A Reconnaissance 

Study of the Beach 

Sands of Puerto Rico

GEOLOGICAL SURVEY BULLETIN 1942-1

Prepared in cooperation with the 

Economic Development Administration 

of the Commonwealth of Puerto Rico

ERRATUM

Bulletin 10^2-1. In table 3 the box heads of the fourth and fifth columns 

are reversed.

A Reconnaissance 

Study of the Beach 

Sands of Puerto Rico

By ROBERT B. GUILLOU and JEWELL J. GLASS

CONTRIBUTIONS TO ECONOMIC GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1042-1

Prepared in cooperation with the 

Economic Development Administration 

of the Commonwealth of Puerto Rico

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 60 cents (paper cover)
CONTENTS

Abstract________________________________________________________ 273

Introduction. _____________________________________________________ 274

Geography ____________________________________________________ 274

Previous work_________________________________________________ 275

Acknowledgements -----______________-________-_-__-__-__-_--_ 275

Scope._______________________________________________________ 275

Procedure ___________ _________________________________________ 275

General Geology._________________________________________________ 277

Description of Beaches.____________________________________________ 278

North coast_________________________________________________ 281

West coast__________________________________________________ 285

South coast.__________________________________________________ 286

East coast.___________________________________________________ 287

Special features _______________________________________________ 288

Description of sand________________________________________________ 288

Tenor of magnetite____________________________________________ 288

Size analyses_________________________________________________ 288

Description of heavy minerals.._________________________________ 291

Frequency distribution of nonmagnetic heavy minerals-____________ 292

Spectrographic study of samples_________________________________ 295

Composite samples (magnetite-free)__________________________ 298

Monominerallic samples.___________________________________ 299

Magnetite samples.________________________________________ 300

Economic geology_ ________________________________________________ 300

Literature cited___________________________________________________ 303

Index.______ ___________________________________________________ 305

in
IV CONTENTS

ILLUSTRATIONS

Page 

PLATE 15. Generalized geologic map of Puerto Rico, showing shore

features __________________________________________ in pocket

FIGURE 31. View looking west from a point east of Palmas Altas________ 283

32. Detail view of the banded magnetite-rich consolidated-sand 

beach rock on the north shore of Puerto Rico, west of 

Puerto Rico____________________________ 283

33. View along the north shore of Puerto Rico, looking west from

Palmas Altas_______________---____-__-_-_-__________- 284

34. Size analyses of representative Puerto Rico sands__________ 290

TABLE

TABLE 1. Description of coasts and beaches of Puerto Rico____________ 279

2. Tenor of magnetite in beach sands of Puerto Rico__________ 289

3. Proportions of the sample fractions and frequency distribution of minerals in heavy nonmagnetic sample fraction in 

beach and dune sands _ ____________________________ 293

4. Percentages of major constituents in beach and dune sands_ _ _ _ 294

5. Semiquantitative spectrographic analysis for minor elements 

and quantitative analysis for niobium in 24 samples of beach 

sands_ ____________________________________________ 296

6. Semiquantitative spectrographic analysis for minor elements 

and quantitative spectrographic analysis for vanadium in 

10 samples of magnetite from beach sands________________ 298
CONTRIBUTIONS TO ECONOMIC GEOLOGY

A RECONNAISSANCE SURVEY OF THE BEACH SANDS 

OF PUERTO RICO

By ROBERT B. GUILLOU and JEWELL J. GLASS

ABSTRACT

A reconnaissance survey of the beaches of Puerto Rico was made by the U. S. 

Geological Survey in cooperation with the Economic Development Administration of the Commonwealth of Puerto Rico to explore the sands for economic 

heavy minerals and to use the heavy minerals as guides in locating their sources 

in hardrock.

Narrow coastal sand dunes behind narrow beaches and rocky areas of consolidated sand are the normal shoreline features of the north coast of Puerto 

Rico from Luquillo to Camuy. Swampy areas and lagoons commonly occur 

between the coastal strip and the flat coastal plain. Magnetite and other heavy 

minerals are concentrated in beach and dune sand deposits near the mouths of 

the major rivers.

From Camuy to Aguadilla, the coast is characterized by cliffs 100-150 feet 

high, with narrow terraces near sea level. The content of heavy minerals in 

the sand of this area is low. Sandy beach predominates on the west from 

Aguadilla to Punta Guanajiba, south of Mayaguez, and concentrations of magnetite are found near the mouths of the larger rivers. The beach sands near 

Mayaguez contain minor amounts of chromite, in addition to magnetite.

The west and southwest shores, from Punta Guanajiba to Ponce, are composed 

of rocks, mangrove swamps, and beaches composed of shell fragments. Noncalcareous material comprises a low percentage of the beach sand, and heavy 

minerals are unimportant.

From Ponce eastward to the Rio Grande de Patillas, the south coastal plain 

is composed of sediments of Quaternary age, except for an isolated area of older 

rocks at Central Aguirre; and the shoreline is a combination of mangrove swamps, 

beaches of sand and cobbles and rocky areas of consolidated sand and consolidated gravel. Heavy minerals are a major constituent of the beach sand in 

many places on the south coast and magnetite often constitutes more than 20 

percent of the sand.

The east coast, from the Rfo Grande de Patillas to Naguabo, consists of outcropping volcanic and intrusive rocks of Cretaceous and early Tertiary age and 

sandy beaches. Rich concentrations of heavy minerals occur locally. In the 

northern portion of the east coast, rocks and swamps predominate but some 

calcareous beaches are present.

273
274 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Recently consolidated sand of eolian and marine origin occurs on the north 

coast of the island as much as 50 feet above sea level. Elsewhere, consolidated 

sand and consolidated gravel are restricted to the littoral areas.

The most abundant heav}' mineral in the beach sands of Puerto Rico is magnetite. It occurs on all shores of the island. Ilmenite is present, associated 

with the magnetite. Chromite, derived from serpentine, occurs in the sands of 

the west coast near Mayaguez. Sphene, although a minor constituent, is widely 

distributed in the beach sands. Zircon is rare.

Channel samples of the uppermost one to three feet of beach and dune sands 

were collected and the locations of the samples on the beach profile were noted. 

Conveniently sized samples were prepared and separated into three fractions: 

magnetic, heavy (sp. gr. ^>2.80) nonmagnetic, and a light (sp. gr. <I2.80) 

nonmagnetic. Each fraction was weighed and studied microscopically. 

Samples of sand from selected localities were sieved and the percentages of 

magnetite in each fraction were determined. The data thus obtained are presented on graphs that show percentages, frequency distributions, and sizes of the 

mineral constituents.

Three-dimensional control in several areas on the north coast of Puerto Rico 

provide sufficient data for rough estimates of tonnages of the magnetite. At 

least 200,000 tons of magnetite, and possibly as much as 300,000 tons, is indicated 

to be present. Some of the lower flood plains of the larger rivers and the shallowwater offshore deposits of sand are suggested as areas for further study.

INTRODUCTION 

QEOGRAPHY

Puerto Rico is an island of about 3500 square miles, roughly 35 

miles from north to south and 100 miles from east to west, situated 

in the Caribbean island arc between Hispaniola and the Virgin 

Islands, at about latitudes 17°55 / to 18°30' N. and longitudes 65°35' 

to 67°15' W. It has a tropical climate tempered by cooling northeast 

trade winds throughout most of the year. The eastern and central 

mountains have a rainfall in excess of 80 inches; the north coast has 

50-75 inches; but most of the south coast has only 25-45 inches 

(Roberts and party, 1942).

PREVIOUS WORK

Puerto Rico has been under control of the United States since 

1898 but few detailed geologic investigations have been made. Parts 

of the island were mapped on a reconnaissance scale by Berkey 

(1915, 1919), Semmes (1919), Hodge (1920), Lobeck (1922), Mitchell 

(1922), Hubbard (1923), Fettke (1924), and Meyerhoff (1931). 

Their work was published as part of the Scientific Survey of Porto 

Rico and the Virgin Islands. These reports served as the basis for 

Meyerhoff's "Geology of Puerto Rico", published for the nontechnical 

reader in 1933. Except for brief reports on various mineral deposits, 

little later geologic work was done until Zapp, Bergquist, and Thomas 

(1948) made a reconnaissance study of the Tertiary sedimentary
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 275

rocks in 1944-1945. The ground-water resources of Puerto Rico 

are described in detail by McGuiness (1948). The most recent summary of the geology of the island is that by Mitchell (1954).

Excellent topographic maps of all parts of Puerto Rico have been 

made on scales of 1:30,000 and 1:10,000. The maps of smaller scale 

are now being revised. Aerial photos of the island taken in 1951, 

at an approximate scale of 1:15,000, are also available. A detailed 

soil survey on a scale of 1:50,000, Roberts (1942), serves as an excellent 

guide to the location of many of the rock types.

ACKNOWLEDGEMENTS

The writers wish to express their sincere appreciation to Dr. R. 

Fernandez Garcia, Director of the Industrial Laboratory of the 

Economic Development Administration of Puerto Rico, for his 

interest and assistance in the study. The members of the laboratory 

staff were also very helpful. Office space and laboratory facilities 

for U. S. Geological Survey personnel were furnished at the Industrial 

Laboratory in Hato Rey.

SCOPE

The reconnaissance survey of the beaches of Puerto Rico on which 

this report is based was made by the U. S. Geological Survey in cooperation with the Economic Development Administration of the 

Commonwealth of Puerto Rico, for the following reasons: (1) The 

large quantities of magnetite reported to occur in some of the beaches 

might be a potential source of iron ore, but the richest areas and the 

size of the deposits were not known, and (2) a study of the beach 

sands might indicate the locations of deposits of strategic minerals 

in parent rocks in the interior of the island. The coastline of the 

island was therefore investigated with special emphasis on the beach 

and dune sands occurring near the mouths of rivers. No attempt was 

made to include more than the unconsolidated sand deposits in this 

study, although some observations concerning the recently consolidated sands were made.

Field and laboratory work were done intermittently in 1953 and 

1954.

PROCEDURE

Whenever possible, samples were taken from the tops and bottoms 

of walls of sand pits and dunes in order to determine the composition 

of the sand at depth. For the same reason, most surface samples 

represent a channel sample of the uppermost 1-3 feet of sand. Exceptionally rich layers were sampled to discover rare minerals, but 

these samples were not used in computing the general tenor of mag-
276 CONTRIBUTIONS TO ECONOMIC GEOLOGY

netite. The samples were collected with a short-handle folding 

shovel.

A few exceptionally salty samples were washed, and then all samples 

were dried at 140° F. Fractions of approximately 100 grams were 

mixed and then quartered down to smaller fractions from which 

samples of 20.0 grams were obtained. The magnetite was removed 

by spreading the sample into a layer approximately one-grain thick 

on a large, smooth sheet of paper and passing a hand magnet oneeighth to one-fourth inch above it. Grains lifted from the paper by 

the magnet were considered to be magnetite and were weighed.

The nonmagnetic portion of the sample was separated into two 

fractions having specific gravities either greater or less than 2.80. 

This was done by using bromoform in a special separatory-funnel 

designed by C. S. Ross (1926) for the U. S. Geological Survey. The 

light nonmagnetic fraction, material floating on the bromoform and 

consisting of variable amounts of quartz, feldspar, and fragments of 

rock and shell, was weighed and briefly scanned under the binocular 

microscope. The heavy nonmagnetic fraction was also weighed, 

and the percentages by weight of both fractions were calculated. 

The nonmagnetic heavy minerals were then examined under a binocular microscope, at a magnification of 27 diameters, and separated into opaque and nonopaque groups of minerals. The nonopaque minerals were identified with the petrographic microscope by 

the oil-immersion method. Opaque minerals were determined by 

their physical properties, aided by qualitative chemical tests. Hematite, for example, is recognized by its characteristic color and 

streak; limonite appears, in these sands, nearly always as a pseudomorph after pyrite; ilmenite is recognized by its dull-black color and 

its weak magnetic susceptibility; and chromite by its occurrence as 

splendent black small octahedral crystals.

The percentage of each of the mineral constituents of the nonmagnetic heavy-mineral fractions was determined by grain count 

and computed as a component unit of the total percentage by weight 

of the heavy nonmagnetic fraction (tables 4 and 5). This was 

accomplished by thorough mixing, using a Jones microsplitter. A 

portion of the fraction, consisting of the number of grains that lay 

on the point of a small spatula, was then placed on a sorting-plate 

and the minerals were separated into mineral groups by hand with 

a pointed tool made of horn. Such work was lessened in a few cases 

by making a partial or even complete mineral separation with a 

Frantz electro-magnetic separator.
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 277

GENERAL GEOLOGY

Puerto Rico is described by Meyerhoff (1933) as being underlain 

by a central east-west belt of Cretaceous rocks flanked on the north 

and south by Tertiary calcareous sedimentary rocks and by generally unconsolidated Quaternary sediments. The central east-west 

belt is mountainous, underlies two-thirds of the island, and consists 

of a complex of highly folded volcanic flows and pyroclastic rocks, 

shale, and limestone. Recent work by Hess, Slodowski, and Mattson 

(written communication) in the area south of Mayaguez and by Kaye 

(1956) southeast and southwest of the San Juan area indicates a 

Paleocene and possibly an Eocene age for some of the rocks heretofore 

considered to be Cretaceous in age. In its southwestern part the 

complex contains several bodies of serpentine, and in the central and 

eastern part, intrusions of granitic, andesitic, and dioritic rocks. 

Owens and Guillou (report in preparation) have obtained age determinations of 51 56 million years for rocks from the San Lorenzo 

batholith in southeast Puerto Rico. The serpentine and larger 

dioritic bodies are shown on plate 15. Meyerhoff (1955) states that 

at least 25 small exposures of intrusive rock, from one half to five 

square miles in area, are scattered throughout the mountains.

The younger Tertiary rocks, according to Zapp, Bergquist, and 

Thomas (1948), consist of clastic deposits in the basal part of the 

section and predominantly of limestone in the upper part, all lying 

unconformably on the Cretaceous and Early Tertiary rocks of the 

complex. These marine sedimentary rocks range in age from middle 

Oligocene to early Miocene.

The Quaternary sediments alluvial deposits, consolidated and 

unconsolidated beach and dune sand, sandy limestone, silt, and clay 

lie unconformably on the Cretaceous and. early Tertiary rocks of 

the central complex and disconformably on the blanketing younger 

Tertiary strata.. San Juan formation is the term that has been applied 

to various portions and types of the Quaternary sediments by Berkey, 

Mitchell, Lobeck, Hubbard, Meyerhoff, and other writers. We 

consider that until these deposits are studied in detail it is best to 

avoid using a formal stratigraphic term. Therefore, the sandstone 

and conglomerate of Quaternary age at or near sea level are referred 

to in this report as "consolidated sand" or "consolidated gravel". 

We believe that these deposits have accumulated during several 

recent periods of deposition and consolidation.

412353 57-
278 CONTRIBUTIONS TO ECONOMIC GEOLOGY

DESCRIPTION OF BEACHES

The terminology adopted by Thompson (1937, p. 725), and McKee 

and Weir (1953, p. 381) will be used to describe the general features 

of the beaches of Puerto Rico. The upper foreshore, the part of the 

beach in front of its crest, is formed primarily by the action of waves 

and characteristically is stratified. The strata, which result from 

the sorting of minerals, range in thickness from a fraction of an inch 

to several inches. Cross-stratification is produced when the beach 

profile of equilibrium, which represents a balance between deposition 

and erosion, is disturbed as a result of variations in the quantity and 

type of material supplied by adjacent rivers, longshore currents, and 

organic life in the vicinity; in the tide level; in the slope of the shore; 

or by the interaction of these factors. As the shore of Puerto Rico 

is complex and continually changing, cross-stratification is common.

The sands of the beach crest and the backshore beach result from 

the action of winds and waves. During storms and times of unusually 

high tides, waves sweep over the crest and deposit sand on the backshore. When this sand has dried, the wind often winnows out the 

lighter grains to form small dunes and leaves a surface layer rich in 

heavy minerals. Backshore sands are a combination of sand-dune 

and storm deposits, and are therefore more complex than upper 

foreshore deposits.

The shores of Puerto Rico are classified on the basis of their character into swamp, rock outcrops, cobble beaches, and sand beaches. 

Swamps are limited to the southwest, south, and northeast shores of 

the island; and cobble beaches are restricted primarily to the south 

shore. Although bedrock outcrops and sand beaches occur on all 

shores, certain generalizations as to types of rock in different areas 

may be made.

Rocks of the central complex and younger sedimentary rocks form 

the outcrops of the east shore, except for a small area of recently consolidated sand and consolidated gravel at sea level in two small coves 

north of Punta Toro, and most of the outcrops of the northwest, west, 

and southwest shores of the island. Consolidated sand, which forms 

the rest of the outcrops on these shores is the principal rock exposed 

on the north shore from Luquillo to Camuy, and with consolidated 

gravel forms the only rock outcrops on the south shore from Ponce 

to Rio Grande de Patillas, except for an extension of Eocene (?) 

limestone (Owens, J. P., written communication) of the central complex 

at central Aguirre.

In table 1 the nature of the coasts, the beaches, and the rocks of 

the source areas are briefly described. A more detailed description 

of the coasts and beaches for the reader who wishes further information 

concerning these features follows the table.
TABLE 1. Description of coasts and beaches of 

Section of coast and 

approximate length

Cabezas de San JuanPunta Uvero (15 miles)

Punta Uvero-Punta Vacia 

Talega (5 miles)

Punta Vacia Talega-Punta 

Salinas (19 miles)

Punta Salinas-Rto de la 

Plata (5 miles)

Rio de la Plata-Rio Grande 

de Manati (18 miles)

Rfo Grande de ManatfRfo Grande de Arecibo 

(12 miles)

Rio Grande de AreciboRIo Camuy (8 miles)

RIo'Camuy-Aguadilla (28 

miles)

Reference 

EO. on 

pi. 15

Nature of coast

Low sand dunes in coves between rocky 

points. Consolidated-sand beach rock on 

and off shore. Swamp east of Rio Espiritu 

Santo.

Consolidated dune sand at Punta Vacia 

Talega. Broad area of unconsolidated sand 

dunes elsewhere.

A complex of low sand dunes and consolidatedsand points and sea cliffs. Offshore rocks 

are common, forming protected bays. 

Swamps in lagoons behind sand dunes and 

in the Bahla de San Juan.

Narrow coastal sand dunes between ocean and 

swamp. Offshore rocks are common.

Predominantly consolidated sand of beaches 

and dunes, the latter are as much as 90 

feet high. Small stretches of sandy beach 

between rocky areas. Swamps common 

behind dunes. Offshore rocks are common.

Consolidated sand of dunes and beaches between ocean and swamp. Unconsolidated 

sand in dunes as much as 90 feet high, in the 

eastern half of section. Offshore rocks 

common.

A sea cliff and a marine terrace make up the 

coast for 3 miles west of Arecibo. Consolidated sand of beaches and dunes and unconsolidated dune sand form the rest of the 

coast. A swamp depression occurs behind 

Punta Maracayo, but in the rest of the area 

the land rises from the coastal strip.

A marine terrace is almost universally present. 

Low sand dunes formed on most of the 

terrace. Sea cliff in limestone of Tertiary 

age near Rio Guajataca, Punta Agujereada, 

and north of Aguadilla. Offshore rocks are 

common.

Nature 

Bedrock and consolid

stretches of beach. 

dominates in beac

terial more abundan

but never importan

Predominantly fine-g

of magnetite small e

Grande de Loiza, w

moderate amounts. 

fragments generally 

Calcareous sand predo

metallic minerals ab

solidated-sand beach 

Sand predominantes; 

ments of volcanic ro

nonmetallic minera

of magnetite.

Most of beach is rock

is composed princip

ments. Anomalous 

and ferromagnesian 

BS-MA-5. (pi. 15, 

Sandy beach predom

beach rock is commo

posed of rock fragm

erals, and moderate 

Content of calcare

westward to be pred

Sandy beach predom

sand beach rock is c

near Arecibo. San

fragments, nonmeta

netite. Concentrat

Arecibo. Calcareou

ments constitute th

in the west.

Sandy beach and c

rock form most of b

in a few places. 

careous material ar

Minor concentration

mouths, major con

Rio Camuv.
280 CONTRIBUTIONS TO ECONOMIC GEOLOGY

03

C3

3

0

o

-o

U 

3

03

fc

Nature ofbeach

o

«.A/3

S

J

?

^

1 1 x, ^

^ 0 

0 0 

a £

a a

3 "S 

K

H

£1

J

a

«j-i

g

n d 10

*3

w J

"'S *S B

d'S

.2 S

§ 21 Me?

,g_i o^J2^ s^iill

w O *-" c3 C "^

S '2 go

"3 *5 *® W c

^ t> ^ QJ

^ o - - o» -S

 o'ie^el d S S3 S ^-^ J5 ^-izs^-g c

^ ^ g yf. D. GO o O o3 '

-*-= D. c3 en

« B -S <D ^ 'g "

.2 o ^Q -4-» "*i c3 ^

Sandy beach predominates but consolidated- sand beachrock Noncalcareous common. sand generally cal- abundant than more sand. Minor concentrations ofcareous magnetite river Chromite atmouhs. 

north Mayaguez. of present 

^ oT N

03 o 3

M^iJ > 

^ -4-3 £ &

^11 tj § * S

5 w g^

^".d^-2

iS^a

B.SSBo u d o)

 d 03 3 te

2 BPntn

PH

o

03

^

'd

03

3

o

£

«x

AS ig'g

Is

<5

I

73

§3

.S

^

§ ><

Q.^2

44M3 ^^

3 CH ®

a g o

f *Q cn ^

Narrow calcareous-sand andbeach swamp 

predominate. Verylittle noncalcareous 

material.

S-gdo M t-'o"

3 ..-o o3 

 0_g.O !j

sand ofstsGuaniji t C and!,tand little

 d rs W 2 03 a3

S?5^

aid ?

§=§ 

"SS^a

jj^S*

fe B °^

03 tr c «d

2 w O3 O2

o

o

0'o

«

o

^5 

03

O

A

£

g"£

rt'S^3

i^ S^i

«

d

 Q

Most beacheither isofbedrock. swamp or The smallamount ofsand ispresent prdominantly calcareous.

 O.J.

1

nsists ofsvand vest 

%£

o C

03 "'

^

3 ^ S

.Sot;

O W *"

' +J !c, O3 ^

_

uchara

O

03

g

PH1 X.

«'s O "- 

.8805-^

O

£

!»

c

c

_c 

t

'o

03

3

o!

a

0

!

offshore !

<D 

?

^

£

73

£1 

02

OT

03

° *§ I ii

S *a> 1 f§

a §£ S u q

3 a;'"1

03 *.2

 o c 

c -53

03 w5

S « 44 "5

y 4) O

s- 21 """S

 'So ° 'S M °

o_g O § ;£ fi

"o*» 'o-S

> >

Volcanic-rock fragments sand cobbles andasdominate beach.the Aoffew swamp. areas Consolidated-sand beach inrock commonvicinity Magnetite of Ponce. theincotnt 

sand isgenerally moderate. 

Most beachis ofsandCobbles andswamp. volcanic grains of rocks beachin constitute de BahfaRincon. Magnetite highcontent 

in places. 

Calcareous sandand consolidated-sand beach ingravel rock portion. Nar- western 

cobble andsand cliff.beach blow row sea Moderate Calcare- amount ofmagnetite. and sandconsolidated-sand beach rockus 

in portion. eastern 

Cobble andsand beaches Con-predominate. solidated-sand andcobble beachrock isBedrock expsed and atwest eastcommon. 

ends ofarea.

,2g«5 « <3!e.Sd<2d fid««

rt 'S!! ^ w^IoSnSg-sl HIS ! . gSi|s ffif|

j=.222g B P | -a S g, w | S o 

Sa^SBdo .gtJ.S'e 28^0 a f3 o||si: e ^i|^ss-is|

a^Buoo.^ wo-gS oCg-S*

§|!iS|if ^li«s°^l^l

iiS-SS^osg ^nS -S^oSd'S*

«''§! § op'* §S^S 2-a§ 2 o3 p 

d° fe'S^'ofl .-'rt >.B'3pH S ^'a""

Iff-Sl0 ^. sg'S |i1 !-a||^

a, M "§ g « "3 a g sT"o - .g "§ E . « |

O3'r3'-t ^_j^p3 ^ j!1iJ3 c^® t-' 03d

pj r- O M.5 h^>PH CJT3oShH'O c3Q<c3O

r i "* ^ "-^ W «3 Z 03

(N CO « 1C

OJ § -O g^

j 1 (-1 OJ

 S <S O-2 03 §

1 ^X -SB *| 1 W w PH 10 o

S-^- fa'" J.-2 ^3~ i-- SS^-1 "O 03

03 03 X d O ^^ ffp-ii ^d -O^ ^03

§B "22 5^ *§§

Ooo "^ «« g ^

11 |l ! § -206

PH 02 TO PH

P

s.s

e X

"^Q,

 Sg

en O

'd o

*s03

u s

.2 °

d a\ i343 ' 

S*"

'So

"S w "^ 2"u|

^ (-H C

S

Although pebbles and cobbles som pres- are beach sandy predominates valley ent,i Bedrock, tumbled blocks, cob- andareas. bleseach make theinof front up seacliffs headlands. inthe Content ofmag- netite high Cal- mouths ofstreams. near sand dominant mouths from careouswav 

!»S«

Pi

§il| if'!

l^i|

33 §2

1-S«5

a § 3 e

°l|-i

-^§3

 M ^ ^~; ®

j O ^3 cd

PS

0

-Punta

03 

W^ £ §

B

i2 -H

^ ^"^

_ §3

o

0

1 

o

0

OJ

X3

1

w

g

T3

"d

10

i-5

a>

i

^j

1

be ommonrocks 3rea

2

D

1'f

"

0?

03!>

1

>> i§

0

S

3

-d

2°

F-H 03 £j

B ==8

S £ S

a> ^ w

.C £ ** ^OJ

:i| JS 03£

2b"S

a l'c

'S'S c

III Q-O 'a

t>

of streams. 

Bedrock andpredominate the swamp on beach. Calcareous-sand beach andcobble 

beach inpresent few areareas.

S a

 "03

O ^ W3

a« 3

ES2 o o y>

pi

«2£

t> c^ W B3 

I3g "aS

ill §£.§

«§o ,2,2°

>. M B

^ 03 S 

o 0 3

O O»-5

PH

1^

o C

I-"1

O) O

.0:3

03 fl 0 H

1 O

a<*

.§a

l_3 M 033

1-5

3dd c3 303

PH
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 281

An explanation of the information depicted on plate 15 is believed 

necessary to prevent erroneous interpretation. Only a very generalized geologic map of Puerto Rico is presented to indicate the possible 

sources of beach sands. The reconnaissance mapping of Zapp, 

Bergquist, and Thomas (1948) and the work of Roberts and others 

(1942) furnish, the information for most of the map. Recent work 

by Hess, Mattson, and Slodowski (written communication) in the 

southwest portion of the island and by Owens and Guillou (in preparation) in the southeast corner result in more detail in these areas. 

Only a few areas of limestone and intrusive rock are differentiated in 

the central part of the island because of the lack of detailed mapping. 

These areas are not to be considered the only ones underlain by limestone and intrusive rock. Some of the contacts drawn between 

Quaternary deposits and older rocks were located by inspection of 

topographic maps and aerial photographs.

The character of the coast is indicated in a general way by combinations of four symbols representing rock outcrops, cobble beaches, 

sand beaches, and swamp. Sections of the coast described in table 1 

and the text are indicated on plate 15 by figures. Localities sampled 

and other specific points referred to in the text are identified by letters.

NORTH COAST

The sands of the east and northeast coasts of Puerto Rico (pi. 15, 

sees. 1 and 17), from Naguabo almost to the mouth of the Rio Grande 

de Lofza, contain small quantities of magnetite and other heavy 

minerals. Rocky points, mangrove swamps, and calcareous beaches 

constitute the shores of this section of the island, which is a coastline 

of submergence. The rivers draining the north and east flanks of 

the Sierra de Luquillo do not bring enough noncalcareous material to 

the beaches to be dominant over the calcareous sand derived from 

fragments of the multitude of organisms growing in the shallow 

coastal waters and from the erosion of consolidated calcareous sands.

From Punta Uvero, about 3 miles east of the mouth of Rio Grande 

de Loiza, to Punta Vacia Talega, about 1 mile west of the river 

(pi. 15, sec. 1), the beaches contain less than 10 percent 

calcareous material and are predominantly fine-grained angular 

quartz. On aerial photographs, broad sweeping arcuate bands indicate 

sandy material as much as 1.5 miles inland,

Calcareous material predominates in the beaches from Punta 

Vacia Talega to the harbor of San Juan (pi. 15, sec. 2). A narrow 

beach, upon which the ocean waves break directly, extends for 4 

miles between the ocean and lagoon from Punta Vacia Talega to 

Punta Maldonado. Punta Maldonado and the island of San Juan 

are composed of hard consolidated beach and dune sand, the sub-
282 CONTRIBUTIONS TO ECONOMIC GEOLOGY

aerial remnants of coastal bars and dunes. Between these points 

submerged remnants of such features and the coral growing upon 

them, break the force of the waves so that the beaches are sheltered. 

From Catano to Punta Palo Seco the beach is in the lee of the island 

of San Juan. The sands between Punta Palo Seco and Punta Salinas 

contain small quantities of magnetite.

A 200-foot-wide strip of coastal dunes, about 30 feet high, extends 

more than 3 miles west of Punta Salinas. The sand of the dunes 

contains a small quantity of magnetite. Behind this narrow strip 

of dunes is a low-lying reclaimed swampy area. Although the present 

mouth of the Rio de la Plata is now to the west, the dunes undoubtedly 

are composed of material brought to the ocean by the Rio de la 

Plata when its mouth was in the vicinity of Rio Cocal.

The coast from Rio de la Plata to Rio Grande de Manati, a distance 

of more than 18 miles, is predominantly rocky, being consolidated 

beach and dune sand, but small stretches of loose beach sand, for the 

most part calcareous, are present. Sands containing magnetite occur only on the banks of the Rio de la Plata, in the small bay just west of it, 

and at one locality west of the Rio Cibuco, where concentration of 

heavy minerals at station BS-MA-5 (pi. 15, loc. 5B) is anomalous, 

because it is the only concentration of heavy minerals that cannot 

be associated with a sizable stream course, and because it contains 

chromite of unexplained origin. This anomalous occurrence is 

probably due to transportation of the material by man, either as 

ballast in ships or during the construction of the nearby Army camp.

Sand deposits are found from the mouth of the Rio Grande de 

Manati to Punta Palmas Altas, a distance of 2 miles (fig. 31). Narrow 

dunes, as much as 90 feet high, extend west of the river mouth for 

more than 1 mile. A depression separates these barely stabilized 

dunes from the consolidated sands to the south. All five of the 

collection localities in the dunes yielded sand samples containing 

abundant heavy minerals. The sand in the walls of the pits was 

clean and appeared fairly uniform with the exception of vague crossstratification. The sand at different levels (table 3). however, varies 

considerably in the size of grains and in the content of magnetite.

Unconsolidated sand rich in magnetite rests on consolidated sand 

containing layers also rich in magnetite (figs. 31 and 32) in the cove, 

west of Palmas Altas (pi. 15, sec. 6). West of the cove are two 1-mile 

stretches of narrow coastal dunes, 30-60 feet high, rich in heavy 

minerals. Consolidated sand is found south of these deposits. The 

rest of the coast to Rio Grande de Arecibo is composed of rocky 

promontories of consolidated sand and beaches of calcareous sand.

Sands rich in magnetite and other heavy minerals occur at the 

mouth of the Rio Grande de Arecibo. The richest deposit is in
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 283

:il. View looking west from a point east of Palrnas Alias, Puerto Rico.

FIGURE 32. Detail view of the banded imignptitc-rich cmsolidated-sand beach rock on the north shore 

of Puerto Kico, west of Palmas Altas.
284 CONTRIBUTIONS TO ECONOMIC GEOLOGY

FIGURE 33. View along the north shore of Puerto Rico, looking west from Palmas Altas.

Barrio La Marina, an island in the mouth of the river. The ocean 

has cut a sea cliff for 3 miles west of the mouth of the Rio Grande do 

Arecibo and the area of beach is narrow and generally pebbly (pi. 15, 

sec. 7). Part of the city of Arecibo is built on sand dunes and on a 

pebbly terrace, 15 feet above sea level, which contains magnetite. 

The terrace extends 2.5 miles to the west and rises to a height of 45 

feet. A sea cliff 3,000-fcct long separates this terrace from a dune 

area to the west which contains abundant magnetite. The creek west 

of the sea cliff does not drain any areas underlain by Cretaceous and 

early Tertiary rocks and therefore the magnetite in the dune sand 

is either derived from the weathered Tertiary rocks of the area or 

it lias been carried from the Rio Grande de Arecibo by longshore 

currents. The latter explanation seems more probable.

From locality IB to Hatillo the beach consists of rocky areas composed of consolidated sand, and dunes as much as 30 foot high, which 

appear to be covering the rocky points.

A marine terrace 3.5 miles long and as much as one-half mile wide, 

and from 50 to 100 feet below the general level of the land to the 

south occurs near sea level at the mouth of the Rio Camuy (pi. 15, 

sec. 7). A narrow coastal dune, 15-40 feet high and about 1,800
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO - 285

feet long, and containing an appreciable quantity of magnetite lies 

to the east of the Rio Camuy. West of the mouth of the river the 

dunes increase in height and width, and are 1 mile long, but the content of magnetite in the sand dimmishes. West of Camuy for 1.5 

miles, low dunes and consolidated sand form the rest of the shore.

A rocky coast extends from the point where the low terrace wedges 

out westward 5 miles to the Rio Guajataca, with narrow terrace, 

just above sea level, separating the 100-150-foot-high limestone: 

escarpment from the oceanl

A low, narrow, sandy terrace extends for 12 miles from the mouth 

of the Rio Guajataca to the northwest tip of the island at Punta 

Agujereada. This terrace also lies between the ocean and the escarpment. At Punta Agujereada and at two places near the Rio Guajataca, the escarpment meets the ocean to form sea cliffs. The sand 

of the terrace is composed of more than 50 percent noncalcareous 

material.

WEST COAST

The shore line from Punta Agujereada to Aguadilla consists of a 

sea cliff 1.5 miles long on the north, 2.5 miles of terrace, largely with 

consolidated-sand beach, and finally sea cliff for a distance of 2 miles 

on the south.

South of Aguadilla the Rio Culebrinas flows into the ocean through 

the middle of a beach four miles long (pi. 15, sec. 8). A few small 

areas of consolidated sand occur at and below the level of high tide 

in this area. The quantity of magnetite and heavy minerals is small, 

although the sands in this stretch contain more than 50 percent noncalcareous material. A similar .beach occurs for 3 miles between 

this beach and the rocks of Punta Higuero. Three small rivers cut 

the beach and surface concentrations of heavy minerals are found 

near their mouths.

Punta Higuero, the westernmost point of Puerto Rico, has three 

miles of rocky coast with very little sand. South of the point is a 

sandy beach about 3 miles long.

Sandy beaches and low, narrow, coastal dunes occur for about 

2.5 miles on both sides of the mouth of the Rio Grande de Anasco. 

The earlier channels of the river are evident in the form of canos, or 

long, narrow, land-locked sloughs almost at right angles to the coast. 

Concentrations of magnetite are found on the surface of the beach 

only in the immediate vicinity of the mouth of the river.

The sands of the beach bordering the Bahia de Mayaguez are 

dark, owing to the preponderance of sand grains derived from volcanic rock.

412353 57 3
286 CONTRIBUTIONS TO ECONOMIC GEOLOGY

The southwest and south coasts of Puerto Rico, from the Bahia 

de Mayaguez to the BahiadeGuayanilla (sees. 10 and 11) have almost 

no noncarbonate beach deposits. Rocky points, mangrove swamps, 

and shell-fragment beaches predominate. No major streams bring 

sediments to the ocean and the materials supplied by the small intermittent streams are dissipated in the mangrove swamps or in the 

flood of carbonate material.

SOUTH COAST

Most of the Bahia de Guayanilla is rimmed with swamp or rock 

terraces cut in limestone of Tertiary age. The small areas of sand 

beach present contain only a negligible percentage of magnetite.

The sand grains in the beach of the Bahia de Tallaboa increase 

progressively in size from west to east. The littoral material changes 

from coarse sand composed of rock and shell fragments, on the small 

peninsula between the Bahia de Tallaboa and the Bahia de Guayanilla, to sand and cobbles in the vicinity of the mouth of the river 

entering the bay. No concentrations of magnetite were found on 

this beach.

The coast line from the Bahia de Tallaboa to Punta Cuchara consists of fine-grained detritus, swamp, and 2 miles of Tertiary-limestone 

sea cliff. Consolidated sand is locally present at the level of low tide.

The beach from Playa de Ponce to the Rio Jacaguas (pi. 15, sec. 12) 

is predominantly sand composed of fragments of volcanic rocks, 

though the beach becomes more cobbly eastward. Consolidated sand 

occurs at the level of low tide and as offshore bars.

The coast from the mouth of the Rio Jacaguas to Playa de Santa 

Isabel, a distance of more than 10 miles, is predominantly a cobbly 

beach, with magnetite-rich interstitial sand. At many places the 

cobbles are restricted to the strip between the levels of low and middle 

tide, the sand forming a layer 1-2 feet thick at, and above, the level 

of high tide. Very low dunes occur behind the beach in several areas.

Mangrove swamps and cobble beaches form the coast from Playa 

de Santa Isabel to the Bahia de Rincon (pi. 15, sec. 13). Cobbles, 

pebbles, and sand with abundant magnetite and other heavy minerals 

occur on the beach of the inner portion of the Bahia de Rincon. 

Arcuate remnants of sand dunes lie as much as 1,500 feet inland.

The coast from Salinas to Central Aguirre is predominantly a 

beach of carbonate sand and mangrove swamps. Except for the 

isolated rocks at Central Aguirre, the Bahia de Jobos is bordered by 

mangrove swamp. Calcareous sand, consolidated sand, and consolidated gravel constitute the arm of land separating the Bahia de 

Jobos from the Caribbean Sea.
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 287

A very narrow beach of cobbles containing interstitial sand below 

a sea cliff cut in red-brown terrace material extends for 5 miles west 

from Arroyo. The sand contains abundant magnetite and other 

heavy minerals, which at several places are concentrated in layers as 

much as 3 inches thick. Three resistant areas of consolidated sand and 

gravels, and beaches composed of carbonate material, quartz, feldspar, and rock fragments, form the low coast from Arroyo to Patillas. 

The beach at the mouth of the Rio Grande de Patillas consists of 

sand, gravel, and cobbles.

The coast from the mouth of the Rio Grande de Patillas to the flood 

plain of the Rio Maunabo (pi. 15, sec. 15), is a composite of rocky 

points and stretches of consolidated sand and consolidated gravel 

at the level of low tide and offshore. Carbonate sand occurs in 

sheltered areas, but sand, gravel, and cobble beaches occur at the 

mouths of small canyons.

EAST COAST

A low barrier dune less than 15 feet high has been built up across 

the flood, plain of the Rio Maunabo.

North of Punta Tuna (pi. 15, sec. 16), a coarse-grained granitic 

promontory, is a smooth beach predominantly of carbonate detritus. 

Pronounced terraces, 30-75 feet above sea level, are present west of 

Punta Toro. The beach below the terraces is narrow and in many 

places cobbly. The beach at the mouths of the small ravines, and the 

beds of the ravines themselves, contain concentrations of magnetite 

that indicate the presence of considerable magnetite in the terraces. 

From Punta Toro to the Valle de Yabucoa, the granitic rocks of the 

Cuchilla de Panduras rise sharply from the Caribbean Sea, precluding 

large beach deposits.

A narrow, low, coastal dune, 6-8 feet high crosses the seaward 

end of the Valle de Yabucoa. Granitic materials are the chief constituents of the sand. The coast for 3 miles north of the Valle de 

Yabucoa is rocky except for narrow beaches in small coves.

Morro de Humacao,a granitic promontory 1,500 feet long, separates 

2.5 miles of granitic and calcareous beach from the Humacao flood 

plain.

The 6 miles of sandy coast from the Rio Humacao to the Rio 

Santiago is interrupted only by El Morillo, a mass of volcanic rock 600 

feet long which rises more than 100 feet above the low, sandy coastal 

plain. Surficial concentrations of magnetite are found on the beach 

near the mouths of the rivers in this area. Near the Rio Humacao 

concentrations of magnetite are also found on storm terraces and on 

the surfaces of vegetation-covered 10-foot sand dunes.
288 CONTRIBUTIONS TO ECONOMIC GEOLOGY

SPECIAL FEATURES

An outstanding feature of the north coast of Puerto Rico is the 

extensive development of recently consolidated "fossil dunes" and 

beach rock. These two distinct features were not separated in 

this reconnaissance survey. They occur irregularly from Luquillo 

on the east to Punta Agujereada on the west, from below sea level 

to more than 50 feet above sea level. Reefs or rocks are commonly 

present as en echelon lines at slight angles to the coast. At some 

localities sandstone displays characteristic eolian cross-stratification, 

but at others it appears to be of marine origin and fossils of corals, 

mollusks, and other invertebrates locally present substantiate the 

theory of marine deposition when the sea was higher in relation 

to the land than now. Nevertheless, the portions of the sands 

exhibiting eolian cross-stratification and the off-shore rocks require 

a lower sea level relative to the present strand during their deposition. 

The conditions for consolidation of the sand are still favorable in 

several places. West of Punta Palmas Altas, blocks of consolidated 

cross-stratified sand, which recently caved off the sea cliff, are now 

cemented together at the base of the cliff.

Consolidated sands and gravels are developed extensively on the 

west and south coasts, but they seldom occur more than a few feet 

above sea level. Coarse conglomerates are found at the mouths of 

some of the south-coast rivers. At sea level near Central Aguirre 

a dump of scrap iron has been converted into a conglomerate by 

iron-rich cement.

DESCRIPTION OF SAND

TENOR OF MAGNETITE

The tenor of magnetite in the beach sands of Puerto Rico is summarized in table 2, which shows the location, type, and extent of 

the various deposits and range in the content of magnetite. Estimated tonnage of magnetite and additional information concerning 

the deposits are included under Remarks.

SIZE ANALYSES

Grain size analyses (figure 34) were made on samples of sand from 

ten localities and give a general idea of the character of the sand, 

the percent by weight of the size fractions, and the magnetite in each 

fraction.

Representative samples were analyzed from the rivermouth 

bars of the Rio Grande de Arecibo, Rio Grande de Loiza, Rio 

Guayane"s, Rio Grande de Afiasco, and Rio Grande de Patillas. 

The coarse nature of sample BS-48 (pi. 15, loc. 16Z>) is attributed to 

the short length of the river and the coarse-grained dioritic source
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 289

rock. Included in the 18 percent of material of -j-20-mesh for this 

sample is less than 3 percent of material of + 10-mesh.

Data for GY-5 (loc. 14J5) illustrate the distribution of size in 

the 10-mesh interstitial material from a typical cohbly beach on 

the south coast.

TABLE 2. Tenor of magnetite in the beach sands of Puerto Rico

Sample

Location 

on pi. 15

2 

2A

30

4 

6A

GO 

7A

7B

8A

9 

12A

12C-12F 

13A

14B 

10

No.

JRG-2-RG-4 ' 

1C.R-1 

fRG-1 

1CR-5

JBM-3 

\BM-5 

fBM-ll-BM-15 

\VA-1 BC-7-BC-12

fBC-5 

\BC-6 AR-3-AR-9

CM-3

fCM-7 

\CM-11 AG-4-AG-7 IRI-I-RT-G

IMY-3-MY-6 

PC-l-PC-3

fPL-l-PL-4 

\SI-l-SI-5

SL-l-SIr-15 

/GY-4 

\GY-5 

[PT 

^PG IBS

Type of deposit

IBeach and broad area 

J of stabilized dunes.

jlli vermouth beach....

! Beach and low stabilized dunes. 

Beach and 30-foot 

dunes. 

Beach and 90-foot 

dunes. 

IBeach and 60- to 90- 

/ foot dunes. 

Rivcrmouth beach. i__

Beach and 30-foot 

dunes. 

yReach and 10- to 50- 

J foot dunes.

S-Bcach and low dunes.

Narrow beach and 

low dunes. 

} Narrow beach and 

low dunes.

Beach and low dunes.

J-Bcach and low dunes.

Length 

(miles)

3 

1 

3 

3 

1.5 

2 

0.1 

0.1 

0.3

1 

11

4 

5

Magnetite (percent by 

weight)

0-2 

2-5 

1-5 

5 

10 

5 

5-30 

10-30 

15

0-10 

5-10 

5-30

5-40

5-30 

0-30

Remarks

Small lenses contain more than 10 

percent of magnetite.

Minimum estimate 90,000 tons

of magnetite. 

Minimum estimate 62,000 tons 

of magnetite. 

Liberal estimate 60,000 tons of 

magnetite. 

Minimum estimate 25,000 tons 

of magnetite.

Minimum estimate 20,000 tons 

of magnetite.

Minor concentrations of magnetite 

near mouths of rivers.

Magnetite abundant in interstitial 

sand -of cobble beach. Possible 

offshore deposits in shallow 

water. 

Magnetite abundant in interstitial 

.sand of cobble beach and in sand 

of low dunes. Possible offshore 

deposits in shallow water.

Magnetite abundant in interstitial 

sand of cobble beach.

Concentrations of magnetite near 

river mouths.

Samples of dune sand from four localities were analyzed. The 

variation in distribution of grain size in the 30-foot sand dune at 

locality QA is illustrated by size analyses of samples from near the 

top (BC-lla), middle (BC-116), and base (BC-llc) of the dune. 

The analyses of channel samples of the top (BM-12a) and bottom 

(BM-126) 5 feet of a 10-foot sea cliff in dune sand at locality 4A 

yielded remarkably similar histograms.

These size analyses furnish data for only one valid generalization 

concerning the beach and dune sands of Puerto Rico. Magnetite 

always occurs as small grains. In every sample the magnetite was 

 40 mesh and in all but two analyses the magnetite of 60 mesh 

was equal to or greater in weight than the magnetite of +60 mesh.

Comparisons in content of magnetite between river-mouth sands 

and dune sands are not justified. Typical river-mouth sands were
292 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Vanadium-bearing magnetite Table 6 shows that magnetite from 

beach sands of 10 localities along the shore of Puerto Rico contains 

amounts of vanadium ranging from 0.24 to 0.44 percent.

Zircon Zircon is a zirconium silicate (ZrSiO4) that may contain 

small amounts of thorium, uranium, hafnium, and the elements of 

the rare earths. The zircon crystals in the Puerto Rico sands are 

colorless and usually about 1 millimeter long. Zircon occurs sparingly in most of the sands, but the samples indicate (table 6) that it 

is more abundant at Punta Puerto Nuevo (loc. 5B) and at Punta 

Guayanes. Zircon normally contains about 65 percent of ZrO2 .

The demand for zircon was small in 1954. Australia has been 

the world's principal producer of zircon concentrates for many years; 

however, the Australian zircon producers are reported to be disturbed about the future markets for their zircon concentrates, because 

of (1) the potential competition of Brazil, Germany (Baltic Sea beach 

sands), French West Africa, and the Union of South Africa; and (2) 

the abundant output of the Florida operations which could produce 

more zircon if the demand warranted it.

FREQUENCY DISTRIBUTION OF NONMAGNETIC HEAVY MINERALS

Percentages of the magnetite-free heavy minerals listed in both 

tables 3 and 4 were determined by counting grains on a plate. A 

convenient sorting-plate is made from a piece of transparent glass, 

such as a lantern slide, to the back of which is cemented a piece of 

coordinate paper divided into squares 3 by 3 millimeters, or less.

Well-mixed fractions are used. Small amounts, 100-200 grains, are 

adequate. The mineral grains are arranged in a narrow thin row and 

are rapidly sorted by drawing the grains together with a microscope 

slide, or any small straightedge, and then drawing them out into a

threadlike line. The dark minerals are now pushed to the right with 

a pointed nonmagnetic grain-separator and the light-colored minerals, 

to the left. The minerals of each species are then brought together 

to cover the 3 by 3 millimeter squares. The number of squares 

covered uniformly by each mineral is a measure of the total number of 

squares covered. Thus the relative proportions of different minerals 

may be determined. If grain sizes have been carefully considered, 

the percentages can be determined with a fair degree of accuracy.

This sorting-plate method was compared with the grain-counting 

method that uses the mechanical stage mounted on a microscope, and 

with the Chayes-point counter. Both these methods required more 

time than the sorting-plate method and show only minor variations 

in the proportions or percentages in the most abundant minerals.

Table 3 shows the varieties and the relative abundance of minerals 

that are found in 24 samples of beach sands collected from river
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 293

mouths, dunes, and beaches around the Island of Puerto Rico. The 

combination of letters and figures in the left-hand column represent 

the reference numbers on plate 15 and the field identification numbers.

Direct percentages of the most common constituents in 13 samples 

of beach and dune sands from Puerto Rico are shown in table 4. 

These samples represent a fair average of the types of heavy minerals 

and the proportions of each in the beach and dune sands of Puerto 

Rico.

Column 3 shows the percentage of the "float", that is, the light 

fraction, composed of rock, shell, and minerals having a specific 

gravity of less than 2.8.

Column 4 shows the percentages of magnetite, in samples which 

range from lean to moderately rich.

Column 5 shows the percentages of nonmagnetic heavy minerals. 

Epidote and pyroxene occur in about the same proportions and are the 

predominant minerals. Hornblende is next in abundance; ilmenite 

and sphene are persistent, but sphene never constitutes more than 3 

percent of the heavy nonmagnetic minerals. These percentage 

ratios coincide with those shown in table 3, but the use of percentages 

rather than symbols permits more detail to be shown.

SPECTROQBAPHIC STUDY OF SAMPLES

The primary objective of the spectrographic study was to discover 

any useful, rare or minor elements, especially niobium, that might 

occur in the black sands of Puerto Rico.

Of 24 samples of heavy minerals, from which magnetite had been 

removed, submitted for analysis, 16 were composite samples and 8 

were samples of separated minerals. In the analyses (table 5) 24 

elements are reported present and 17 others were looked for but not 

found. The column headed Nb gives the quantitative determination 

of niobium. Also examined spectroscopically (table 6) were 10 

samples of magnetite.
TABLE 3. Proportions, in percent by weight of sample fractions and frequency distrib

dune sands of Puerto Rico

Type of sample: Cs, Channel sample; S, surface sample; frequency-distribution symbols and range in perce

cent; C, common, 40-60 percent; C , less common, 25-40 percent; M, moderate, 15-25 percent; R, rare, 1

Sample

Location 

on 

pi. 15

2A 

2A 

3C 

4A 

4A 

4B 

6A 

"A 

7A 

8B 

8C 

9A 

9B 

9C 

11A 

11B 

110 

11D 

12B 

12C 

12D 

12E 

13A 

13A 

14A 

14B 

14C 

15A 

16A 

16B 

16D 

16E 

16G 

16G

No.

RG-1 

CR-5c 

BM-5 

BM-12a 

BM-12b 

VA-3 

BC-la 

AR-3 

AR-5 

CM-lOa 

QB-2 

AG-4 

RI-2 

MY-5 

CB-2 

PA-2 

PV-2 

PC-6 

PL-7 

PL-3 

SI-5 

SI-2 

SL-2a 

SL-6a 

CA-3 

GY-5 

GY-1 

PT-6 

PT-2 

PG-la 

BS-49 

BS-29 

BS-8 

BS-6

Type

Cs 

S 

Cs 

Cs 

Cs 

Cs 

Cs 

S 

Cs 

S 

Cs 

S 

Cs 

Cs 

S

s

Cs 

Cs

S

s

Cs 

Cs

S

s

Cs 

S 

Cs 

Cs 

Cs 

S 

Cs 

Cs 

Cs 

Cs

Percent by weight

Light 

nonmagnetic 

fraction 

("float") 

sp. gr. 

<2.8

4 

14 

6 

10 

11 

11 

11 

33 

8 

17 

5 

21 

4 

10 

5 

8 

6 

6 

37 

27 

44 

27 

61 

25 

15 

29 

13 

18 

13 

89 

6 

14 

2 

4

Magnetic 

fraction

81 

45 

71 

57 

54 

50 

46 

27 

65 

48 

74 

51 

78 

59 

84 

79 

81 

81 

23 

30 

6 

40 

6 

24 

77 

29 

74 

44 

61 

1 

83 

74 

78 

41

Heavy 

nonmagnetic 

fraction 

sp. gr. 

>2.8

15 

41 

23 

33 

35 

39 

43 

40 

27 

35 

21 

28 

18 

31 

11 

13 

13 

13 

40 

43 

50 

33 

33 

51 

8 

42 

13 

38 

26 

10 

11 

12 

20 

55

Frequency distribution of individual minerals 

0

"o

.9

o

 <

R 

P 

P

P

MP 

P

o 

'C

C3 m

p

p

p

p

0

5

p

p

p

'C 

^o

6

p

R

P

MR

P 

P

S

1 

-a 

O

B

P

 -

«

'o

c 

c

O

K

P

P

o

a 

o

5

c

R

R 

R

R

o 

"o

"2

'3. 

w

CccM

M

C

Ccc

MC 

R

A

R

R

P

P MCA

A

C

CM

MCM

<a 

. c&.

cl

O

p

R

MR 

P 

R

P

P 

P

P

P

P

MP 

P

o

S

o 

O

cR

M

M

M
TABLE 4. Percentages by iveight of major constituents in beach a

Sample

Location 

on 

pi. 15

7B 

90 

9D 

13A 

I6C 

16D 

16E 

16E 

16F 

16G 

16O 

16O 

16H

No.

CM-3 

MY-5 

MY-3 

SL-6 

BS-54 

BS-49 

BS-25b 

BS-29 

BS-21 

BS-3 

BS-5 

BS-9 

BS-15

Light 

nonmagnetic 

fraction 

("float") 

sp. gr. 

<2.8

26.0 

60.0 

40.0 

27.0 

68.0 

83.0 

82.0 

74.0 

93.0 

59.0 

89.0 

. 21.0 

75.0

Magnetic 

fraction

34.0 

10.0 

15.0 

25.0 

9.0 

6.0 

6.0 

14.0 

1.0 

2.5 

1.0 

54.0 

2.5

Heavy 

nonmagnetic 

fraction 

sp. gr. 

>2.8

40.0 

30.0 

45.0 

48.0 

23.0 

11.0 

12.0 

12.0 

6.0 

38.5 

10.0 

25.0 

22.5

Individual minerals in heavy n

Actinolite

3.0 

5.0 

2.0

1.0

Ohiorite

1.5

P

P

P

Chromite

4.5

Clinozoisite

1.0

P

Diopside

P

E pidote

16.0 

1.5

14.0 

19.0

6.0

3.0

1.0 

. 1.0

12.0 

5.0 

12.0 

7.0

Oarnet

1.0

P

P 

1.0

P

P 

P 

P

Ho

ble

P

2
TABLE 5. Semiquanlitative spectrographic analyses for the minor elements, and quan

sands of Puerto Rico 

[Analysts: H. J. Rose and J. D. Fletcher]

Sample

Location 

on pi. 15

1A 

4A 

5B 

6A 

9B 

9C 

11B 

12A 

14A 

14C 

8A 

2A 

12B 

12E 

12D 

15A 

9A 

16B 

14B 

11C 

5B 

16D 

5B 

16B

No.

RG-9 

BM-12a 

MA-5 

BC-lla 

RI-2 

MY-5 

PA-2 

PC-2 

CA-3 

GY-1 

CM-11 

CR-Sc 

PL-7 

SI-2 

SI-5 

PT-6 

AG-4 

PG-la 

GY-5 

PV-2 

MA-5b 

BS-49 

MA-5c 

PG-la

Description

. .do..... . .......

 do.. 

 do.. 

 do.. ...

 do-- 

 do.... ..... -

..--.do-. ... -----------

 do.. 

 do-- 

 do..--. ----------

- do--... ----------

-.. .do.-.. ----------

 do..-.. -----------

 do... ...

 do-...- .

.. do____. - -------

Nb

0 

0 

0 

0 

0 

0 

.006 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

.009 

.02 

0 

0

Cu

O.OOX 

.OX 

.OX 

.OX

.ox 

.ox 

.ox 

.oox 

.ox 

.ox 

.oox 

.oox 

.ox 

.ox 

.ox 

.oox 

.ox 

.ox 

.oox 

.oox 

.oox 

.oox 

.ooox 

.ooox

Mo

o.oox

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0

.oox 

.oox

0

.oox

0

w

0 

0

.ox

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 0 ' 

0 

0 

0

Sn

0 

0

.ox 

.ox

0

.oox

0 

0 

0 

0 

0 

0 

0

.oox

0 

0 

0

.oox

0 . 

0

.oox 

.oox

0 

0

Pb

o.oo

0

.ox 

.oo

.oo

.oo

0

.oo

0 

0

.oo

0

.oo

0 

0 

0 

0 

0 

0 

0 

0 

0

.ox

0
Sample

Location 

on pi. 15

1A 

4 A 

5B 

6A 

9B 

9C 

11B 

12A 

14A 

14C 

8A 

2A 

12B 

12E 

12D 

15A 

9A 

16B 

14B 

UC 

5B 

16D 

5B 

16B

No.

RG-9 

BM-12a 

MA-5 

BC-lla 

RI-2 

MY-5 

PA-2 

PC-2 

CA-3 

GY-1 

CM-11 

CR-Sc 

PL-7 

SI-2 

SI-5 

PT-6 

AG-4 

PG-la 

GY-5 

PV-2 

MA-5b 

BS^9 

MA-5c 

PG-la

Description

 -do....... .... . .

-....do-.....-.--..-.

-.-.do.... ........... .

 do.._- ------.-...

. .--.do-...-, ._....... .

.--.. do.. ..............

do

.--.-do................

---..do.-..-.------..

-. do.-... ...........

.....do.. ..............

..-.do-.. .........

 do.-. 

. do... .

... ..do.-. 

- do.-.-. .

 .do.... ...

Cr

O.X

.X 

X. 

.X 

.X 

.X 

.X 

.X

.ox 

.ox

.X

.X 

.X 

.X 

.X 

.X

.ox 

.oox 

.oox 

.ox 

.oox 

.ooox 

.ooox 

.ooox

V

o.ox

.X 

.X 

.X 

.X 

.X 

.X 

.X

.ox 

.ox 

.ox 

.ox

.X

.ox 

.ox 

.ox

.X 

.X

.ox 

.ox

.X 

.X

.oox 

.oox

Sc

o.oox 

.oox 

.oox 

.oox 

.oox 

.oox 

.oox 

.oox 

.oox 

.oox 

.oox 

.oox 

.oox 

.ox 

.oox 

.oox 

.ooox 

.ooox 

.oox 

.oox 

.ooox 

.ooox 

.oox 

.oox

Y

o.oox

0

.oox

0

.oox

0 

0 

0

.oox 

.ox 

.ox 

.ox 

.oox

0 

0

.ox

0 

0

.oox 

.oox

.X 

.X 

.X 

.X

La

0 

0

.oox

0 

0 

0 

0 

0 

0 

'0

.oox 

.oox

0 

0 

0 

0 

0 

0

.oox

0

.ox 

.ox

0 

0

Ti

X. 

X. 

X. . X. 

X. 

X. 

X. 

X. 

.X 

X. 

.X 

X. 

X. 

X. 

X. 

X.

xo. 

xo.

.X 

.X

xo. 

xo.

.X 

.X

Looked for but not found: Ag, Au, Hg, Pd, Ir, Pt. Ge, As, Sb, Bi, Cd, Tl, In, Th, Ta, Be, P; 0 in unit col
298 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 6. Semiquantitative spectrographic analysis for minor elements, 1 and quantitative spectrographic analysis for vanadium in 10 samples of magnetite from 

beach sands of Puerto Rico.

[Analyst: J. D. Fletcher]

Location on 

pi. 1

7A__-.._

6A_.---.

8A- .

2A .-.-.

9C.----

lGB.--_- 

16B---. 

12D-' 

ICE---.

Location on 

pi. 1

7A- .

4A.---_-

8A__..__

2A----_

9C-----

16B---. 

16B-... 

12D .

16E-.--

Fie.ld no.

AR-3---..

BC-lla-..

BM-12a 

CM-11....

CR-5c-.-_

MY-5.. 

PG-2...._ 

PG-la---. 

SI-5_......

BS-29 -.

Field no.

AR-3-.--.

BC-lla.-.

BM-12a...

CM-11-..

CR-5C....

MY-5 

PG-2..... 

PG-la.--. 

ST-5. ...

BS-29 -.

V

0.42

.28

 JO

.24

.35

.31 

.33 

.41

.28

Sc

0

.OOX

.OOX

0

0

.OOX

.OOX 

0. 

.OOX

0.

Cu

0. OOX

.ox

nnY no"V

.OX

.OOX

.OOX 

.OOX 

.OX

.OX

Ti

X.

X.

X.

X.

X.

X.

X. 

X. 

X.

o.x

Pb

20

.OOX

0

0

0

.OOX

.OOX 

0 

0

0

Be

0

.ooox

.ooox

0

0

.ooox

0 

0 

0

0

Mil

0 X

X

X

X

X

X

X 

X 

X

X

Ca

0 X

X

X

X

X

X

X 

X 

X

X

Co

0. OOX

.OOX

flOY

.OOX

.OOX

.OOX

.OOX 

.OOX 

.OOX

.OOX

Sr

0

.ooox

0

0

.ooox

.ooox

0

.ooox

0

Ni

o.oox

.ox

.ox

.OOX

.OOX

.ox

.OOX 

.OOX

.ox

.OOX

Ba

0. OOOX

.ox

.ox

.ooox

.ooox

.ox

.ox 

.ooox 

.ox

.ooox

Al

o.x

X.

X.

.X

.X

X.

X. 

.X 

X.

.X

Mg

o.ox

.X

.X

.ox

.ox

X.

.X

.ox

.X

.ox

Cr

0 X

X

X

X

X

X

X

ox

X

ox

On

o.oox

.OOX

.OOX

.OOX

.OOX

.OOX

.OOX 

.OOX 

.OOX

.OOX

1 Looked for but not found: Ag, Au. Hg, Bi, Ir, Pt, Mo, W, Ge, Sn, As. Sb, Zn, Cd, Tl, Y, Yb, La, 7.\\ 

Th, Nb, Ta, U, P, B.

2 0 in unit column means element was not detected.

COMPOSITE SAMPLES (MAGNETITE-FREE)

In order of abundance, the elements in the composite samples are: 

iron, calcium, titanium, magnesium, chromium, manganese, and 

vanadium. This may well have been predicted from tables 3 and 4, 

which give data on the mineral composition of the black sands. 

Silicon is not reported because an equal weight of quartz was added 

to all of the samples to improve their arcing characteristics. Samples 

that show more than 10 percent calcium contain major amounts of 

epidote, pyroxene, and usually some sphene.

The amounts of niobium are too small to have economic importance, 

but the occurrence of niobium in these samples, always in sphene, is 

significant. The only composite sample (PA-2) which contained 

detectable amounts of niobium was unusually rich in sphene.

Chromium and vanadium occur in about the same proportions. 

Most of the chromium occurs in chromite, but magnetite contains 

tenths of a percent of the element and traces are found in other 

individual minerals. The unusually high content of chromium in 

composite sample MA-5 is due to its relative richness in chromite. 

Vanadium, in amounts of tenths of a percent, is common in mag-
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 299

netite, hematite, ilmenite and sphene. Nickel in the composite sample 

is mostly attributable to its presence in. magnetite. Cobalt, copper, 

and scandium are not sufficiently concentrated in any one mineral 

that any of them could serve as an index element for that mineral. 

Zirconium percentages are indicative of the amounts of zircon, 

although small amounts of zirconium may also be contributed by 

sphene. The sources of molybdenum, tungsten, tin, lead, zinc, 

yttrium and lanthanum are problematical.

Strontium, although not abundant, is reported in the analyses of 

all samples. No strontium-bearing mineral was detected in the heavy 

fractions, so the element undoubtedly is a component of calciumbearing minerals of the light fraction.

MONOMINERALLIC SAMPLES

Ilmenite Spectrographic analysis indicates much more iron than 

titanium in sample AG-4, which suggests that the sample is an 

intergrowth of ilmenite and magnetite, a suggestion that receives 

some support from the presence of nickel. This sample from the 

extreme northwest coast of Puerto Rico also contains less manganese 

than does sample PG-la from the southeast coast.

Much of the ilmenite is altered to a yellowish opaque material 

that may be leucoxene (in this instance, sphene). A trace of niobium 

confirms the presence of sphene. This alteration product can then 

account for some of the calcium in sample AG-4; however a small 

trace of calcium could be due to adhering particles of marine shells.

Epidote One sample of epidote from Puerto Rico and two samples 

of epidote from different localities in Virginia (contributed by J. B. 

Mertie, U. S. Geological Survey) were analyzed under the same conditions. The results coincide with standard chemical analyses, with 

the addition of strong traces of titanium and strontium in the Puerto 

Rico sample and in one from Virginia, and faint traces of these elements in the other sample from Virginia. Faint traces of vanadium 

occur in all three samples.

Pyroxene A sample of typical pyroxene augite shows \veak 

traces of chromium, vanadium, and strontium, and strong traces of 

titanium.

Sphene Table 6 shows the percentages of niobium found in two 

selected samples of sphene from different localities on the beaches of 

Puerto Rico. Besides niobium, the spectroscopic analyses show from 

one-tenth to one percent of yttrium.

Zircon In addition to the normal'constituents of zircon, titanium, 

calcium, iron, and yttrium are present in amounts up to one percent.
300 CONTRIBUTIONS TO ECONOMIC GEOLOGY

MAQNETITE SAMPLES

Ten samples of magnetite from various localities along the beaches 

and from the coastal dunes of Puerto Rico were examined spectrographically, particularly for the vanadium content (table 6). The 

highest percentage of vanadium obtained is 0.42 and the lowest 0.24, 

with an average of 0.33 for the ten samples. The percentages of 

vanadium were compared with results from a recent study of the 

magnetite deposits of the Adirondack Mountains of New York 

(personal communication from J. J. Fahey, U. S. Geological Survey). 

A series of seven samples showed variable amounts of vanadium, 

from 0.04 to 0.10 percent, with an average of 0.07 percent. Semiquantitative analyses were made for 15 other elements (table 7). 

Titanium is consistently high; all but one sample contains 1 percent 

or more titanium. Half of the samples contain 1 percent or more 

aluminum and the other half, 0.1-1 percent. Manganese and calcium occur in all samples to the extent of 0.1-1 percent; and 

chromium nearly the same. Magnesium is variable, 0.01-10 percent. 

Other elements shown in table 7 occur in minor or negligible amounts.

ECONOMIC GEOLOGY

The heavy minerals described or elements obtained from them' 

when they occur in sufficient abundance, have several uses and are 

therefore economically important.

Chromium metal and its alloys are used in making stainless steel, 

chrome plating, and in high-speed and high-temperature steel. 

Chromite is used in the manufacture of chromium brick and refractory cement and in making chemicals used extensively in leather 

tanning. Chromium is a constituent in certain green, yellow, orange, 

and red pigments, and of similarly colored dyes.

The paint and ceramics industries consume most of the world's 

supply of ilmenite, using it in the form of titanium dioxide, an opaque 

white pigment. Titanium and its alloys, because of their strength, 

lightness, and resistance to heat and corrosion, offer particular 

advantages for civilian uses as well as for uses in military, marine, 

and aircraft equipment. Some ilmenite is also used as flux in the 

steel industry and as roofing granules.

Vanadium is used for its alloying properties, and for its grainrefining effect on steel. Much vanadium is used as ferrovanadium 

in the manufacture of tool steels and in high-strength structural 

steels. Vanadium oxide is used as a drier in paints, and as a catalyst 

in the glass and ceramics industry.

Zircon is used as a refractory and opacifier and is an important 

constituent of heat-resistant glass and porcelain. Metallic zirconium 

has many specialized uses in electrical and surgical equipment.
In recent years considerable interest has been developed in the 

relative abundance of certain heavy minerals that occur in minor 

amounts in some beach sands. The rare-earth metals and other rare 

elements found in these minerals are playing an important role as 

alloying elements in modern metallurgy. No significant amount of 

useful heavy minerals other than magnetite was indicated by this 

preliminary study of the black sands of the beaches of Puerto Rico. 

These sands yield small amounts of sphene and zircon, both of which 

contain small fractions of the rare-earth metals and other rare elements in addition to the important minor elements, titanium in 

sphene, and zirconium in zircon. Other minor minerals are chromite 

and ilmenite. Chromite is present in several samples of concentrates, 

and ilmenite occurs in varying amounts associated with magnetite.

Heavy minerals other than magnetite are not sufficiently abundant 

in the Puerto Rico beach sands to have any present commercial 

value, but in consideration of the rapidly increasing uses of these 

minerals, even small, easily accessible deposits, and byproducts of 

magnetite-mining operations may yield some minor minerals in the 

future for local use.

Black sands are generally mined by power shovels or suction dredges. 

The heavy minerals are recovered by gravity concentrators, such as 

the Humphrey Spiral or the concentrating-table. Batteries of 

concentrators are used in two large-scale operations in Florida. The 

heavy minerals, are then kiln-dried and are separated by electromagnetic and electrostatic machines, though gravity separators also 

are used.

The reconnaissance survey of the Puerto Rico beaches could not 

yield enough data for accurate estimates of the tonnage of magnetite 

present in the sand. The purpose of this survey was to determine 

those areas deserving further study. Figures on tonnage have been 

roughly estimated for those areas along the north coast where sampling 

in sea cliffs and sand pits provided three dimensional control. An 

accurate estimate of reserves in these areas could be made, based on 

information inexpensively obtained by hand auger or light-power 

equipment.

In the following estimates of tonnage, 1 percent of magnetite in 

a sample was assumed to indicate 1 pound of magnetite in each cubic 

foot of sand within limits of the deposits. This figure is conservative 

but moderately accurate for low percentages of magnetite.

Two channel samples in a narrow dune, at the mouth of the Rio 

Camuy near Hatillo (pi. 15, loc. 8 A), more than 1,800 feet long, 

contained 20 percent magnetite. On the basis of a very conservative 

estimate of 15 pounds of magnetite per cubic foot, this dune contains 

20,000 tons of magnetite.
302 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Sand averaging more than 10 percent magnetite is found between 

the mouth of the Rio Grande de Manati and Punta Palmas Altas 

(pi. 15, loc. 6^4). All samples collected in this belt of sand dunes, which 

extends for 6,000 feet, contain from 10 percent to more than 20 

percent magnetite. Using the smaller figure, it is estimated this area 

of coastal sand would yield more than 62,000 tons of magnetite.

The several samples collected in Barrio La Marina, an island at 

the mouth of the Rio Grande de Arecibo (pi. 15, loc. 7 A), indicate 

that the sand of the island contains between 5 and 10 percent magnetite. Assuming 5 pounds of magnetite are in each cubic foot of 

sand, this would mean a deposit of 25,000 tons of magnetite

A narrow coastal dune area more than 3 miles long lies between 

the Rio Cocal and the Rio de la Plata (pi. 15, Iocs. 4A-45). Sample? 

of sand from this area indicate 90,000 tons of magnetite present, 

based on a 5 percent magnetite content.

These figures are believed to represent minimum tonnages of 

magnetite. A more optimistic but still conservative appraisal of 

the reconnaissance data would increase the Hatillo deposit by 25 

percent to 27,000 tons, Punta Palmas AJtas by 25 percent to 78,000 

tons, and La Marina -by 50 percent to 37,000 tons. In addition, if 

two large areas of coastal dunes near Los Negritos (pi. 15, loc. 6(7) 

are assumed to contain 5 pounds of magnetite per cubic foot of sand, 

they would contain 60,000 tons of magnetite. Magnetite-tonnage 

figures are tabulated below :

Magnetite 

(pounds

per cubic Minimum Possible 

Area foot) tonnage tonnage

---- -_-_-- -- _- ------- 15 20,000 27,000

LaBoca_ ___ __ - -- 10 62,000 78,000

La Marina __________________________ 5 25,000 37,000

Rfo Cocal - __ __ _ _ 5 90,000 90,000

Los Negritos __ 5 __.._.____ 60,000

197, 000 292, 000

No other large areas of magnetite-rich sand dunes are known on 

the island, but very rich beach sands and small dune areas occur at 

many places. If economic interest in the beach and dune sand develops, the lower flood plains of the large rivers on the north coast 

should be prospected for magnetite.

Abundant magnetite is found on the south coast from Ponce to 

Pa till as. Most of this magnetite occurs in beach sand and in the 

sand between cobbles, rather than in dune sand. Offshore sampling 

was attempted in one locality on the south coast and revealed a 10- 

percent content of magnetite for the sand of the sea floor 300 feet 

from shore in ten feet of water. Although the visible deposits on
RECONNAISSANCE, STUDY, BEACH SANDS, PUERTO RICO 303

the south coast are small, when the length of the beaches and the 

width of the shallow water zone are considered, this area may warrant 

further study.

LITERATURE CITED

Berkey, C. P., 1915, Geological Reconnaissance of Porto Rico: New York Acad. 

Sci. Annals, v. 26, p. 1-70.

 1919, Introduction to the geology of Porto Rico: New York Acad. Sci.,

Scientific Survey of Porto Rico and the Virgin Islands, v. 1, pt. 1, p. 11-29. 

Fettke, C. P., 1924, The Geology of the Humacao District, Porto Rico: New

York Acad. Sci., Scientific Survey of Porto Rico and the Virgin Islands,

v. 2, pt. 2, p. 117-197. 

Hodge, E. T., 1920, The Geology of the Coamo-Guayama District, Porto Rico:

New York Acad. Sci., Scientific Survey of Porto Rico and the Virgin Islands,

v. 1, pt. 2, p. 111-228. 

Hubbard, B., 1923, The Geology of the Lares District, Porto Rico: New York

Acad. Sci., Scientific Survey of Porto Rico and the Virgin Islands, v. 2, 

pt. 1, p. 1-115.

Kaye, C. A., 1956, Lower Tertiary of Puerto Rico: Am. Assoc. Petroleum Geologists Bull., v. 4.0, p. 108-121.

Lobeck, A. K., 1922, The Physiography of Porto Rico: New York Acad. Sci., 

Scientific Survey of Porto Rico and the Virgin Islands, v. 1, pt. 4, p. 301-379.

McKee, E. D., and Weir, G. W., 1953, Terminology for stratification and crossstratification in sedimentary rocks: Geol. Soc. America Bull., v. 64, no. 4, 

p.381-390.

Meyerhoff, H. A., 1931, The Geology of the Fajardo District, Porto Rico: New 

York Acad. Sci., Scientific Survey of Porto Rico and the Virgin Islands, 

v. 2, pt. 3, p. 201-360.

 1933, Geology of Puerto Rico: Puerto Rico Univ. Mon., Ser. B, no. 1,

p. 306. 

McGuinness, C. L., 1948, Ground water resources of Puerto Rico: Puerto Rico

Aqueduct and Sewer Service, San Juan, Puerto Rico. 

Mitchell, G. J., 1922, The Geology of the Ponce District, Porto Rico: New York

Acad. Sci., Scientific Survey of Porto Rico and the Virgin Islands, v. 1,

pt. 3, p. 229-300. 

Mitchell, R. C., 1954, A survey of the geology of Puerto Rico: Puerto Rico

Univ. Tech. Paper B. 

Roberts, R. C., 1942, Soil Survey, Puerto Rico: U. S. Dept, of Agr., Bur. Plant

Industry Soil Survey Report, Series 1936, no. 8, p. 1-503. 

Ross, C. S., 1926, Methods of preparation of sedimentary materials for study:

Econ. Geology, v. 21, p. 454-468. 

Semmes, D. R., 1919, The Geology of the San Juan District, Porto Rico: New

York Acad. Sci., Scientific Survey of Porto Rico and the Virgin Islands,

v. 1, pt. 1, p. 33-1.10. 

Thompson, W. 0., 1937, Original structures of beaches, bars, and dunes: Geol.

Soc. America Bull., v. 48, p. 723-752. 

Zapp, A. D., Bergquist, H. R., and Thomas, C. R., 1948, Tertiary Geology of the

coastal plains of Puerto Rico: U. S. Geol. Surv. Oil and Gas Inc. Prelim.

Map 85.
y
INDEX

Page 

Anomalous concentration of heavy minerals. 282,291

Barrio la Marina.-.-------.--.-.----------- 284,302

Batholith, San Lorenzo.___.___-._-----.--...- 277

Beach rock..-.-.-.............-..-----.-..--- 288

Beaches, cobhlc...--------------------- 280,287.290

Black sands.-----------------------------.- 291,301

Canos............--..-.......--...-.-.--...-- 285

Central complex...........--...--.-._.... 277,278

Chromite ................ 276,291,294,295,298,301

Chromium...-.-_._................ 297,298,300

Coastlines, character....-.---.-.-----__.. 278-288

Consol idatcd gravel................ 277, 278, 286, 288

Consolidated sand...-...-.. ..-.--- ..... 277-288

Dune sand, analysesDunes, fossil.........289

Epidotc...............---...----.-- 293,294,295,299

Geologic investigation, previous............ 274-275

purpose of study....-..--.....---...------ 275

Gold....----.--..-........-..-.--....-.-..... 291

Grain size.....---.--.--...-.......---..---- 288-290

Hematite---.---------.._................ 270, 294

Identification of minerals-.-------.--.-..-.--- 276

Ilmcnite.................... 276,291,293,294,299,300

Intrusions__................._..-.__ 277,281

Limestone.--.---.---....-.-.----------- 277,278,286

Limonite---..--.------------.--..--.--- 276, 294, 295

Magnetite, content of sands at selected localities....-. --............-.--..-.- 301-303

Maps-....---.-----..-............---------- 277,281

Page 

Nickel.-..--.--,---...-..-...-..-----....... 296-298

Niobium..---..--..-..-.-...-..----....-... 290,298

Percentages, mineral constituents........... 270, 292

Puerto Rico, summary of geology_.......... 277

Pyroxene...--..---.------------..-....- 294,295,299

Quaternary sediments.-...................... 277

Heels......................................... 288

Reserves...---.-...--..----....-..-.-..--.. 301-302

Sample fractions, heavy magnetic....-.---- 276,300

heavy nonmagnetic...-......---.... 276,292,298

light nonmagnetic...-...---.----...-... 276, 293

Sampling procedure.................... 275-276,292

San Juan formation..--....................... 277

ScaclifTs...---..---..--..---............ 284,285,286

Separation of samples....................... 270,292

Shores, classification,.-....................... 278

Size analyses...--..----.--...--.---........ 288-290

Source areas.....-...--................. 279-280,291

Sphcne............. 291,294,295,296,297,298,299,301

Stratification, beaches....-..--..----......--- 278

Strontium.---..--.------.-....-.---..-.-... 297-299

Swamps, mangrove......................... 281-286

Terraces,................................ 284,285,287

Tertiary rocks, early...-..-........--.--.,.... 277

later...................................... 277

Titanium........-----...-........-.. 297,298,300,301

Vanadium.--....................... 292,297,298,300

Zircon. 292,294, 295, 296, 297, 298, 299

305

U. S. GOVERNMENT PRINTING OFFICE: I9S7