2,
Ponce-Penue-
las Road, Aguila Pt.
Quebradillas
limestone.
Leda
cf.
pcUella Dall
K.
75.2,
Ponce-Peiiue-
las Road
Resembles
this
species,
which
is
very
typical
of
the
Quebradillas
lime-
stone.
Metis
trinitaria
Dall
Cuiebrinas
Pt.
Quebradillas limestone.
Ap-
parently
rare
on
the
south coast.
Ostrea cahohasensis
var. portoricana
n.
var.
Guanica
Boring Sponge?
Cuiebrinas
Pt.
The
casts
of
the
burrows
of
some
organism
like
CMonc.
These
are
so
widespread
and
charac-
teristic
of the Quebradil-
las
limestone
that
they
may
be considered as
in-
dex of this horizon.
Their
burrows are chiefly in the
large
gastropod
shells.
It
is believed that the last list includes fossils from all horizons of the
limestone overlying the Juana Diaz shale, and hence ranging from Ijares
to Quebradillas in age.
It is inadvisable with the data at hand to attempt
listing the fossils from the limestone as Upper or Lower Ponce.
Never-
theless,
it
is evident that most
of the more typical Quebradillas
species
occur near the coast and therefore in the upper part of the Ponce lime-
stone.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
71
COMPARISON WITH IMPORTANT ANTILLEAN
LOCALITII-S
Antifjua.
The Island of Antigua, in the Lesser
iintilles, southeast of
Porto
Eico,
is
considered
the
type
locality
of
the
Middle
Oligocene
(Antiguan)
of the Caribbean region.
The island consists of an igneous
basement
njion which
rests
a
series
of
tuffs
with
interbedded marine
strata, and finally an uppermost limestone
series known as the Antigua
formation, and considered by Vaughan as the type section of the Middle
Oligocene.
The evidence given by Vaughan (1919, p. 259)
is based npon
the fossil corals which he finds chiefly in a GO-foot fossil reef at or near
the base of the Antigua formation.
The evidence of
tliese corals shows
that the
x'\ntigua formation
is equivalent
to the lower mem])ers
of the
Tertiary series in the Lares District of Porto Eico.
On the basis of the
€orals, Vaughan puts the entire iintigua formation in the Middle Oligo-
cene, and estimates
its thickness at 350
feet.
These conclusions appar-
ently do not agree with evidence furiiished by others who have studied
the geology of this island.
Of a
list of 10 molluscan
species from
the
Antigua formation,
collected and
described by Brown
(1913,
p.
598),
6 are identical with, and
3 are similar or related to species occurring in
Antillean formations of Bowdcn age, while another species
is character-
istic of the Anguillan, or so called Upper Oligocene.
The thickness of
tlie Antigua formation
is estimated by Spencer (1901)
as '^at least many
hundred
feet," and by Brown as
"^^upwards
of 1500
feet
at
least.''
It
forms a belt about 5 miles in width, and Vaughan estimates the seaward
dip to range from 10°
to 15°.
Taking the average at 10°, the thickness
must be several times greater than Vaughan estimated, and lience more
nearly comparable with the north coast series of Porto Eico.
Consider-
ation of the above conflicting evidence seems to show that more than one
formation
is represented in the Antiguan formation, and detailed
strati-
graphic work will have to be done before the Antigua formation can be
regarded as the type section of the Middle Oligocene in the West Indies.
Santo Domingo.
The
highest
Tertiary
formations
are
the Aphera-
Sconsia formations described by Maury
(1917).
These formations are
approximately
equivalent
to
the Bowden marl
of Jamaica and
to
the
Quebradillas
limestone
of
Porto
Eico.
Correlation
of
the
Orthaulax
Zone
of Santo Domingo and the pre-Quebradillas formations
of
Porto
Eico cannot be made
with
certainty
because no
detailed
stratigraphic
work has been done in the older Santo Domingo formations.
Haiti.
In
Haiti, the Maissade beds
(uppermost horizon)
correspond
with
the
Quebradillas
limestone.
The
underlying
formation
(Las
Cahobes)
is probably equivalent to the San Sebastian shale, and overlying
72
SCIENTIFIC SURVEY OF PORTO RICO
limestones below the Quebradillas, as indicated by
its stratigraphic posi-
tion.
Among the fossils listed by Jones (1918, p. 738) from this forma-
tion are
Turritella planigyrata Guppy.
Turritella tornata Guppy.
Vener^wardia scal)ricostata Guppy,
which in the Porto Eico
section
are limited to the San
Sebastian and
Lares formations.
Other
localities.
It
is quite
likely that future work
will greatly
in-
crease the known number of localities in which the uppermost portion of
the Tertiary
is of Bowden age.
The small key of Sombrero, 140 miles
east of Porto Eico, is built ap of a white limestone of Bowden
(or Que-
bradillas)
age.
The following is a partial list of the fossils:^
8trombus proximus Sowerby.
Xenopliora conchyliiphora Born.
Bullaria granosa Sowerby.
Tellina
cf. strophoidea n.
sp.
Cijathodonta
cf. reedsi Maury.
Cardium haitense Sowerby.
Cardium cf. samhaicum Maury.
Chione woodwardi Guppy.
In addition to these, the casts of the Sponge
(?)
burrows, character-
istic
of the
Quebradillas limestone,
occur abundantly in the Sombrero
limestone.
The proximity
of
Sombrero
to
Anguilla,
Vaughan's
type
locality
of the Upper
Oligocene,
suggests
the need
of
detailed
strati-
graphic work in this vicinity.
The
conclusions
regarding
the
correlation
of
the
Porto Eican with
some of the other Antillean localities
is given in the accompanying table
(8).
This table
is based on those by Vaughan
(1919^, p. 595), Maury,
and Jones, with minor changes.
The chief departure from the authori-
ties above named
is the placing of the Bowden formation and
its equiv-
alents in the Upper Oligocene.
OLIGOCENE OR MIOCENE
The question of the Miocene or Oligocene age of the formations equiv-
alent or approximately equivalent to the Bowden marl of Jamaica is one
on which authorities do not agree.
Within the last few years Yaughan,
Maury and others have maintained that the age
of these formations
is
' Determinations by
the
writer from
material
in
Paleontological Museum,
Columbia
University.
Table 8
PROPOSED CORRELATION OF ANTILLEAN TERTIARY FORMATIONS
Age
Porto Rico,
North
Coast
PoBTo Rico,
South Coast
Santo
Domingo
Haiti
Jamaica
Cuba
Panama
Sombrero
Anguilla
Antigua
Florida
AND
Georgia
Maryland,
Vir-
ginia, North
Carolina, and
South Carolina
Pliocene
Caloosahatchie
marl
Jacksonville
formation
Waccamaw marl,
etc.
Duplin County
marl,
etc.
Upper Miocene
liower Miocene
Shoal River
marl
Oak Grove
Chipola
Upper Oligocene
Quebradillas
limestone
800
feet
Los Puertos
limestone
lOOO
feet
Ponce limestone
3000
to 4000
feet
?
Juana Diaz
shale 3000 to
4000 feet
Sconsia forma-
tion
Aphera forma-
tion
"Orthaulax
in-
ornatus
zone"
Maissade
beds
1000
feet
Las Cahobes
beds 6500 feet
Bowden marl
La Cruz marl
Baracoa marl
Miscellaneous
limestones
Guantanamo
coral lime-
stone
Gatun
formation
Emperador
limestone
Sombrero
limestone
Anguilla
limestone
Antigua
formation
(undivided)
Thickness
more
than 1500 feet
Chief coral zone
at base
Middle
Oligocene
Cibao limestone
250-1000 feet
Lares formation
350-1200 feet
San Sebastian
shale 700 feet
9
Tliomonde beds
1500
feet
9
Culebra
formation
Tampa
Flint River
Lower Oligocene
Limestone series
8000 feet
7
Montpelier
limestone
?
•
Limestone
?
Tonosi
?
Tuffs
with
ma-
rine beds
Seaforth
limestone
?
Igneous
basement
Vicksburg Group
Upper Eocene
Cambridge
formation
Richmond
formation
St. Bartholomew
limestone
Ocala limestone
HUBBARD, GEOLOGY OF THE LARES DISTRICT
73
Lower Miocene.
On the other hand, Dall and some other authorities
still
take the opposite view and
place them
in
the Upper
Oligocene.
The
writer believes that the position taken by Dall
is correct.
The evidence
favoring the Miocene age of the Bowden
is given by Vaughan and need
not be reviewed here.
The evidence favoring the Oligocene age may be
summarized as follows
1. The genus Orthaulaa) has until recently been considered an index
of Oligocene age.
It is now regarded by Cook and Yaughan as indicative
of
either
Oligocene
or Lower
Miocene.
This change
in
the
status
of
Orthaulax
is not due to any new discoveries of that genus
in horizons
higher than
it has previously been known
to
occur, but
is due
to the
placing of previously recognized Upper Oligocene formations in the Mio-
cene nomenclature by Vaughan and others.
The genus Orthaulax is one
of the most abundant fossils in the Quebradillas limestone, as well as in
the Los Puertos limestone.
In the former horizon, there are at least two,
and very likely as many as four species of Orthaulax present.
According
to Vaughan the Quebradillas limestone
is Lower Miocene in age.^
If this view be accepted, then we must admit that the Lower Miocene
fauna of Porto Eico is largely Oligocene in its general aspect.
Thus one
of the strongest arguments of the Miocene advocates is weakened, namely,
that the Chipola fauna (and presumably its time equivalents) in its aspect
"looks forward to the later Tertiary and Eecent, rather than backward"
(see Vaughan, 1919a, p. 573).
It is true that Orthaulax has been noted
as an exception to this statement, but every new discovery of abundant
Orthaulax in strata of Bowden-Chipola age tends to increase the signifi-
cance of this exception, and the Quebradillas fauna with
its myriads of
Orthaulax
shells mixed with the
host
of Bowden-Aphera-Sconsia types
furnishes one more difficulty in the way of any argument based upon the
above quotation.
2. The presence of Ostrea antiguensis in great numbers at the top of
the Quebradillas limestone.
Maury regards this fossil as so excellent an
index of the Oligocene that
its presence in the Guanica shaly limestone
is sufficient evidence for correlating this formation with the Antiguan.
3. The gradual appearance of some of the Quebradillas species,
start-
ing with the Lares formation (Antiguan).
4. The absence of any unconformity or marked faunal hiatus between
the Quebradillas limestone and the lower limestones.
The entire
series
is a structural unit.
5. Erosion
interval and faunal break occurs
in Porto
Eico,
as
else-
where, at the end of the Bowden time.
^ Personal communication.
74
SCIENTTFJG SURVEY OF PORTO RICO
6. Large representation in the San Sebastian shale of Gatun, Chipola,
and Bowden types shows relationship with the Bowden horizon.
7. Two species in the San Sebastian shale (Glycimeris collazoensis and
TurUnella chipolana prectirsor)
apparently have
closely related deriva-
tives, showing
slightly greater
specialization
in
the
Quebradillas
lime-
stone.
8. The marked
difference
of
aspect between
the San
Sebastian and
Quebradillas
faunas
is
due
chiefly
to
change
in
bionomic
conditions.
Thus
the
San
Sebastian
fauna,
typically
a
lagoon
or brackish
water
facies, was forced to migrate because of changing conditions.
It reap-
peared in Bowden time in the Canal Zone (Gatun fauna), with consid-
erable changes in species, but with the same general aspect
it had in San
Sebastian time.
The San Sebastian and Gatun faunas have in common
such characteristic species as Clementm dariena and Area dariensis.
9.
Inspection of the fossils listed from the Canal Zone shows that the
Gatun formation has many species in common wdth the underlying lime-
stones.
Most of the change in faunal aspect may be explained by change
of conditions
of deposition.
In Antigua, Bowden molluscan
fossils
are
apparently
so
intimately mixed with
Oligocene
corals
that no Upper
Antiguan or Bowden horizon
w^as differentiated th3re by Vaughan.
In
Florida, the Alum Bluff series is so closely associated with the underlying
Tampa and Chattahoochee horizons that authorities are
still not in agree-
ment regarding the existence of a physical and faunal break, even after
considerable stratigraphic work has been done.
All are agreed, however,
that there
is a break
at
the top
of the Alum Bluff group (= end
of
Bowden time).
10.
In the San Sebastian shale
is found species
of Cirsotrema which
is hardly distinguishable from certain species in the Helvetian (Miocene)
of Europe.
Species of Campanile and Plectosolen in the San
Sebastan
shale have
allied forms
in
the European
Eocene.
Facts such as
these
suggest caution
in attempting to decide the age
of an American fauna
by comparing
it, in
its general aspect, with European faunas.
The only
infallible criterion
for
correlation with European
or other
distant sections
is the recognition of a world wide crustal movement or
change of
sea
level.
If
the
elevation
of the
South
Atlantic-Antillean
region
at the
close
of
the Bowden-Alum
Bluff time
can
be
definitely
correlated with movement
in other
parts
of
the
world,
a
definite and
logical division line can be drawn between the Oligocene and the Miocene.
In the American-Antillean Province
this
line of division
is
distinct,
as
Ball
(1898^
p. 329)
long ago
pointed
out.
The evidence
obtained
in
HUBBARD, GEOLOGY OF THE LARES DISTRICT
75
studying the sections in the Lares District supports the early conclusions
of Dall rather than the recent conclusions of Vaughan.
Tertiahy Histoky
The entire group of Tertiary formations of Porto Eico were deposited
during
a
continuous
period
of
gradual
submergence^
which began
in
Middle Oligocene time.
The formations
are therefore
all conformable
with one another, and show a progressive overlap over the Older
Series
rocks.
The relationship
is brought out in the ideal section
(Fig.
23).
With the initiation of submergence^ the sea encroached in the valleys of
the old land surface, forming embayments, as for example at San Sebas-
West
Lares
District
San J (JAN District
East
Vieques
Is.
5
+
_____^=..=^
3
—
—"^5
^^4-:^ ^
Fig.
23.
Ideal section along the north coast
at the time of maximum
siihmerfjcncc
(late
Oligocene)
Showing
transsTessiA''e
overlap
of
tho
Tertiary
formations
on
tlie
old
land
surface.
The San
Sebastian
shale
is confined
to
the Lares-San
Sebastian embayment.
The San
Sebastian shale and basal shale
facies of the higher horizons are shown by the broken
lines.
The formations are numbered consecutively:
(1)
San Sebastian
shale,
(2)
Lares
formation.
(3)
Cibao limestone,
(4)
Los
I'uertos
limestone,
(5)
Quebradillas
limestone.
tian and Juana Diaz, in which the chief deposits of basal shales accumu-
lated, under an
alternation
of
brackish, freshwater and marine
condi-
tions.
At San
Sebastian
coral
reefs formed
across
the mouth
of
the
embayment.
The large
'^heads^^ or colonies of
corals, broken from
this
reef by the surf, are found imbedded in the marls of the San Sebastian
formation,
exposed
along
the
Lares
Eoad.
In
connection
witli
the
theories
of origin
of
coral
reefs,
it
is
interesting
to note
that
several
hundred feet of shale, in part marine, were deposited before the actual
reef
itself was formed.
Comparison
of
sections
at
Collazo and
Lares
would seem to show that the Upper
shales and
coral marls
of
Collazo
grade into coral-reef limestone when traced toward Lares.
This would
indicate
that
the
green
coral-bearing
marls
at
Collazo
were
formed
behind an off-shore or fringing reef.
7g
SCIENTIFIC SURVEY OF PORTO RICO
With continued submergence
corals ceased
to be the dominant
reef-
building
organisms,
molluscs
and
foraminifera
taking
their
place.
Throughout the entire series, the deposition kept pace with subsidence,
as shown by the fact that shallow water organisms predominate
in
all
the
formations.
In the
eastern
part
of the Lares
District,
there was
apparently frequent emergence, with
oxidation
of the newly
deposited
strata.
The frequent occurrence of red limestone at various horizons in
the
eastern
part
of
the
district may
be
evidence
of
this.
The
chief
evidence, however, is to be found in the thinning of the Cibao limestone
and Lares formation toward the west, indicating near shore deposition,
with a peninsula in the longitude of Moca.
The submergence was prob-
ably accompanied by a seaward down-warping movement
illustrated by
the
fact that the Quebradillas limestond dips
at lower angles than the
lower formations.
This statement
is true even where the
Quebradillas
limestone
is the basal formation, as in the San Juan District.
MAGNITUDE OF THE SUBMERGENCE
One of the unsettled problems has been the extent to which the island
was sugmerged in Upper Oligocene time.
Lobeck concludes (1922) that
the island was not entirely submerged, as shown by
1. Presence of gravel and clay at various horizons in the Tertiary beds.
2. Abrupt termination of Tertiary beds against the upland slopes of the
old land surface.
The
evidence
found
in
the
Lares
District may
be
summarized
as
follows
1. The faunal
difference between the lower formations
in the Lares
District and the lower formations of the south coast is considerable, and
indicates a land mass quite extensive in an east-west direction, with no
connecting passages close at hand.
2. The Bowden faunas
of the
Quebradillas and
IJpper Ponce show
much greater similarity than do the older faunas, but
still show greater
difference than the present beach faunas of the north and south coasts.
The extreme abundance
of certain
shells
(like Metis
trinitaria)
in the
Quebradillas limestone, and their rare occurrence in the Ponce limestone
is very significant.
3. The great thickness of the Tertiary series (nearly 4000 feet) would
seem to show that the island was completely submerged.
It has been
show^n, however, that the formations near the old (Oligocene)
shore line
are only a fraction of their maximum or off-shore thickness.
The beds
were undoubtedly
deposited with
a
slight
initial
dip
to
seaward, and
HUBBARD, GEOLOGY OF THE LARES DISTRICT
77
hence are of the nature of fore-set beds.
The maximum accumulation of
limestone was in an outward direction, rather than upward.
This may
be one reason why the enormous thicknesses recorded for Oligocene reef
limestones
like those of Haiti and Porto Rico are not comparable with
measurements of beds the same age deposited in Florida and elsewhere
on the continental shelf.
4. A earful search was made for outliers of Tertiary rock in the higher
portions
of interior mountains.
The only evidence found was a single
houlder of Tertiary limestone (probably Lares) in the channel of the Eio
Guayaba, two miles southeast of Aguada.
However, this does not show
that the Tertiary beds covered the Cordillera Central to the south.
From the above evidence,
it may be
stated that Porto Eico was not
•completely submerged in Upper Oligocene time.
THE UPLIFT
The uplift was differential.
It amounted to at least 1300 feet in the
vicinity
of Lares, and
less than
100
feet
in
the extreme
eastern
part
of the island.
This differential uplift resulted in the truncation of the
w^est end of the
island,
chiefly by warping.
The northwest dip
of the
Tertiary strata along the west coast has already been referred to in dis-
cussing the
structure.
It
is probable that there was some
faulting in
connection with this truncation of the island, and that the zone of fault-
ing lies somewhere to the west of the island, where faulting is now taking
place.
This was shown by the 1918 earthquake.
PHYSIOGRAPHY
There
are
three major
physiographic
provinces
represented
in
the
Lares District:
1. The Complex Mountainous Oldland.
2. The Elevated Coastal Plain.
3. The Playa Plains.
These are the most important physiographic units of the island, and
have been
described by
Hill,
Berkey,
Lobeck, and
the authors
of
the
geological reports on the
different
districts.
The geologic map
brings
out the contrasts of
relief and topographic
characteristics
of the
three
provinces as they occur in the Lares District.
The Complex Mountainous
Oldland comprises
the
central mountain
chain; the core or backbone of the island.
In the Lares District
it
is
the mountainous area south of the Lares Road; that
is, the area making
78
.
SCIENTIFIC SURVEY OF PORTO RICO
up
the southern
half
of
the
district.
The maximum
elevation
{2i)0(f
feet)
is
in the southeast corner of this
area, and the
relief here
is 500
to 700 feet.
The Coastal Plain comprises the area north of the Lares Road.
It
is
a plateau in a youthful or submature stage of dissection.
From a maxi-
mum
elevation
of about 1500
feet
(east
of Lares )^ the plateau surface
slopes very gradually to the north
coast, termiiuiting there
in sea
cliffs
50 to 100 feet high.
The rocks are Oligocene limestones with some basal
shale beds, the entire series lying nearly horizontal, or with a slight dip
to
the north and
northwest.
These
Oligocene
formations
overlap
the
mountainous oldland as far as the Lares Eoad, where they culminate in
a more or less distinct cuesta, in places more than 300 feet high.
This
cuesta marks the boundary between the Coastal Plain and Mountainous
Oldland provinces, and
its position can readily be seen on the geologic
map.
The Playa
Plains
are
the
nearly
flat
alluvial
plains
at
or near
sea
level, occurring along the coast at the mouths of the
rivers.
They
are
especially large on the west coast of the Lares
District^
as for example,
the
Culebrinas and Ahasco
Playas.
Smaller
ones
occur on
the
north
coast of the
district,
as for example, the Guajataca and Camuy Playas.
The Playa Plains
(locally known as Playas)
are of comparatively recent
origin, and are found only along the coast.
Some occur adjacent to the
mountainous oldland
; others adjacent
to the coastal
plain.
The Complex Mountainous Oldland
This province
in the Lares
District, includes practically
all the area
south
of
tlie Lares Eoad
(see map).
The
relief
is
very
considerable
throughout
the
greater
part,
especially
in
the
southeast
corner
of
the
district.
The central mountain range of Porto Rico, known as the Cor-
dillera Central, can be traced westward across the island to the vicinity
of Adjuntas.
West
of Adjuntas
it
divides
into
a
southern
range and
a
northern
range.
The
southern
range
extends
along
the
southern
border
of the Lares
District, through Maricao and Consumo,
to Maya-
guez.
The northern range passes south of Lares, where
it
is indistinct,
to Atalaya Peak, north of Afiasco.
Northwest of Point Jiguero,
it cou-
tinues as a distinct submerged range, of which the highest summit forms
Desecheo Island, 15 miles from the west coast at Rincon.
The Mountainous
Oldland
in
the Lares
District
is
drained
by
two
master
streams,
the
Rio
iViiasco-Rio
Blanco,
and
the
Rio
Culebrinas.
These two rivers flow west and northwest through the area to the west
HUBBARD, GEOLOGY OF THE LARES DISTRICT
79
coast^ following in a general way^ the rock structure.
The Eio Anasco-
Eio Blanco system forms the parting valley between the north and south
branches of the Cordillera Central.
The Eio Culebrinas forms the part-
ing valley
(or inner lowland) between the oldland on the south, and the
overlapping Tertiary formations of the coastal plain on the north.
The Mountainous Oldland
is maturely
dissected.
All of the streams
are
in the stage of youth, and are characterized by narrow,
steep-sided
valleys, and numerous
falls and rapids.
As compared with the playas,
or lowlands of the coast, the climate of these interior mountains
is
cool
and humid.
Eains are of almost daily occurrence, but are typically of
short duration.
The moimtain
slopes, almost everywhere developed on
clay or
laterite, are exceedingly
steep, and
as a rule,
are covered with
forest
trees.
All of the
trees
are second growth,
utilized
as shade
for
coffee.
Thus what appears as wild forest land,
is in reality highly
cul-
tivated coffee land.
Travel in
this mountainous area
is very
difficult.
Automobile
roads
are rare, and the native cart roads (caminos)
are often impassable after
showers.
Grades
of 30°
or more are commonly met with in travelling
these roads, and in the areas of greatest
relief,
coffee, bananas, and
all
other
products must
be
transported
by
pack
animals.
Some
of
the
richest
coffee
districts of Porto Eico are in the most
inaccessible parts
of the mountains.
THE PENEPLANE SURFACE
The
summits
of
the
Cordillera
Central mark
the
remnants
of
a
formerly
continuous
surface
of moderate
relief
(Fig.
24).
This
old
surface
is the upper peneplane described by Lobeck.
Above the pene-
plane surface rise a few scattered monadnocks of quartz diorite, or other
relatively
resistant rock
(Fig.
1).
The peneplane has been maturely
dissected
in
the
Lares
District,
and
is
not
a
striking
feature
except
where viewed from certain points of advantage.
In the southeast corner
of the
district, where
it has an elevation of 1700 to 1900
feet,
it slopes
gradually to the west.
Near the west coast it
is preserved only on por-
tions of the Atalaya Eange north of Anasco, and possibly on the Mesa at
Mayaguez.
In central and eastern Porto Eico, Lobeck has distinguished
a lower peneplane, marking a second erosion
cycle.
No
traces
of
this
lower peneplane are found in the Lares District, probably because
it
is
buried here by the overlapping Tertiary formations.
The upper peneplane was formed some time
after
the
close
of
the
Cretaceous Period and before
the beginning
of
the
Oligocene
Period.
This
is proven by the fact that
late Cretaceous formations make up
a
gl)
SCfE.XTIFie SSUJIVKY OF POKTO RJVO
portion
of
tbo kMed
Ohh^r
Scrit'H
rock?
on
which
tlie
poiiephiiu'
k
(levelopw:!, and
arc the jinm^m
fornuitioiis known
to exkt
in the Older
Beries.'
That
ilu! peneplaLiatioii
ttKjk
plaeo before the
Oligoeeiie
Pi>ri<HJ
ir^ shown
by
the
fact
that
tlic
earliest marine
formation.^
doposite-l on
the
ptmepIajHMJ area are of middle Oligoeene age. M
previously
nr.Uid,
Lobeek has shown the e.xisionet* of two penoplanes in Forto
Jiieo.
Jiotli
of tbew most
have
been^ made during
tlie
interval
Iw'twecm
llie
close
of the Cretaceous
PericKJ and the
Ix'ginniiig of
midrlle
Oligoeene
time.
Ilie most probable date for the upper peneplane h the Palooe<nie Ferin,!
;
for
tlio
lower
peneplane,
the
Eocene
Period.
Tlw.
fornuUi<)M
ol*
the
view
lookltii
.^fiiricii<>
Kiwd.
lower pen(?plane seems
to
liave
Iteon
interrupted^
l)y
an,
uplilt, initiating
a
ihir<l
eyel{>
uf
erosion, and
resulting
in
the
dissection
of
b'oth
ptna-
planes.
This
is
sliown
by
tlie
fact
that
the
basal
formations
of
the
Tertiary Series
lie upon an extremely irregular surface,
in, some
])laces
filling
old
valleys
in
the
oldlaud
snrraee.
From
their
locutiou,
these
hurit'd
valleys seem
to
Inive been
cut
in
tin; lower
peneplan,e.
Coarse
gra\i:!ls occiipyin,g one of tlicse buried valksys near MotMi, ami immediately
underlying
tlic basal uurrine Cligoeene
fornmtions in
this
locality,
p<sint
to
tb.o' existence
of youtb,rnl
streams
in
this
.region
Just
preceding
the
in,vasion
of
the sea
in middle Oligoeene time.
Thus
the
ttdrd
erosion
cA^ele belomjs to
th,c
late
Flocnvne or early
Oligoeeiu^ .Periods, or possildy
HUBBARD, GEOLOGY OF THE LARES DISTRICT
gl
both.
This cycle had apparently reached a stage of late youth or early
maturity before it was interrupted by tlie middle Oligocene submergence.
The
following
are
the
important
physiographic
events
which
took
place during
the time
interval between
the
folding
of
the
Cretaceous
formations
of Porto
Rico
and
the
deposition
of
the
Tertiary
coastal
plain strata:
1.
First
erosion
cycle,
ending
in
the
formation
of
the upper
pene-
plane.
(Late Cretaceous to end of the Paleocene.)
2.
Uplift, with dissection
of the upper peneplane, and the formation
of a second or lower peneplane.
(Eocene.)
3.
Uplift, with dissection
of the lower peneplane and further
dissec-
tion of the upper peneplane.
(Late Eocene
tcf early
Oligocene.)
4.
Subsidence,
resulting
in
partial
submergence
of
the
island,
and
interruption
of
the
third
erosion
cycle.
(Middle
Oligocene
to
early
Miocene.)
The geological dates given in this outline are to be considered as the
probable approximate
dates
of
these
events.
The
date
of
the
close
of
the third erosion cycle, however,
is based on good
strati graphic evideiice.
At the close of the Oligocene Period, tbe entire island was uplifted, the
amount of vertical movement being differential, but reaching a nuiximum
of 1500 feet in the eastern part of the Lares
District.
I'hus erosion
of
the present cycle was initiated in early Miocene
tinu^, and has coutinued
to the
present.
Those
portions
of
the
interior
uu)Ujitains which were
not subnu^rged
in
Oligocene time
liave been
subjected
to
erosion
since
the
close
of
the
Cretaceous
Period.
Other
])ortions
nearer
the
coast
have been stripped of some
of
tlieir mantle
of Oligocene
strata during
the present cycle.
This
is true
of most of the area drained by the Pio
Culebrinas and
its tributaries.
Drainage Features
All the streams of the mountainous oldland are in the stage of youth.
Falls and rapids occur in abundance
in streams of
all
sizes.
The ouly
outcrops of fresh, unweathered rock in the oldland area are found in the
stream channels.
This, in a region where rock decay
is extremely rapid,
is
a good
indication
that
the
rivers
are
still
actively
deepening
their
valleys.
Eiver terraces occur in many places along the Pio Ahasco, Rio Blanco,
and Rio
Culebrinas
valleys.
None
of theme
are
continuous, and they
occur
at
all elevations above river
level, from 10
to 100
feet.
Most of
them are built of river gravel; some are rock cut.
Li most places they
82
SCIENTIFW SURVEY OF PORTO RICO
are obscured by forest trees which cover the valley walls^ and their true
nature
is therefore not evident.
A special feature of the drainage of the oldland area
is the presence
of hanging
valleys where
small
tributaries
enter
the
valleys
of trunk
streams, such
as the Eio Blanco.
In the upper Rio Blanco
valley
in
the
southeast
part
of
the
Lares
District,
hanging
valleys
are
very
numerous.
The
small
streams
enter
the
deep, narrow
valley
of
the
main stream with rapids and in many cases falls of considerable height.
The same feature may be seen in the upper Rio Culebrinas valley,
and,,
in
fact, seems
to be the
rule wherever small
tributaries
enter
a trunk
stream of large volume.
Any
attempt
at
interpretation
of
the
origin
of
the
streams
of
the
mountainous oldland
is made difficult because the relationship of stream
to
rock
structure
is
in most
places
not
apparent,
and
the
structure,
where not obscured by
soil and vegetation,
is as a rule
so complex that
a long and careful study would be required to unravel
it.
This
state-
ment applies
especially
to the southeastern part
of the Lares
District.
A glance at the map will show that in the western part of the oldland
area, the main streams
follow the
strike
of the
Cretaceous formations
more
or
less
closely.
Such
streams
are developed
in
belts
of the
less
resistant
rocks,
especially
tuffs and
shales, and should be
classified
as
subsequent streams.
Examples
of these are the Rio
Casey, Rio
Cafias,
Rio
Santiago, Rio Grande, Rio
Culebrinas
(in
part), and Rio Blanco
(in part).
The Rio Aiiasco-Rio Blanco
is a stream of complex origin, and being
the
largest
stream
in
the
district,
deserves
further
description.
The
upper portion, known as the Rio Blanco, follows the structure where
it
leaves the areas
of massive igneous rock and
enters
belts
of
stratified
rock.
In these portions,
it may be regarded as subsequent.
The lower
portion, known as the Rio Aiiasco, cuts across the strike of the forma-
tions without regard to the relative resistance
of the different types of
rock.
That part of the mountainous oldland drained by the Rio Aiiasco
was probably submerged during the
Oligocene
Period.
If
so,
the
de-
posits formed at that time have been removed by post-Oligocene erosion,
and the area
is to be classed as a pseudo-oldland.
This theory
is sup-
ported by the fact the nearest outliers of Tertiary strata
(at Pt. Jiguero)
are of middle
Oligocene
age, indicating that some 2000
feet
of upper
Oligocene strata have been removed by erosion.
If this portion of the
area is indeed a pseudo-oldland, then the Rio Aiiasco is probably a super-
imposed stream.
Additional evidence favoring this view may be obtained
from
a study
of two
other streams
flowing
to the west
coast.
These
HUBBARD, GEOLOGY OF THE LARES DISTRICT
§3
are Calvaehe Creek and Pueblo Creek, both southeast of
llincon.
The
map shows that the upper portions of these streams follow the belts
of
weak
rock, and
are thus adjusted
to the
structure.
In each
case, the
stream
disregards
the rock
structure
in
the
lower
part
of
its
course,
exactly as the Eio Ahasco does.
As a whole, the mountainous oldland area in the Lares
District pre-
sents
two
contrasting
types
of
drainage
pattern
(see map).
In
the
eastern part, the pattern
is dendritic
; in the western part,
trellis.
The
explanation of this is apparent.
In the eastern half, the country rock
is
largely igneous intrusive bodies and massive tuffs and agglomerates.
In
the western
half,
stratified
rocks and interbedded
lava
flows and
sills
predominate, and the beds
are
nearly everywhere
folded and
tilted
at
high angles.
The influence of hard and soft beds on the drainage lines
is very marked.
The Elevated Coastal Plain
This
province
includes
the
area
between
the
Lares
Eoad
and
the
north
coast, and
a narrow
strip
along
the
west
coast from Aguadilla
to Point Jiguero.
The
Coastal Plain
is developed on a
belt of nearly
horizontal
Oligocene
strata,
mostly
limestone,
which
overlap
the
old-
land.
It has a topography wholly
distinct from the topography of the
complex mountainous
oldland
on
the
south.
Because
of
its
elevated
position
and
nearly
horizontal
strata,
the
coastal
plain
belt may
be
classed
as
a plateau.
It
is marked
off from 'the oldland by
a more or
less
distinct
cuesta, which
faces south and overlooks an inner-lowland.
The inner4owland
is developed on
the
Older
Series
rocks, and
in
the
Lares District, owes
its existence to the erosive action of the Rio Cule-
brinas and
its tributaries.
From the summit of the cuesta, which marks the highest elevation of
the
Tertiary
coastal
plain
strata,
the
plateau
surface
slopes
seaward
(north
and
northwest)
at
an
average
angle
of
less
than
1°.
This
plateau surface
is in a youthful or submature stage of dissection.
Only
two master consequent streams cross the belt, from the oldland area to
the
north
coast.
These
are
the
Rio
Guajataca and
the
Rio Camuy.
They have cut narrow canyons
in
the limestone
to
a maximum depth
of more than 300
feet.
River
erosion, however, has not been the only
destructive agency at work in this limestone area.
Surface solution and
underground solution have been exceedingly
active, and have produced
a
peculiar type
of Karst topography,
characterized
by
sink
holes and
conical mounds
or
hills
of
limestone known
as pepinos
(Spanish
for
cucumbers),
or haystack
hills.
This sink-hole
pepino
hill topography
S4
.vC/f^lV/'iF/C HUMTEY OF FOIfTO EfVO
k m niggfd
t'h<U
travel
across
it
is
ponsibk'
only
by
a. few
I'avorahle
routes.
Thus,
wliiie tbc
jyiateaii
is in a yontlifol
ste^yv of
disi^er'tion. as
far
as stream erosion gues,
tlie topograj>hy
in certain
belts
is
as rough
a,s tliat of
a. maturely
<lis.se(-tc'(l country, because of the a(;tivity of
uiulcr-
grouud soluriou.
Th(; pepino
liilk
are the uiost inten»stiug and
uiriqiu;
feai.ure of
th,e
eoaslal
plain.
Jllustratioos
o1'
theju
are shown
in
Jigures 28
aiui
'ii),
and
their
distribiitiou
in
cast-west
belts
across
tlu;
plateau
surface
is
brought out on the geologic map.
Tliey
.have been
sliowu on this map
by
hiiehures, because
to contour
theui
would
be an
cud less
task,
oord:our iid^unul of 100 hot would fail to bring out the extreuudy
i
topograpliy
ol; tliese belts of
liills.
As may be seen from
the
ilJustrations, these
hills are roughly
(
or inoujul-shaped.
Jji
size, they
range
fr()m small mo'uiuls
l(»ss
t!'
feet high, to liiJIs at least ;IO0 fe«t liigh.
i:b,ey are, wluu-e best dev(
cdosely crowded, the
intervening spaces being
o('<;upied
by
sink-Iu
elongate
or irregular
pattern.
In
these;
belts
of
p>cflno
hills,
oi
often not find
level spaces large euougti
to
pitch
a
tent on.
All
and cart roads iiuiander around the steep sides of
tlie
hills, avoid ii
onieal
.loped,
dcs of
w
can
trails
nc tlie
MVBBAEIK (SEOLOaV OF THE LAMES IJliSTKICT
So
Hin.k^li(>l(jH.
TJie
traiL< are
(luiig(!r()ii.<
io
tra.vcl
aStoT
eliower.<. when
the
rcsidiLul clay
.^oil is iruule extremely
sli])}M'ry.
Soil, however,
h. not thick
or
widch|tre;el
in these areas, the
oiiteroj).s and
talus
ot*
wliiti-
liiiis'stoue
eovering
luiieli
(if the Kuiiace.
Ill
vimviiig these
hills from a
disiafict', ur from the siiinmii of
oin- of
tlieii),
it h noticeable tliat the 8nmmiis
(rf
all the
hii^iier hills have a
lair
aeeordaiiee
in
Jevelj presumably
mui'kiiig the
plateau
snrt'aee
ii>
it was
before being cut up into
tliis rough topography
l>y inidcrgroirnd
Hnlnti<iii.
(hi the plateau surfaee, adjaeeut
to
tlie
belrs^ of pepifio
hills
tliere are
belts
of
relatively smooth,
urujissectt'd
eoinrtry.
Typieal
views
of
tliis
type of topography arc; shown in figure 2T.
The prairie belts are covered
witli black or red residual clay soils, excellent tor growijig sugai" cane and
tobacco.
Wlu>re not cultivated, they nmke good, grazing land.
Idicy are
naturally
grass-covered,
anel the
trees afe
wirlely
seattcjred, and
consist
cfii(>ny of the Koyal Palm.
The smooth or rolling country
is broken here
and there by canyfms, siuk-holes, and low cuestas which mark
tiie outcrop
o( relatively resistani; limestone tieds dipping
li" or 4°
to the north.
The
principal
cuesta
occurs
aloug
the Lares
lload,
overlooking
the
3(3
SCIENTIFIC SURVEY OF PORTO RICO
inner-lowland,
as
already
noted.
It marks
the
southern
limit
of
the
Tertiary
coastal
plain
strata which
overlap
the complex mountainous
oldland.
Figures 25 and 26 show the variations
in the topography
of
this cuesta.
Where the basal shale beds of the Tertiary Series are capped
by
resistant limestone,
the
cuesta
is prominent and not maturely
dis-
sected.
Where
only
Tertiary
shale
is
present,
however,
the
cuesta
is
so thoroughly dissected by the tributaries of the Kio Culebrinas that it is
not
a prominent topographic
feature.
The inner-lowland
is developed
throughout on shales, tuffs, and other rocks of the Older Series, which in
general,
are more
easily
eroded than
the
Tertiary
limestone
forming
the summit of most of the cuesta.
At some points along the cuesta, the
basal shale of the Tertiary Series
is absent, and the higher horizons of
limestone
lie directly on half buried spurs of the rugged oldland surface.
At such places, there is no inner-lowland, and the cuesta is low and indis-
tinct.
Other
cuestas,
of minor
size and
importance,
occur
in
various
parts of the plateau surface north of the main cuesta.
They occur where
"resistant reef limestones alternate with soft, chalky or argillaceous beds.
The regional dip
is north or northwest at low angles; thus
all of these
cuestas face south or southeast.
THE COASTAL PLAIN BELTS
The
belted
character
of
the
coastal
plain
has
been
referred
to
in
describing the belts of pepino hills, and it
is brought out on the geologic
map.
Each belt owes
its topographic characteristics to the rock forma-
tion on which
it
is developed, hence the boundaries of the different belts
are almost the same as the boundaries
of the
different Tertiary forma-
tions.
K description of the Tertiary formations has already been given
and the topographic expression of each was summarized in Table 4.
1'he
belts,
named
in
stratigraphic
order,
from
the
lowest
to
the
highest, are
1. The Lares Pepino Belt.
The belt of pepino
hills developed in the
Lares limestone, on the plateau surface north of the cuesta in the eastern
half of the Lares District.
2. The
Cibao
Prairie
Belt.
Adjacent
to,
and
north
of
the
Lares
Pepino Belt.
Developed on the Cibao limestone.
3. The
Los
Puertos
Pepino
Belt.
North
of
the
Cibao
Belt,
and
developed on the Los Puertos limestone.
4. The
Quebradillas
Plateau
Belt.
Developed on
the
Quebradillas
limestone.
Extends from
the Los
Puertos
Pepino
Belt
to
the
north
coast.
IIUBBAIilK (JEOLOaY OF THE LARES DlHTh'lCT
S7
To
()l)iaiii a clear eoiieeptioii of the relation.<liip of thci^e boltn, and
tlie
obvious
reusoii
for the
belted ehaxaeter of
tlie coastal
plain,
the
readtu-
is referred
to
tlie geologie cross-sectious
(Plate
I), showing the north-
tfontli profiles across these topographic belts.
The Lm-es Pcpino Belt.
Tlie bent development of thJs l)elt i? the area
ni>rth of
tlie
town,
ot
liares.
From,
the Guajataea Eiver
eastwar<l
be-
yond
tlie east border of the Lares District, the })lateau surface jiwt
biH:k
of the cuesta
is cluiraeterizcd by high and densely crowded pepino
bills.
These Idlls are formed in the pure white ami pink T.ares limestone, both
of the nnissire and tlnndji'dded types.
West of the Guajataea lliver, the
Lures fornnition grades from pure limestone into
soft, chalky and argil-
laceous limestones, and finally into
slniles and gravel
be^ls.
It
is
tlu'rc-
fore signitlcant to note that the pepino hills do not exten.d very far west
of the Ginijataea Kiver in the Lares Belt.
Thfi Cilmo Prairiti BeM.
Tliis
helfc
is a relatively
flat,
rolling, grass-
covered prairie, situated between the two l)elts of pepino h'M^
(see njai)),
and
at
a
slightly lower elevation tlian the adjacent pepino
hill country
on eitlKiT side.
"The Cibao limestone
is
clrielly a soft, cluilky roek^^wntli
intercalated
beds
of hard
limestone, winch
form
low
cuestas.
Figur<!
21 shows typical views of the topography of this
belt.
,SH
^f'lEXTIFlC SFRVEY OF f'OKTO KIVO
The Lijs Fufiios I'l'inno Belt.
This belt
is essentially
like the Lares
J^M'lt, but ihe liep'uio
hills are larger, and the to|)o<i;Taphy cerresptmdiiig'ly
.more
riig<ied and
difficult
to traverse.
The
Lets
l^iiertos Belt
is
inueli
more
eoiitiriiioits
eiisf
aiid
west
ttuoi
is
ilie
l«ires
Belt.
The
striking
eont.rast
bi'twetsii
this
iiepitio
liill
topography
and
th(^
Cibao
|.»rrtirie
tcrp(»grfi]»by is shown in figures 28 and W.
The
QvehrcuMllas Plakmi
Bell.
ThJs
belt
is
for
t\w,
most
part
a
plateau
in
a
yuuthfiil
stage
of
dissection,
sloping
gradiially
seaward,
and
terii'iiriated bv sea
cliffs along most of the coast
line.
The surface
is
reniarkat)ly
flnl,
especially
in
the
northwest
corner
of
the
dii-^trict.
.Abo\'e
ibis surface
rise a few
hills of the pepino
tYf)(>, grouped
iriHHrn-
larly,
or
in
long,
ridgc^like
cluiins some
of
whi<;li
are quite cemtimious
east and west.
8i,nk holes are eoimoon, but not as large or numerous as
those of the Cibao
prairie.
The coastal
plain
is
traver.-ed by two master consequent streams,
t'ae
Ki(i (luajataca and
liio Camay, each of which has
its headwaters in
tlu;
(Vklland
area
to
the
south.
With
the
exce})tioii
of
these two
streams,
which
cut deep
canyons
(Fig.
30)
tli rough
tlie
limestone
belt,
about
HUBBARD. (iEOhOGY OF THE L/IJIHS DrSTEIOT
89
niiio-tonths of
hII
tlic drainage
is
KobtCTriiiieHii.
Kvcii
fiic Camuy takes
lu
a
subterraiieari
eourBe
in
ernsHing
the
Cibrto
l)clt
(Fig.
Ml).
In
jiiaii}'
plac(K<, partJciilarly
in the
(1l)a(i
priiirie, Ihe low
riijnl)le of iiiider^
gTOund rivets' can
bc^' Iieard: in otlier places
tJicy are seen in t\w l.olloms
of
llie
larg-C!
sink4u:des, where
tlie water
eoiiu'S
briefiv
to
view,
boiling
a> tiiuiigh in Bomt giant eaiildroii.
After a t-ontiiiuouB
2-1 tionrs
ol' rain,
it^
w'.rn iioUhI
tliat tbere wag
i)raetically
lut surface rim-olf on
tbe
t^Xibao
prairi(», not even the smallest rivnlet bein,g in eviden(re.
The rain waiter
.bad
e.^t:apiHl almost immediately
into
tlie undergToiind
ebiinmds.
It
is possible thai the
JJio (Inajataea and
JJio l^anuiy were at one Tin»e
biriridy
subterranean,
in
tbeir
coursi's
tlinnigli
tbe
limestones, and
tbat
the fH-esent canyons liave Iseen profhieed
in jnaiiy phices
Ity tnuing of
tlie
surfaee.
Tbe
lollowing consideratitrns snpport
this theory;
1. Most of the present drainage line? arc siibterrantian.
2.
Sinkdiok'H
are
evithaitly forming
at
tbe
present
tim(>
iibove
these
subterranean ebiinnels. and given time enongh, should
resnlt
in tbo
e<>in-
plete caving in of the rock overlying such dnuniels.
3.
Lar^c unisses
nt
lini(»stone,
nniny
(»f them
nnire
than
KKi
feet
in
diameter, oeeur in
the Eio Cainuy canyon, nnn'c or
hi'ss obstrueting tbe
stream.
Tliey
are
espe(-ially
nnmennis
in
tbat
part
of
the
canyon
adjacent to
tlie snijterranean
route o[
llie
river.
They
imlieate tbat
at
<)()
SCII-:M'JFI€ f(UR]'EY OF PORTO RICO
one tiine, a larger
portit^ni of the Rio C'amuy was subtcrraiioan, and that
the |}reseiit siil)t(>iTaiiean course is io process of heiiig de^t^()yed.
Tlie future drainage hi'story of the eoastal plain
will very probably be
an opening up
of underground
water
courses
by
caving,
until
surface
drainage of i\w area
is (sventnally established.
OJilGI-N OK Tl'
EPJNTO HILLS
The
tof)Ogra.pliy of these
hills
lias been described, but
a
discnjssioii of
ilieir origin has been
left
for tlie last,
b(.'eansc
it
is a siibjeet (dosely con-
nected
with
the draining of the coastal
plain.
It
is
believed
that
the
foHowing
U'bservntions
are
of
critical
iniijortaiice
in
formulating
any
theory of tlic origin ot this type of toijography:
1.
1lie
C'ihao and
(|uebra(lillas limestone
belts do
not have
a
typieal
development of
pcprtio
hills.
Likewise, these
hills do not occur
in
the
liarcs formation in
the western part of the Lares District.
In aiudjzing
these associations,
it
is evident
tliat
(pertain
tyi^es of
formt»ti(ins are not
favoral)k'
to
their
development.
These
types
are
(«)
shales,
marls,
argillaceous
or impure
limestones,
(h)
eoniinuoos
linie.<tone
strata
of
the
hard,
fine-grained,
flinty
type,
cliaracteristic
of
the
Qnebradillas
fornurtion,.
HUBBARD, (JEOIAXtY OF THE LABES DIHT'ltlCT
9i;
2. In the most arid portion of the I'onstal pkiu, the northwest corner,
there are no pepino
hJJ.ls.
Thi.« k
illustrated by the even
plateaiitf back
of Point BoriiMintm.
'riie more arid the climate, the
less the aincnini of
siirl'aee sol'itir^o wbirh ha-
iaki^n
nlnrn.
^M.;
:;1.
//'.
The
iiv«r
ll<>«s
tliv->ij:;b
a
.lo-p
>-i!u;,..ii
wall, and there takes to a stibterranean
clii
belt.
Tlic "bUie
trnle"
Is seen at
the estn
in
tlie foregrownd.
in passing tlirough
the filiao
limestim«»
•iglit.
nearly
bJocked
by
the timber jam
3. The pepino
liills are not individual reefs, since
tticiv are not limited
to the massive
reef type of lime«totie.
Some of them
are developed
in
5,2
f^ClKXTIFIiJ HVRVKY OF PORTO HIOO
well stratifuMl lunostoijc, mul m meh vmc?,
llie mnw strata mm
Ih> traeocl
ill detail from one
hill to another
(¥ip:.
'.I'i).
4.
Fr.pino
liiJIs
tofjograpby
is
best
tiovelupwl where
the
lirrn'siono
i^
the
mo.-^t
I'iiveriious
; that
is,
in those
^nlle^te]lert which
have
.-utferecl
liu
inaximuDi amount of ?^ohitioii bv gj-niUHl watiT.
The hills are not
invari^
alily
fuiirxl, liowever, in
tlris type
ol' limestone formation.
5.
In. many
loealities
the
hills
sliow
a
north^Houth
linear
.uTouping-
with
intervoning
lanes
of
sink holes
(Fig'.
3;j).
Ttiis arrangement
sink
hnlt.'s ean
bo |>rodn<'od
in no other way than
by caving along m^h^
i.erranoaii drainage lines.
b.
C'arel'nl observation sliows that the hills have a tendeney to he steep-
est on
the side toward the
west.
This feature
is so noticeable
in plaees
as to
«(ivo a sawtooth effect to tbc sky
line
(Fig.
:tt).
Further observa-
tion shows that tlris asynnnetry has no rcdation
to skimping or tilting, no
reflation
to
difference
in
structure or
eonrposition
of
ttic
ro(;k, and
no
relalieni
to vegetation covering the
hills.
Ttie only apparent explanation
left
is that of
ditferential
weatbei'ing am!
solntion.
Tin;-
daily showers
u<-eur usmilly
in. ihe aftenn.on, when the
sun. has
been
shining on
the
bill^
rr«.)in the west, antl
he.u(!e while the rock on
the west
is at a higher
jniBBARD, Gf-JOLOGY OF THE LARES DISTINCT
m
toniitcratirre tlum
that on the
east
<»r
.shaded,
slides.
This
iiu;roasc>;s
tlw
rate ot solution, and results in
a more cavernous stnieture hidng devel-
oped
cm
the
west
siden.
Suxdi
dilferential
surfaee
>ohitioJ)
would
he
notjeealde only
mdiere
th,e proe(;ss
is extreiiielj
raj)id.
From the ahove ohservation«,
it
is (Mdieve*! that the iM'plNO hills are the
hvproduct
ot; exten.iive iiiiderground sulntiun and extremely ra|)id Hurfaee
solution, eombiiied with
the
other
neeessary
faetors,
lithologie and
<-li-
njatie, above dcseribed.
T.'nder these
eondiiions.
pi-fyino
hills might
he
<le\"eloped anywlif^re in
tlu:' tropical 2oin\
:
loul-iilij
n<,)ih fn
TiiK Playa
Pi.ain.s
Tlie term Flaya
h^ u.-^ed in the West Indies to deiu.te
tlu'
Jhii
oi- gently
s1oi)irig alluna]
plaiJis whi(,'h oe-enr along
lh,e i-oasb< of nearly all
(he large
islands at the mouths of
rivers.
Siieli playas are
in nurnv cases
sfx-cral
ndles wide, but none
is elevated more
tiniii a few
bn't above sea
level.
In
the
liares
District,
large
pbiyas
oeeu,r
at
the
nnuith
of
the
Hio
Afuiseo and at the mouth of the Kio (hilehrin,as.
There are- many smaller
om?s on
the
nortli and west
coasts.
The
playas,
or
playa
[jlairis
dilTer
ehielly
in
size and
outline,
anxl a deseri|)tion of the
largest
(the
.\nasco
Pbivat
will include all the features to he tnuml
in
ajiy of the others.
f>4
SVlEyTIFlC SUIf'VEY OF PORTO RICO
'riic
xiilosco
Plavii
occupies the mouth
of
a
drowned
TaJlev, and
is-
hoimdod on its inland sides hj the luuiintaiiis or Irilis of the oldlaiul.
In
gToiiud plaij, tlie playa
i.'^ faji-shapedj wliich is
fiic form of a typical (hdta
or alluvial
fan.
TJio surface in most places
is
srnool;li and apparently
hn'eJ, but lia]id-lc\-cJ measurements show a gradual slope to eeawaixl froju
tlie interior portion, where the (ilcvation
is SO to 30 feet above high tide
level.
Tlie eoastal margin is juarked by a broad sandy beach, sand dunes
back
of the
beach, and a zone of coeoarmt palms
back
of
the ridge
of
ihnie sajuh
Then;
is a narrow zoue back of the zone of pahus where
the-
(liUa-Isa'behi Komi
in
toregnmrn}.
pepino
hills
of
the
Los
Piiertos
lirne^
• illustrates
Use "saw-tooth" skyline, the steeper
si(l«s
land
stands
at
or
slightly below
sea
level,
with
small
patches
of
salt
marsh, hut no large
<tr coid;inuou8 lagoon.
The remainder of the playa
is planted
in sugar caoe.
The town of Ailasco
is located in
the intcirior
part of the playa, at an elevation of about 80 feet aliove sea level.
The Bio Ailasco, leaving the narrow flood plain between the mountains,
oast of Anasco, takes a meandering course
across the playa to the
sea.
In the interior part of the playa, the
river
is
intrenelied
10
to 15
feet
below the
playa
surface,
exposing and
actively
eroding
the
layers
of
unconsolidated
m,aterial of which
the playa
is
hiiilt.
Numerous gullies
have been erodcnl into adjac-ent parte? of the playa surface by small inter-
mittent tributaries of the main stream.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
95
The lower
(seaward)
portions of Anasco Playa are said to be flooded
by the river during periods of very high water, but this
is of rare occur-
rence.
On the north coast, however, small playas of intermittent streams,
as, for example, the playa of Los Cedros, are flooded more frequently and
more completely.
This
is because the river water
is ponded behind an
unbroken ridge
of dune sand
at the outer margin
of the
playa.
The
playas, while making up but a small part of Porto Eico, are very impor-
tant
economically,
since
they
are
the
best sugar
cane
districts
of
the
island, and the sites of nearly all the large sea-port towns.
FExVTUEES OF THE COAST LINE
Terraces
Terraces
were
first noted by Berkey
(1915)
and
classed
as
coastal
(marine) and river.
The marine terraces are prominent features in many
places along the coast line of the Lares District and on Desecheo Island.
They are more or
less
local
in
extent and occur
at
elevations ranging
from 2
or 3
feet above high tide
to about 200
feet.
Most of the lower
terraces carry ^^fossiF' beaches or sands and gravels of undoubted marine
origin.
The higher terraces, however, rarely show exposures of gravels,
and where such gravels occur, they are not always recognizable as marine
deposits.
Along
tlie automobile road west
of Camuy
are extensive
ex-
posures
of conglomerate, composed
entirely
of fragments
of
Oligocene
limestones, highly weathered and disintegrated.
Similar deposits occur
elsewhere along the north coast on the higher terraces.
No marine Pleis-
tocene or Eecent fossils are found in these conglomerates, but their ma-
rine origin is suggested by the fact that they are never found except near
the coast at altitudes up to about 200
feet.
On the coast of the Ponce
District, Mitchell (1922) has found coastal terraces carrying undoubted
marine
deposits.
Pleistocene
to Eecent
in
age, and ranging in
altitude
from
10
to 200
feet.
In comparing
the
terrace
elevations
listed
by
Mitchell with those recorded in the Lares District, it was found that the
elevations covering the stretch of coast from Camuy to Juana Diaz fall
into four rather distinct groups, indicating as many stages in the Pleis-
tocene-Eecent uplift of this portion of the island.
These stages may be
described as follows
1. The
Isabela
Stage.
Named from the well-preserved
terrace
(175
feet)
at Isabela on the north coast.
Other well marked terraces of this
stage were found near Camuy
(125
feet), Quebradillas
(160 feet),
Pt.
Jiguero
(165
feet), and by Mitchell near Guanica
(150
feet), Yauco
R(,UKNTIFH' i^UKVMY OF I'OHTO HIVO
(2<>0
feet),
?
(UJO
I'eet),
i
t,erniees on
\
Tlie averauv
Pleistocene.
ith of Yuiiw)
(150 feet), southwest
ol'
Oniiyaiiilla Harbor
1
w(>st of Police (U30 to
180 feet).
These are the liigliest
ieh
(letMjsits of uiuloiibted
iiuiriue origin have
Ijeeii
foioid.
evatioji of
tlie group
is probably
ehjsc
to
ir,t»
footj*
The
(J
bv
intchcll
show
tlu:it
the
ati'e
k
not
ohk^r
tliiiii
2.
Tlie Ciijjo Rojo Stage.
Named from the well luarkerl
tcrrat-e
f'oinnl
by ^Ptehell
ois C'abo Hojo
(75 feet).
Other turracH"* of this groirp wwe
"Tlio
fJiscrciwnc.v
in eh-vatlon
of
terraces
.eroupefl
togptlicr as
Ix'iMtisinc
to
tlip
same-
sdiev. may
be
I'xulnlned
in one or more of
tfiw followkiK wn.vs
:
:i.'
Errors
in
aiiproid
retullngs, by which
the
cle-s^arions were
«lf.teriuliKM].
2.
Errors of
Jiidss.iein
in
selt'ctiiig the
lop of
the
tei'race
for
moasureuioiit:.
Many of
these
terrsiees are not
level, but slope
1<> seaward.
iMirtiierniore, they have
hv-en
cxtT-n^
sivelv eroded by wave and
river
action.
Thus
tJie eroded remnant
of
tlie outer
marjrin
mislit Rive a
iniich
lower aneroid
reading than wonUl
a
remnant
i»t;
the
inland border
of
the same
terrace.
3.
Loeal warping, affecting
tlie terraces along
tlie
coast,
4.
Tlsc terraces grouped under any
single
stage,
as under the
Isahela
Stage
of
npllfr,
may
not
tieeessarily
huve
been
exactly
eonteinporaneous
lu
orisln.
Tims
during
the
time interval
in winch
all
iJie terraces of the Isahela Stage were formed,
there may have
been a
series of mimn-
nplifts.
total ins 50
t:eor.
HUBBARD, (JEOIJMIY OF THE LARE8 DISTBtOT
97
found iMsar Qiicbradillas
(:>0
to 100
feet),
l:»t. Jiguero
(45
to 70
feet),
and by ACitchell ^outJiweKt uf Mayti^iicz, at iho Icform Behool (50 feet),
Enseiiada (75 foot), Guanica (50 foct),
i-iiiaiiicij Light House (50 feet),
Ft. Broa
(1)5 to 100 ftsct), and soutliwo^t of (liuiyaiiilla Harbor (00 feet).
The.ic are ttie id^ghost ttirract't carryiiiir the typical indurated duiio sands
and beatdi cong-loriiorates known an
tlie San .Tuan
forniatiori
(Fig. 35).
3. The Upper Doseolieo 8ta.ge.
Xamcd from
rlie ('X('(dlent rock
terrafro
with
coiisolidatod
l)ea.cl)
gravel
(eloYatiof),
20
to 25
feet)
preserved
in
many places on Descfdieo Island
(Fig.
36).
Terraocs
of
ihi's stage on
Porto
liico were found
at
the old Port of (Juebradillas hetweeii (Jamny
I
iHllllKl
on.
twent-r-flve
feet)
and
rftDHMiiit.'ii of the
the
liigli
tidet
o
and
Qiiehradillas
(JO
U)
t5
feet), near
lsal)ela
(20
feet),
Pt. diguero
(10
to 30 feet), and
by Miteludl at Pt. Aguila
(25
feet), and Guanica
Light Hxnise
(10
feel;).
^lany of these
terraces,^ as on
Desetdieo Island
and at Pt. Jiguero .Light House, are (;arved in
liighl}^ tilted Glder
S(>ries
rocks, and their mariru; origin
is
indisf)u,tal)le.
•t. The Lower Desechco
Stage.
Kained from
the
well defined
h:)Wt;'r
roek terraces
(elevation, 3 U) 5 feet)
on, Deseeheo Island
(,Fig. ;jr),
Lvi-
dence on
the Porto Bico coast consists
chiefly in
tlie
exj)osures
of con-
solidated beach sand and gravel a few leet above liigli tide levtd at various
points along the
nxirth and
west
(;oasts of the Lares
.District.
On
the
beacli at Camuy, these consolidated beach deposits are made up in part
98
SCIENTIFIC SURVEY OF PORTO RICO
of calcareous
alga^^ marine worms^ and
corals^ apparently
still
in
situ,
but slightly above high tide level.
On the lower portions of the terrace
at the old Port of Quebradillas, many corals were found
in position
of
growth at elevations of from 3 to
6 feet above high tide level.
Similar
occurrences were found at higher elevations
in other
localities, but no-
where offering such unmistakable evidence of recent uplift as here.
The
Sa^" Juax PoRMATioisr
The name San Juan Formation was first applied by Berkey to the con-
solidated dune sands of Pleistocene or Recent .age found at many locali-
ties along the north coast, notably at San Juan.
The term has since been
extended to include the firmly consolidated elevated beach gravels, such
as those on Desecheo Island.
Both types are found along the coast in the
Lares
District.
As these
deposits have an important bearing upon the
question of Pleistocene and Recent changes in the coast line, a somewhat
detailed description of them will be required.
THE CONSOLIDATED DUNE SANDS
These deposits occur in three different forms
1. As a solid core, anchoring the modern unconsolidated dune ridges.
This can be seen at a number of places, notably
at Camuy, and at the
mouth of Los Cedras canyon.
Consolidation of the grains seems to com-
mence around the roots of palm trees (Fig. 38).
These dune ridges are
so thoroughly anchored by the
solid core and fixed along the rear slope
by cocoanut palms that migration
is apparently impossible.
2. As promontories,
reefs, or small islands a short distance from the
shore.
Most of these have the form of
spits.
A glance at any map
of
Porto Rico
will show the great number
of these
spits along the north
coast,
all
solidified,
all
elevated
well above
sea
level, and
all pointing
toward the west in the direction the
littoral currents move.
Excellent
illustrations in the Lares
District are Pehon Pt. near Camuy, Sardina
Pt. near Isabela, and Jacinto Pt. near Los Cedras canyon.
Fossil palm
roots
(molds)
are found
in
the upper portions
of
these
outcrops,
but
never occur below 4 or 5 feet above sea level.
3. Capping the
elevated beach
gravels.
The best illustration
of
this
type
is found at Pt. Jiguero Light House.
In this occurrence, molds of
cocoanut palms are well preserved
(Fig. 39).
In general, consolidated dune sands may be distinguished from
con-
solidated beach sand by the following criteria:
IIVBBdlll), GEOLOiJY OF THE LAUKS DimiiKJT
Oij
1. The
(liino
.sands
have
crus.s-beddiiig
(jipping
f-Jiaractori.stieallv
as
high as :i()% while tlio clips cf the bciich saiul he<hling are very inueli
le.^s.
2.
Tlie consolidated dime sands
iitn'er eontfiiji large peljbh^s.
The only
fossiJ.s are small
sliells wliich eould
be
eas^ily moved by wind.
3. The consolidated dune sands contain caleareons mohjs of palm roots
(Fig. 38).
4. The consolidated dnne sands contain
relHtively few
fossils; the con-
solidated beach sands
ha\x»
cxniasional
strata
nui<h> up
cd'
little
else but
fossil shells.
Jt
is very impurtant to
distinguish, between dime and
bcsacli sand in
i
mW^^"t::j^-X^i:..-.^.
Terrace of
t.lie Lower DeseelH-n
SSI
">f
the
T'pypr
Deseclie
necessarily show evidence
of
nplift.
since
the
process
of
solidification
(cementing wdtli CaCO,/)
takes place
at»o\-e and
l>elow sea
level.
Till-: COXSOLIDATI-:!) BMICII
DJ-M'OSITS
These consist of both
sajids and
gravels,
in.
all stages of cementation
occnrring from below
sea
level
to
elcva,tions
1'ar above
tlic
rcacli
of the
waves.
The elevated gravels are generally
strati fietl and
vei-y
fossil iter-
ons.
Thc'y differ m no
r(>spect from the gravels on the
presejif:
l)eael)cs.
The yame
is
J.rue
of
the
i>levated and
present day
Iteaeli
san<ls.
Both
sands and
gravels
are nnule
up
in
part
of material
derived from
the
100
SCIENTIFIC SURVEY OF PORTO RICO
Cretaceous
rocks, mixed with material from
the
Oligocene
limestones
and Pleistocene or Eecent shell fragments.
The rock is in some localities
so firmly cemented that it might readily be mistaken for basal sandstone
or conglomerate of the Tertiary series.
On Desecheo Island, guano is the
chief cementing material, but in other localities
it
is calcium carbonate.
EVIDENCE OF UPLIFT
Lobeck
(1922)
has advanced the theory that many of these elevated
gravels may have been deposited by storm waves, and hence are not evi-
dence of uplift of the coast.
There are two considerations which make
this theory doubtful
1. At the
old Port of
Quebradillas, on
the marine bench previously
described, there stands an
old
stone
structure
(Fig.
40)
in which
the
roof, cross-beams, and other wooden parts have long since turned to dust
and disappeared.
This old building is at least 100 years old.^^
It stands
upon the high inward portion of the elevated marine bench, at an eleva-
tion of about 12 feet above the sea level.
Firmly cemented to the
floor
of the surrounding rock terrace are gravels, fossiliferous sandstone, and
large
coral
heads
(msandra)
above
the
reach
of
the
average
waves.
Within
the
old
stone
building,
however,
none
of
these
materials
are
found.
Although the open doorway faces the
sea, only wind blown sand
has found entrance.
Furthermore, the building fails to show any injuries
which might have been caused by hurricane waves.
2. The beach gravels and fossiliferous beach sands are in many places
too well stratified and too well assorted to be the work of hurricane waves.
They show no differences from the present beach deposits at sea level and
below sea leveh
THE AGE OP THE SAN JUAN FORMATION
It has been pointed out that there are very few exposures of San Juan
formations
in
the
Lares
District on
terraces
higher
than
the Upper
Desecheo Stage.
Mitchell (1922) has shown that the highest fossiliferous
gravels and the highest outcrops of typical San Juan formation
(Cabo
Eojo Stage) contain fossils of Pleistocene or Eecent age, in any case not
older than Pleistocene.
In the Lares District, the writer made a collec-
tion of more than loOO specimens from the modern beach, the San Juan
formation
of
the Lower
Desecheo
Stage, and
of
the Upper Desecheo
Stage, representing a total of 25 localities on the coast of the Lares Dis-
10 Personal communication from
Sefior Ramon Cordova, Sub Commissioner of Agricul-
ture and Labor, Porto Rico.
'HrHHARTK (iHOLOar OF TJIE LARES DhSTh'ICT
JOl
triet mnl
I)e>e(;lu'o
Island.
Of
tlris
nuiteriiil, 103
npeeies and
varieties
were
iflentificd.,
conipriHiiig" moHiiHcs,
c-erals, echinoderms, and
enistacf^a.
The data thus gatliererl niav
bo snrnmarized as follcws:
mem,
JmiMki.
t palm
JoilietJdc'd
in
the
roek.
1.
Xiiuiher of genera
jdenlifie<l
in modern beaches was
5^>, and 47 in
San Jnaii formation.
2.
A'unibor of speeies and varieties
idcntifietl
in modern
Ijeaehijs was
!>2. and 50 in San Jiiaii formation.
a.
Xiind.ier of molliisctan
sp(H;ie.s not reeordiH] by Dall
(1901)
is
in
San Juan formation and 2
in modern beaebes.
103
t^ClHKTIFlC SURVEY OF PORTO RICO
4.
^X iiiTiber of «|MKi'es or varic'ticH
in
Haii
Jiinii forinatiuii of the
I'ppt'i*
Desi'clux) Staj^v whitth wiAre not
I'oiuh] on tho
riio^Juni
bcrtclios was
11.
5.
Xiirnl>crs of species or varieties in SaJi fjuaii formHtion of tho
fjinver
Disscelioo Stage which were not i'ouiKi on the ;n,UHlern
l)eachi;ti wan
4^.
G, NmntNT of inolluscaii species
in
the iiiuileni
heacrht'K which
Hceiii
to
nhow
nligiit mutational changes
w'lien
coinpared
with
s|)eeiinej).s
of
the
same speejes from
tlie San Juan foi'matioii
is
Ci.
T.
Other
faunal
diifereuees wh,ieh
CHOOot
be recorded
quantitatively.
(n)
Tlie marked difference in
faunal content of certain ehn-ated
beacli
gravels and the modem or present beach gravels
innnediafely
adjacent.
(b)
Some
of
the
most
abundant
molliiscan
species
loiind
on
the
nioicm
beaches
are
rare
in
the San Juan
formation,
and
vke
vemi.
Tin's
dilTnrence
is more prononn(!d
for the Upper
Desecheo
Stage
ihaii
for tho Ijower.
{(•)
(/crtain
corals, particularly Mmmdm eUi'osa (EH. and Sol.)
and
B-Ia'andm lahyrirdhiformu
(Linn)^^^ arc extremely abimdarit in
tlie San
.Jiian formation ami in situ on
tlie (Jevated. benclies; bnt on the adjacent
snbsea
flats,
where
Pariies
sp., Sidermlrma
nid'ums.
(J:'alla?).
and
« Bc'termuiatiuns of
c
HUBBARD, GEOLOGY OF THE LARES DISTRICT
103
Agaricia
sp. grow
in great
abiindance, Mwandra
is conspicuous
by
its
absence.
CONCLUSIONS
The faunal differences above noted are too numerous and persistent to
be accidental.
From a careful study of the fossils and from the various
data described above, the following statements seem highly probable
:
1. The deposits of the Upper Desecheo Stage are distinctly older than
those of the Lower Desecheo Stage, and both are older than the present
beach
deposits.
The higher
gravels,
therefore, cannot be
attributed
to
hurricane waves, without ignoring certain
facts
of
critical importance.
2. The following ages may be assigned
to the elevated beach deposits
of the different stages of the uplift
(a)
Isabela Stage—Early Pleistocene.
(h)
Cabo Eojo Stage—Late Pleistocene.
(c) Upper Desecheo Stage—Post Pleistocene.
(d)
Lower Desecheo
Stage—Eecent
(Historic?).
The Plata Deposits
The Playa Plains have already been
described.
The material under-
lying these plains
is
for the most part fine-grained alhivium,
de})osited
in horizontal and very regular, extensive
strata.
Typical exposures are
to be seen in many places along the lower courses of the JUo Culebriuas
and
Ilio Ahasco, where
these
streams
have
intrenched
tbeir
channels,
exposing
in some
places
(as due sonth
of Ahasco)
at
least
15
feet
of
strata.
Except
for
a few pebble
beds here and
there,
the material
is
very fine grained, consisting of carbonaceous chiy,
silt, and sandv loam.
The
fossils are chiefly fresh water or land
nu)lhiscs and plant remains,
but occasionally one may
fiiid beds carrying marine
shells
(Slro tubus,
Area, Mytilus, and others) and brackish water forms
(chiefly Nerifirm).
Near
the
coastal margins,
the number
of
intercalated marine
beds
is
greater.
Marine and land molluscs are found mixed in the same strata.
Old
fossil beaches
(San Juan formation)
are found almost
buried
by
Playa deposits near Camuy, at Columbus Monument, and at other })laces,
demonstrating that the Playas have been extended
withi]i
recent time.
This
feature might
conceivably
be
the
result
of
recent
uplift,
but
it
cannot be used
as one
of the
criteria.
No marine
terraces or
deposits
were found around the interior borders of the larger Playas of the west
coast
in
the
Lares
District.
The
smaller
Playas
(Los
Cedras
and
Guajataca in particular) show evidences of marine action
(caves)
in the
limestone cliffs which form their inland boundaries
,i^ig. 41).
HUBBARD, GEOLOGY OF THE LARES DISTRICT
105
A.
Plistocene.
Formation
of
the
highest
terraces during the
inter-
glacial epochs, and the submarine benches during the glacial epochs.
B.
Post-Pleistoeene.
1.
Eise of sea level and formation of the present playas.
2. A series of uplifts, affecting the western part of Porto Eico, initiated
possibly in early Pleistocene time, totaling at least 200 feet, and possibly
continuing at the present time.
In the earthquake of 1918, which wrecked so many towns on the west
coast of Porto Eico, the tremors travelled from west to east
(Cordova,
1918).
The probable source was somewhere in the submarine banks not
far
from
the
west
coast.
This
earthquake
is
of
special
significance
because
it may indicate that the movements which elevated the western
end of the island in Eecent time, are
still going on.
However, further
evidence
is needed to settle this question.
MIXEEAL EESOUECES
lliON
No
iron
ore
in
commercial
quantity
occurs
in
the
Lares
District.
Several small patches
of limonite
soil
Avith limoite concretions, derived
from serpentine,
are found south of Aguada.
The locations are shown
on the geologic map.
This ore is like that of the Mesa at Mayaguez and
is
of
the same
origin.
From
samples
obtained
in" the
area
south
of
Aguada, Fettke
(1918, p. 673)
determines the content of iron as 18.76
percent, which shows that the ore is too low grade to be used, even
if
it
occurred in large enough quantity.
Copper
Copper
is found at a
locality
21/^ miles south of Aguada in the area
of
volcanic
flows.
The
southern
portion
of
this
volcanic
area, shown
on the geologic map,
is largely made up of a dark scoriaceous, vesicular,
or amygdaloidal
augite
andesite.
It
doubtless
represents
a
succession
of surface flows, but these are too irregular to distinguish in the expo-
sures.
Locally the amygdaloidal
cavities
are
filled with
calcite, amor-
phous silica, and zeolites, but throughout the greater portion, the cavities,
where present, are empty.
In a small portion
of this volcanic area near
its southern limit, and
covering only a few
acres,
is the mineralized zone in which the copper
occurs.
The approximate location of the test pits and adit where
it
is
mined
is shown on
the map.
The
rock
at
this
particular
location
is
100
^SCIENTIFIV SUKVKY OF PORTO KIVO
eonsiiierably
faulted, and the
aiuygdaloiriai
eaYilios
are
entirely
tilled,
priiK'i pally with
cak-ite,
aiul some
zeolites.
The
etipper
eecurs
in
ami
adjacent
to
the crunh
ZAXiies,
wliieh
are
lU'ver
nnieh more than
1
Foot
wide, and
a\"era,u;e oidy 2 or 3
inclies.
'Flic richest vein, whieh has hdH-n
folhnvcd
J'or
a
,<h(>rt
disiauee
in^
the
ailit,
averages
r^i/^
iiiebeg
thick,
strikes
north
25"^'
west, and
dips
58 '"
to
ihc
northeast.
Otlier
veins
eiieounlercd
in
the
exeavation
range
I'rtrni
i/o
to 2
inetuis
thick,
strike
east and
wes^t, and
dip
at
angles
ot from
?0''
to
DO".
The
adit
has
penetrated the side of the small hill
for a
distaiiee of only 27
feet and
View
1 hM
It?
KO
Kea
cuvi.'
»<.
H
tJl
C'MJras y an
iritrri
to a ileia h
t>
two
erlg<^ at
i w
i la.va
dune
rMgp,
kIi.h
HNl
<Jiii'i!ig
r
The diiiH*
f
jKl
n
has
a
eross-seeti(jii
of
9 by
6
feet.
Besides
the
(uipper-bearing
veins,
many
minor
veiulehs
weri'
euconnt;ered,
carryi-iig
ealeite
ami
stilbite.
Tlw,
ore
emiHists
(d'
native
copper,
chaleoeitc,
malachite,
tetrahedrite,
and
ayarrite, of which
the
first
is hy far the most ahiiiidant.
It occurs
in small irregxilar masses in the veins, and
with, the nnihiebite and
ebal-
cotute, forms a matrix for the crushed fragments of wall roek.
Aziirite
is frequently found enenisting the
otlier copper minerals, hut never
in
any
great
quiintity.
Occasional amygdaloidal
cavities and
blow
holes
adjacent
to the mineralized
erusli
zones
are
filled
with
native
copper,
nVUHARJ), (ffjOIA)aY OF THE LAIiHS DlHTIilCT
107
iihually
iiiterruixod
witli
iiiirior
airioiiiits
oi'
the
oilier copper minerals.
Ojie ui tlvose large nuggets, almost cjilirelv of iiativo coppur, u sliown in
Fig.
12.
This
prcipcrtv;,
covering 160
acres, h owned by
Sefior Antonio
San-
eliez and
,ju(jge Luis Vadis of Agnada,
Seilor Poali of (Jatano and
l>r.
dinieirez oC Agnadilla.
'JMiese gentlemen Inivo rented adjoining land dor
exploration.
In
addilJon
to
the small
adit,
wlrieh
represent.s^^he
ehiid'
excavation on
llu! property;
several
lest
pit-? have been sunk
at
various
Fio.
42.
Nugget of iiatin
in
Ific localhm stmth of
Aff
found.
Altogether, alK/ut five Ions of ore have becjn ship|ied
to Agnada.
All work has been done by hand
labor, no niaelvinery of any kind having
been used.
In the opinion
ol' the writer, this prospcid will not warrant the distalla-
tiun of ebiborato maeliiiKsry or expendituR!
of
large sums of luoiHyy for
developing
and
exploitation.
While
tlus
ore
is
rich
and
the
veln.-i
nuruerons, the area involved seems
to be very limited
in
extent.
How-
ever,
tlie property will doubtless yiehl eoiisiderable profit
if worked on a
small
scale with
sinrple methods,
u.'^ing hand
bdjor and
tra^isportatioii
by
oxcarts.
TJic
topograj)liy
between
this
property
and
tluj town
ol'
Agnada on the Anunlcan
railroad,
is not excessively
hillj, and
the
dis-
ta,iicc not great.
Improvements
in
ttic present
cart, road could
1)0 nnide
108
SCIENTIFIC SURVEY OF PORTO RICO
at small expense.
It
is to be further recommended that more
test pits
be dug in this area, most of which is covered by clay soil.
Kaolin
Kaolin
of high grade of purity
occurs
at many
places
in the Older
Series, but especially along the road south of Lares^ near the Eio Blanco
(Fig.
2).
For the present, however, with the scarcity of fuel, distance
from railroad, and lack of any nearby market, the material will remain
of potential value only.
Brick Clay
Clay
is the most common type
soil
in the
district,
especially
in the
moimtainous
portions
developed on
the
Older
Series
rocks.
Most
of
these clays would require mixing with sand before using for brick manu-
facture.
Considerable red clay and black clay loam
is developed in the
Tertiary limestone
area north of
Lares,
particularly along the
belt
of
Cibao limestone.
Generally this material
is too high in organic matter
to
be
of any
value, and otherwise
it
is
altogether
inaccessible.
Along
the belt of Quebradillas limestone, particularly over the plateau west and
southwest of Isabela are many areas of red sandy clay which may some
time prove valuable for brick.
Some brick has been made at Lares, and
doubtless at other localities from time to time, chiefly for local use.
The
only place where brick
is now being made
is
in and around Mayaguez,
where, according to the reports, there are three plants, all of smaU size.
One of these was visited by the writer and the following notes regarding
it may be of interest: The material used
is taken from the small flood
plain
of the Yaguez Eiver,
close
to the plant.
The
section exposed
in
the
excavations
shows
a
dark
red
clay
overlain by
6
feet
of
lighter
colored reddish brown clay w4th a capping of 1 to 4 feet of river gravel.
The plant contains three compound ovens, a mixing pit, and drying racks
with wooden molds.
The mixer consists of a circular pit in which
is set
a
center-post.
On
this
post
as
pivot
is mounted an
horizontal
shaft,
counterweighted at one end and at the other hitched to a pair of oxen,
who operate on a circular path surrounding the pit.
• A large cart wheel,
mounted on the horizontal shaft ploughs through the clay and water in
the
pit and
serves
as
the mixing
device.
This wheel
revolves on
the
beam, and
is regulated
in position from center to periphery
of the
pit
by
a system
of ratchet and worm gears operated from
the end of the
shaft.
In the pit, with the clay and water,
is mixed sand from the river
channel to the extent of one cartload per 1000 bricks produced, or for
HUBBARD, GEOLOGY OF THE LARES DISTRICT
109
a single filling of the pit^ 12 cartloads of red clay (known locally as the
"black
clay^" because of
its
relatively dark
color),
12
cartloads
of the
reddish brown clay, and 6 loads of sand, with water to give proper con-
sistency.
It
takes
1
day
to
fill
the
pit,
2
days
for
the mixing, and
then the material
is dumped by hand
into the molds
(wooden frames
laid on planks), and
left
to
dry.
This
takes
8
days
in dry
weather.
Frames are then removed, and the bricks loaded into the ovens, requiring
two days to fill.
Fires are built during the third day, starting with wood
charcoal, and then adding coke
(one-inch size).
Two cartloads of coke
are said to be required for every 15,000
bricks.
The bricks
are baked
for 24 hours.
The shrinkage of the sun-dried brick was found
to be high, between
20 and 30 per cent by volume, with further very slight shrinkage after
firing.
The price obtained
(in 1916)
in the local market was $10 per
thousand.
The
quality
of
the
brick
is
very
poor, owing
to
the
poor
material used and lack of proper mixing.
The apparent success of this
enterprise, with
its more or
less primitive methods, would seem to war-
rant further development
of the brick industry, using more up-to-date
methods, and taking
a
full advantage
of the more
valuable
clays and
other materials occurring abundantly in this portion of the island.
Lime
Pure limestone for burning purposes may be obtained at so many easily
accessible points on the American
Railroad
along
the north and west
coasts from Camuy to Aguadilla that no specific localities need be men-
tioned.
The material as a whole, however, will not be found as good for
this purpose
as the limestone
farther
east from Camuy
to San Juan,
and on the south coast near Ponce.
Building Stone
No first-class building stone is to be found among the Cretaceous rocks
of the Lares District.
The lime-shales are too thin-bedded and fractured
to
obtain
slabs
of proper
size.
The
igneous
rocks
are
of
little
value
owing
to
the weathered
condition,
difficulties
in
quarrying,
and
high
content of ferro-magnesian minerals and oxides of iron.
The best build-
ing stones are to be found in the Tertiary limestones.
At various places
along the north
coast
in
the
Quebradillas
limestone,
hard,
flinty beds
alternate with chalky layers.
These hard strata can be
easily quarried
into large blocks.
In many places, however, the rock will be found too
massive and riddled with solution cavities to be of much value.
110
SCIENTIFIC SURVEY OF PORTO RICO
KoAD Metal
Eoad metal of
first-class type
is being quarried in many parts of the
Lares
District,
chiefly along the antomobile roads where they traverse
rocks
of the proper type.
On the Lares Eoad, andesite porphyries are
used in the neighborhood of Lares.
N^ear San Sebastian, the large coral
heads, imbedded
in
the
soft marl
of
the San
Sebastian
shale horizon
are accessible in cliffs along the road
; they are easily quarried, and make
an
excellent and
durable
material.
Farther
west,
around
Aguadilla,
the massive Los Puertos, and underlying limestones are quarried exten-
sively.
Xorthwest from Ahasco, the lime shales are quarried along the
road at several places.
These hard beds are easily broken to proper size
and the
material
is
of
lasting
quality.
On the Mayaguez-Las
Marias
Eoad the various andesitic rocks are employed, and
at a few places the
black pyritic shale, which
is quarried near Consumo.
This shale
is hard
and fresh, and apparently a good material, but the high content of pyrite,
and the depth to which this shale
is characteristically weathered, makes
it doubtful if
it will last any length of time as road ballast.
In general,
the Tertiary limestones produce the smoothest and best road beds of any
rock to be found in the district.
Where igneous rocks are used, a cover-
ing of this limestone would produce ideal conditions.
Lignite
At Lares, San Sebastian, west of Moca, and many other points along
the Lares Eoad, the basal shales (San Sebastian)
carry lenses of lignite
of no great extent or thickness.
They are seldom over six inches thick
at
a maximum, and
average much
less.
They
are extremely high
in
marcasite, and hence more or less completely oxidized where exposed at
the surface.
These coal lenses are valueless except occasionally for local
use.
An enterprising Lares blacksmith has dug out considerable quanti-
ties for use in his forge.
The frequency with which these lignite lenses
are exposed
in the San
Sebastian
shale along the Lares Eoad has
led
many of the local residents to believe that coal must occur in commercial
quantity.
This false impression has spread to other parts of the island,
where
people
will
often
refer
to
the
coal
of
the Lares-San
Sebastian
locality.
Such lenses of this material as occur would not repay exploita-
tion unless they were continuous over considerable distance.
That they
are not continuous can be seen from observation, and from a considera-
tion of the conditions of origin of the San Sebastian shale in which they
occur.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
HI
Guano
Bat guano, taken from caves in the Tertiary limestone
area,
is used
to some extent for
fertilizer by local farmers.
Bird guano
is found on
the Desecheo
Island,
chiefly
as a cement matrix
in the
elevated beach
gravels
(San Juan formation).
However,
it has
not
accumulated
as
abundantly
as
on Mona
Island,
where
it
is now
being
quarried
and
shipped to Porto Rico.
Oil
1^0 oil shales or surface indications of oil w^ere found in the Lares Dis-
trict.
The San Sebastian shale
is the only formation observed in which
hydrocarbons might originate, but lignite seems to be the only carbona-
ceous material present.
In view of this, there
is very
little to warrant
an exploration for
oil in this part of the island.
Nevertheless, the San
Sebastian formation is not essentially different from formations in Trin-
idad which carry bituminous material, and therefore the possibility of oil
in the Tertiary series of the Lares District must not be overlooked.
The
most promising location for test drilling is the area north of tlie Tertiary
cuesta between the meridians of CoUazo and San Sebastian, since
it
is in
this area that the San Sebastian shale attains its maximum ttrickness, and
greatest development of carbonaceous clays.
Summary
The
chief
assets
of the Lares
District
are the
varied
soils, and
the
agricultural products.
Important
studies
in
this
field
are being made
at the Agricultural Experiment
Station
at Mayaguez.
Of mineral
re-
sources,
clays, road metal, and natural
fertilizers
will prove
to be
the
most important.
Copper is the only metallic mineral which shows possi-
bility
of commercial importance.
Tlie
reported
presence
of
coal
beds
proved to be thin seams of lignite in the San Sebastian
shale, and are
of no value.
iN^o
oil shales Avere found, and there
is no good evidence
that oil exists in any of the rocks of the district.
BIBLIOGRAPHY
Alexander, W. A.
1902.
Porto
Rico,
its
climate
and
resources.
Bull.
Am.
Geog.
Soc,
XXXIV,
p. 401.
Allen,
J.
A.
1916.
An
extinct
octodont from
the
island
of Porto
Rico, W.
I.
Ann.
N. Y. Acad.
Sci., XXVII, pp.
17-22.
112
SCIENTIFIC SURVEY OF PORTO RICO
Anthony, H. E.
1916.
Prelimiiiury
report
on
fossil mammals
from
Porto
Rico.
Ami.
N. Y. Acad.
Sci., XXVII, pp.
l<K5-203.
Berkey, C. p.
1915.
Geological
reeoniioissance
of
Porto
Rico.
Ann.
N.
Y. Acad.
Sci.,
XXVI, pp.
1-70.
1919.
Introduction
to
the
geology
of
Porto
Rico.
Scientific
Survey
of
Porto Rico and the Virgin Islands,
I, part
1,
i)p. 11-29.
Brixton, N.
L.
1919.
History
of the
survey.
Scientific Survey
of
Porto
Rico and
the
Virgin Ishmds,
I,
i)t.
1, pp.
1-10.
Brown, A. P.
1913.
Notes on the geology
of the
island of
xVntigua.
Proc. Acad. Nat.
Sci.
Phila., LXV, pp. 598-610.
Cleve,
p. T.
1871.
Geology
of
the northern West
Indies.
Kungl.
Sven.
Vet.
Akad.
Handl., IX, No.
12.
1883.
Outline
of the
geology
of
the
northeastern West
India
Islands.
Ann. N. Y. Acad.
Sci.,
II, pp. 185-192.
Cordova, Ramon
G.
1918.
Earthquakes.
Bull. Dept. Agric. and Labor, San Juan, P. R., No.
G.
Crampton, H. E.
1910.
I»orto Rico.
Am. Mus. Journ., XVI, Jan.
Dale, W. H.
1898.
A table of the North American Tertiary horizons, correlated with
one another and with those of western Europe, ^vith annotations.
18th Ann. Rept.
J].
S. Geol.
Surv., part
2,
pp. 323-348.
Dale, W. IP, and Simpson,
C. T.
1901.
The mollusca of Porto Rico.
U.
S. Fish. Comm. Bull, for
1900,
vol.
1, pp. 351-524.
Dinwiddie, W.
1899.
Physical features of the island
(Puerto Rico).
Harper's Weekly,
XLIII,
p. 248, IMarch
11.
1899a. The
great
eaves
of
Puerto
Rico.
Harper's
Weekly,
XLIII,
p.
293, March 25.
Domenecii, M. K.
1890.
Mineral resources
of Porto
Rico.
Mines and
Minerals, XIX,
pp.
529-532.
Dorse V,
C, W.
1902.
Soil survey from Arecibo to Ponce, Porto Rico.
Report of Bureau
of Soils, U.
S. Dept. Agric, pp. 793-839.
Falconer,
J. D.
1902.
Evolution of the Antilles.
Scot. Geog. Mag., XVIII, pp. 309-370.
Fettke,
C.
R., and
IlLn'.BARD, B.
1918.
The limonite deposits of Mayaguez mesa, Porto Rico.
Bull. Amer.
Inst. Min. Engin., No.
135.
1920.
Geology
of
the Ilumacao
District,
Porto
Rico.
Scientific Survey
of Porto Rico and the Virgin Islands.
(MSS.)
FisKE, Amos K.
1899.
The West Indies.
G. P. I'utnam's Son, N. Y.
HUBBARD, GEOLOGY 01^ THE LARES DISTRICT
113
Gannett, Hknry.
1901.
(Jazeteer of I»orto Rico.
Bull. U.
S. Geol. Survey, No.
183.
Gregory,
J. W.
1895.
Contributions to the paleontology and physical geology of the West
Indies.
Quart. Jour.
Geol.
Soc. of London,
51, pp. 255-310.
GUPPY, R.
,T.
L.
1909.
Geological
connections
of
the
Caribbean
region.
Trans.
Canad.
Inst., pp. 373-391, January.
Hamilton
S. Harrert.
1900.
Note on some
of the ore
deposits of
Porto
Rico.
Eng. and Min.
Jour., LXXXVIII, p. 518, September.
Hill, Robert T.
1898.
Cuba and Torto Rico, with the other islands
of the West Indies.
New York.
1899.
The value of Porto Rico.
Forum, XXVII,
pp.
414-419, June.
18990^. Porto Rico.
Natl. Geog. Mag., X,
pp.
93-112.
1899&. The geology and physical geography of Jamaica.
Bull. Mus. Comp.
Zool., XXXIV.
1899a The forest conditions of Porto Rico.
Bull.
25,
U.
S.
Dept.
Agric.
1899f?. Mineral Resources of Porto Rico.
20th Ann. Rept. U.
S. Geol. Sur-
vey, part
6.
Hodge, E.
T.
1916.
Report of progress.
Ann. N.
Y. Acad.
Sci., XXVI, pp.
434-435.
1917.
Report of progress.
Ann. N. Y. Acad.
Sci., XXVII, pp. 277-278.
1920.
Geology
of
tlie Coamo
District,
Porto
Rico.
Scientific
Survey
of
Porto Rico and the A4rgin Islands,
I, part
2.
PIUBRARD,
P>.
1920.
The Tertiary formations
of Porto
Ri;-o.
Science,
LI,
pp.
395-396,
April
1().
1921.
The Tertiary molluscs of the Lares District, Porto Rico.
Scientific
Survey of Porto Rico and the Virgin Islands, III, part
2.
Jones, W.
F.
1918.
(kH)logical reconnaissance in
Haiti.
Journ.
Geol., XXVI,
No.
8.
LOBECK, A. K.
1919.
Five land features
of Porto Rico.
Natural History, XIX, pp.
522-
540.
1922.
The iniysiography of
IN)rto Rico.
Scientific Survey of Porto Rico
and the Virgin
Islands,
I, part
4.
Matthew, W. D.
1916.
New sirenian from
tlie Tertiary of Porto Rico.
Aim. N.
Y. Acad.
Sci., XVII,
pp.
23-29.
Maury,
C.
J.
1917.
Santo Domingo,
type
sections and
fossils.
Bull. Amer.
Paleont.,
V, Nos. 29 and 30.
1919.
On the correlation of Porto Rican Tertiary formations with other
antillean and mainland horizons.
Amer. Jour.
Sci., XLVIII, pp.
209-215.
1920.
Tertiary
mollusca
from
Porto
Rico.
Scientific
Survey
of
Porto
Rico and the Virgin
Islands,
III, part
1.
114
SCIENTIFIC BURV^Y OF PORTO RICO
Mitchell, O.
J.
1922.
Geology
of
the Ponce
District,
Porto
Rico.
Scientific
Survey
of
Porto Rico and the Virgin Ishmds,
I, part
3.
Newberry, J.
S.
1882.
Geology of the West
Indies.
Trans. N.
Y.
Acjid.
Sci.,
I, pp. 23-24.
NiTZE, H. C.
B.
1899.
Investigation of some mineral resources of I*orto Rico.
20tli Ann.
liei)t. U.
S. Geol.
Surv., part
il pp. 779-878.
Reeds,
C. A.
19K).
Report of progress.
Ann.
N.
Y. Acad.
Sci., XXVI,
pp. 435-438.
1917.
Report
of progress.
Ann. N.
Y. Acad.
Sci., XXVII, pp. 280-282.
1919.
New base map of Porto Rico.
Scientific Survey of I*orto Rico and
the Virgin Islands,
I, part
1, pp. 30-31.
Semmes,
I). R.
1910.
Report of progress.
Ann. N. Y. Acad.
Sci., XXVI,
pp. 433-434.
1917.
Report of progress.
Ann. N.
Y.
xicad.
Sci., XXVII, pp. 279-280.
1919.
Geology
of
tlie
San Juan
District,
l*orto
Rico.
Scientific Survey
of Porto Rico and the Virgin Islands,
I, part
1, pp. 33-100.
Stevpirs, W.
1895.
Zur Kentniss Puerto Ricos.
Mittheil. Geog.
Gesell., Hamburg, Heft.
11. pp. 217-230,
pi.
4.
Spencer, J. W,
1903.
On
tlie geological relationship of the volcanoes of the AVest
Indies.
Victoria
Inst. Jour.
Trans., XXXV, pp.
189-207.
1904.
Tlie Windward Islands of the West Indies.
Trans. Canadian Insti-
tute,
\'II,
pp. 351-371.
Vaigiian, T. Waylani).
1902.
Earliest
T(»rtiary
coral
reefs
in
Antilles and United
States
(Ab-
stract).
Science, XV, March 28.
190G.
Some
littoral and sublittoral physiographic features of the
A^irgin
Islands and northern lA^eward
Islands, and their bearing on the
coral reef problem.
Jour. Wash. Acad.
Sci., VI, No.
3, pp. 53-00.
1919.
Fossil
corals from
Central America,
Cuba, and
Porto
Rico, with
an
account
of
thc^ American
Tertiary,
I*leistocene, and
Recent
coral
reefs.
Bull.
U.
S.
Natl.
Mus.,
No.
103,
pp.
189-524.
Ii919(/. The
biologic
cliaracter and
geologii*
corrcdations
of
the
sedimen-
tary formations of Panama in their relation
to the geologic
his-
tory of Central America and the West
Indies.
Bull.
IT.
S. Natl.
Mus., No.
103, pp. 547-612.
Weather Rej^ort (Annual).
1914.
Climatologlcal
data,
Porto
Rico
Section
U.
S.
Dept.
of
Agric,
Weather
Bureau.
W^harton, W.
J.
L.
1894.
The physical condition of the ocean.
Geog.
Jour., IV,
Sept., p. 255.
Willis, Bailey.
1912.
Index
to
the
stratigraphy of North America.
Professional Paper
71, U.
S. Geol. Survey, pp. 13, 342,
348, 794, 801-803, 887.
HUBBARD, GEOLOGY OF THE LARES DISTRICT
115
Wilson, H. M.
1899.
Water resources in Porto Rico.
Water Supply Paper No.
32, U.
S.
Geol. Survey.
1899a. The
engineering
development
of
Porto
Rico.
Eng.
Mag.,
XVII,
July, pp. 602-G21.
1900.
Porto
Rico,
its topography and
aspects.
Bull. Amer.
Geog.
Soc,
No. 32, pp. 220-238.
Anonymous.
1778.
North America
and West
India
Gazetteer.
London
(6), XXIV,
p. 216.
1898.
Porto Rico, its natural history and products.
Scien. Amer.
Suppl.,
XLVI, July 30.
1916.
Overcrow^ded Porto Rico.
Geog. Review, March.
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