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oo
Gallagher, Wm. G., Jr.

The geology and petrology
of the Hunter Point Region

1923

 
 

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GLADIS NUMBER: 184788008C FORMAT : BK AD:991012/FZB
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CUScCU

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SbDISS.GALLAGHER.GEOL 1923

Gallagher, Wm. G., Jr.

The geology and petrology of the Hunter Point Region.
$cl1923.

[11], 37 p. :$bill., col. maps ;$c29 cm.

Thesis (B.A. in Geology) -- University of California, Berkeley, May
1923.

Includes bibliographical references.

University of California, Berkeley.S$bDept. of Geology and
Geophysics$SxDissertations.

Dissertations, Academic$xUCBS$xGeology$y1921-1930

Microfilmed by University of California Library Photographic Service,
Berkeley, CA

 
   

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THE GEOLOGY AND PETROLOGY
OF THE
HUNTER POINT REGION

 

LIBRARY COPY

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THE CEQLOGY AND PETROLOGY

HUNTER POINT REGION

Index

THTRODUCTION ee ss evesvonescesssosoetesscsccscnsonsonnss
PURPOSE ei evsecssersssrseorsssosessscsssssssssnnsnnse
ACKNOWLEDGMENT Se vassessvveesssssesscssssncssens
LOCATION eeveessasessnssstosssssesnsscsnsssenasnsss

PROBLEM OF THE ACE OF THE FRANCISCANeeeoevevsoevenns
GENERAL SP A El Teves esnncrssnsvevasssnvsssnvne
FOSSIL EVIDEICEesesessesassssscsscssssssscssssse
STRATIGRAPHIC EVIDENCE IN OREGON

AND NORTHERN CALIFORNIA weer terssnsvensnnven
EVIDENCE OF TRIASSIC AGEe¢esrtveesrssrsccscsnsanses

CGE OLOGY eco cessvssrestesssstssssvssesesvassssesnsssnny
GENERAL STATEMENT e cee eesaveesscssccscsscssesens
CONGLOMERATE ¢ tes eects vets ssscct tor vsssecsesssneny
RADIOLARIAN CRERT sevessrsnssserssessvsss tonnes
SANONSTOME. . vs erst tnnrs svssenssossverrms sone ves od
IGNEQUS BOOKS ess estersssvsnsnssossnvbevennnvevsdd
SRE TIRE veer sovarvansvetosssesnsvnrnesnsennsentd
LR ROLY Th ev bev sruseesnssestsssnssnssorssnsrseeie
DIABASE wc. exvavsasrasrivs astern sesarsarseslp
RELATION OF LHERZOLITE TO SANDSTONEeseessessseel
RELATION OF SERPENTINE TO DIABASE evs vers veevesdil
PILLOW BASAL Fe vecsnsvnevanssstmntssiesnistsseverd
DISCUSSION OF PILLOW BASALTeeecsssovnvvssnnnssnd]
BRO OT acne ncsvsrtssnnsts tenets snssavesssvevis
STRUCTURE «vse crsvessestsnvnvssessvsvernnvsnsevedd

General FeatuTeB.. +r cevtrcsssssvinstrssvssndy
PALE Be svc crores rants sasnsesnvesnsvenvetwls

e~d~d Ul PDN pee

 

SUMMARY EE e « tesvtvanssssrtsssres vy rion snes eros

 
INTRODUCTION

PURPOSE

The purpose of this report is to record a detail
study of rocks encountered at Hunter Point, and in partial
satisfaction for the degree of A, B., geology mjor, Col=-

lege of Letters and Science. .

ACKNOWLEDGMENTS

The writer wishes to express his grateful appre-
ciation to Dr. R. R. Morse for his interest in the work
and helpful suggestions. To Mr. T. P. Andrews of San Fran-
cisco, the writer is indebted for the map used, and for

data on the various bore holes.

LOCATION

Hunter Point is & peninsula located in that part
of San Francisco known as South San Francisco, and about
one and three Quarters miles south of Potrero Point. A
long ridge runs the length of the peninsula terminating in
a steep bluff at the point. The highest point on the pen-
insula attains the altitude of 275 feet approximately at
the intersection of La Salle Avenue and Keith Street.

The Union Iron Works Company's dry docks are lo-

cated on the northeast portion of the point and are actively

engaged in business at the present time.

PROBLEM OF THE AGE OF THE FRANCISCAN GROUP

GENERAL STATEMENT

In view of the fact that the study of Hunter
Point area involves rocks of the Franciscan group entirely,
a resume of the present status of the age of this series
is in order.

The age of the Franct sean group presents an un-
usual and interesting problem which has never been satis-
factorily solved. The chief difficulty being, first, the
almost complete absence of fossil evidence and, secondly,
the contradictory results obtained by running down various

sets of field data to their natural conclusion.

FOSSIL EVIDENCE

One of the characteristics of the group is the
lack of fossils. In the various sedimentary members there
are but few marine mollusca reported ur to the present.
An Inoceramus was found by Whitney (13) in the sandstone of
Alcatraz Island. A fragment of either an Aucella or an
Inoceramus was found two and three quarters miles south of
San Mateo. (1) A fragment of an Inoceramus was found in
a schooner load cf rock, which had come from either Goat
Island or Angel Island.

In Monterey County, at Slates Springs, on the

west flank of the Santa Lucia Range there occurs an area

of Franciscan rocks from which various fossils have Deen

found.

 
 

Professor J. C. Merriman(3), in 1896, sent four

epecinmens of plant remains to L. F. Ward, whc in turn sent
them to W. M. Fountain to be determined. The result of
this investigation showed that the plants were in a highly
transitional stage and probably belonged to the lowest Cre-
taceous.

~ Charles H. pavis(¥), after studying the inverte-
brate fauna of the Franciscan at Slates Springs, concludes
that the forms are not pre-Jurassic, and that they are
closely related to the described forms from the Middle Ju~
fassic of Cook Inlet and the Alaska Peninsula, and from the
Jurassic of Queen Charlotte Islands. He believes the beds
to be nct higher than the lower Upper Jurassic. b

Fairvanks, in 1855, with F, MU. Anderson visited

Slates Springs and collscted a nurber of plant forme and
five species of Lamellibranchs. The latter were submitted
to T. VW. Stanton(16) for determination. He reported that
none of the specimens could be referred to described species,

thet the Inoceramus was very probably not older than, and

 

probably of Jurassic age, ut might be Cretaceous. The |

other fossils, while indeterminate, do not conflict with

this inference.

i an. cm cn. om A

STRATIGRAPHIC EVIDENCE IN
OREGON AND NORTHERN CALIFORNIA

 

In Oregon there are a series of rocks that appear

ee

to be the equivalent of the California Franciscan group. | 3

Professor Loudervack(?) reported on this series for the

Carnegie Institute in Washington and mentions that: The
"Jurassic" shales of the Port Oxford area are scmetimes re-
ferred to as slaty shales. This partially metamorphosed
character, the association with the Dillars formation and
the faot that they could not, in the field at any locality,

be separated in characters or appearance from the "lyrtle"

formation, would suggest that they are really Dillard, or

that the Dillard (Franciscan) is, in part at least, Juras-
gic. It is possible, however, that these beds may be stra-
tigraphically below the Franciscan.

In all its characteristics and peculiarities, the
Dillard series is practically identical with the Franciscan

and it fits into the same stratigraphic position.

Diller (8) compared the formations of Oregon and
California in the vicinity of the Klamath lbountains to ve-

rious Californie formations and correlated them as follows:

Oregon California

Chico oe Chico Upper Cretaceous

"Lyrtle" Horsetown Lower Cretaceous
" Knoxville

Dothan Franciscan

Galice Maripcsa

The Galice series is lithologically like the
Mariposa slates and contains fauna similar 0 that of the
Mariposa.

The Galice (Monte de Oro) beds are composed of
olaites, sandstones and conglomerates in small layers, ir-

regularly bedded. The fossil evidence indicates Jurassic

 
 

rocks to the Jurassic or Triassic, s0 Smith did the same in

age, equivalent to, or older than the Mariposa formation, 4

in an opinion given by Stanton.

In another paper on this same series pi11er(9)
statee that the mass of volcanicswhich overlies the Dothan
series is the same that has been shown to lie beneath the
Galice series. This succession illustrates their age unless
they have been overturned, a condition which is suggested by
the planes of displacement. Diller cites several localities
where there is definite evidence of thrust faulting, there-
fore the Dothan (Franciscan) is younger than the Galice

(Mariposa) even though it is etratigraphically below it.

EVIDENCE OF TRIASSIC AGE

smi th (10) examined the cherts in Oregon in a 1lo- »
cality 25 miles east of Roseburg, and found several distinct
genera of radiolaria. Identical forms were found by nim(11)
in material 1ithologically similar in the Philippine Islands.
The same formations are found in Borneo, Java, liollucoas,

Ceram, eto. Martin(12) and Hinde(1%) have assigned these

the Philippines.

The cherts of Rote and Savu are associated with

 

veds of limestone containing Halobia and Daconella and are
therefore Triassic. Smith cites ~Hinde(l7) to the effect
that these cherts, as described, are strikingly similar to

our Pacific Coast rocks of this character.

pavis (8) calls attention to the fact that there ; i’ ()

is a possibility that Diller may be mistaken as to the re-
lation of the Galice to the Dothan. In case these two se-
ries are now lying in their proper historical sequence, the
Dothan is the older formation, sc the Franciscan must be pre-
Mariposa. It may be lower or middle Jurassic or Triassic.

professor A. C. Lawson(l) sums up the problem in
an extremely clear and concise manner. He pointe out that
the Franciscan is overlain unconformably by the Knoxville
(Lower Cretaceous) and is therefore pre-Cretaceous. This
contact may be seen in several exposures in the immediate vi-
cinity of Serkeley.

In several folios of the U, S. G. S. the granite

“tof the Sierras has been mapped as of post-Jurassic age. Law-
gon believes this granite extends continucusly around the
southern end of the Great Valley of California into the
Coast ranges, where it forms the complex upon which the
Franciscan rests unconformably (18); the granite is nowhere
geen intrusive into the Franciscan and includes no fragments
of Franciscan rocks.

It follows then that the Franciscan series is not
older than post-Jurassic and cannot .be younger than pre-
Cretaceous - the two conclusions being in decided conflict.
Lawson suggests that: (1) either the geologic time scale
would have to be extended or (2) assume a period of batho-
lithic development in the Coast ranges was distinct from,

and older than that in the Sierras. He states that neither

 
assumption may be made without further and fuller investiga-
tion.

The problem will have to stand at that, for the
complete paleontolozical data to date shows nothing definite -
that the Franciscan is in a "transitional stage."

Though no further data on the age of the Franciscan
group was obtained in the study of the rocks of Hunter Point,
there may be found there some strong suggestions as to the
sequence within the Franciscan, particularly in reference to

the various igneous rocks.
GEOLOGY

GENERAL STATEMENT

This region has been mapped in the preparation
of the San Francisco Folio, and, as partially indicated in
the folio, the sedimentary formations are as follows: oon-
glomerate, Marin sandstone and Sausalito chert. The igneous
rocks consist of serpentinized peridotite, diabase, pillow
vasalt, and andesite. With the exception of the recent al-
luvium, all the formations mapped are of Franciscan age.

In general the structure consists of broadly fold-
ed sedimentaries, which have been intruded by a great sill
of peridotite, now highly serpentinized. Diabase dikes cut
the peridotite in various localities. On the point of the
peninsula a reverse fault has elevated a block of chert

against a peculiar spheroidal greenstone, locally called pil-

low basalt.

CONGLOMERATE

The conglomerate consists of Loulders and pebbles
of various size, well-rounded and waterworn. Fragments of
chert, Franciscan sandstone, diabase, serpentine and basalt
may be recognized. The am trix consists of a very well cem-
ented sand. This formation is exposed in two localities as
indicated on the map. At locality #1, the exposure is quite

limited and is cut by a curved joint system. (See pictures

1 and 2.)

The ma jor portion of the exposure at Sandstone

Point (locality #2) consists of a coarse, moderately indu-

rated, brownish colored sandstone, which is, in part, highly
conglomeratic. The included rock fragments are the same as
in the locality described above. This exposure bears no
lithologic resemblance to the Marin sandstone. This fact

is mentioned, for in the San Francisco Folio this locality
is mapped as Marin sandstone, descrived later under Francis-

can sandstone.

The writer does not agree with this, for, as stated
vefore, this conglomeratic sandstone contains fragments of the
Marin sandstone as well as fragments of at least two young-

er formations, the chert and the serpentine. (See picture 3).

RADIOLARIAN CHERT

These cherts are noticeable for their hardness and

 
 

 

their resistance to weathering and decay. The cherts are 1

very well bedded and consist of layers of chert alternating
with shale. The chert layers being one inch to five inches
thick with a general average of two inches. The usual thick-
ness of shale varies from one-eighth of an inch up to one

inch.

There is no gradation between the shales and cherts,
the separation being clean-cut. The prevailing color is
red and brownish-red with minor amounts of green,

The chert veds are locally highly contorted, show-
ing intricate plications, The presence of many layers of
ghale undoubtedly makes the chert incompetent and suscep- |
tible to folding. :

A common feature is the pinching and swelling of »
the chert beds, one ved swelling a little and the next com
pressed a little to compensate for it.

The fracture is generally smooth and conchoidal,

though sometimes the fracture is irregular or hackly. The

ee a I. Po

lustre is dull vitreous, texture dense and compact.

Within the red siliceous matrix there are darker

spots of pure silica, barely perceptible to the eye, which

Ld a

represent the remains of radiolaria, a minute sea organism

SE ——

having a silicious test or skeleton.

The color is in all probability due to red iron

oxide.

J

In chemical composition it is about 95 percent

. ish-gray to oluish-gray in color. The texture is dense,

10."

silica. (510,).

SANDSTONE
When fresh the sandstone is typically dark, green-

with closely packed grains. The strength of the cementing
material holding the grains of sand is shown Ly the fact that

in breaking the rock the fracture passes through the mineral 4

grains as readily as through the cement.

In general, the sandstone is medium to coarse
grained. In hand specimens quartz, feldspar, muscovite and
bictite my be recoonized under the lens. Sandstone of this
description my be found along the beach in the vicinity of
locality #3 on the map.

When exposed to action of the weather, the sand-
stone decomposes. The process of alteration begins with a

discoloration of the rock, it rapidly loses its bluish or

greenish color and becomes first yellowish and then brown,

At this stage the cement is considerably weakened. On break-
ing such a discolored sandstone, the grains pull out of the
cement instead of fracturing as they do in fresh rock. The
decomposition continues until a sandy soil results.

In the fresh state the denseness of texture, color,
mode of fracture and the presence of fresh feldspar cause
it to resemble an igneous rock.

The sandstone formations contain very minor amounts

of shale which are not extensive, but appear locally.

 
 

Under the microscope, its clastic texture is at
once apparent, the grains being very angular with remark-
ably sharp boundaries. The rock is composed of about 50
percent quartz, 30 percent feldspar in the form of ortho-
clase characterized by a few recognizavle Carlsbad twins
and plagioclase, probably oligoclase. Many of the oligo-
clase crystals are quite cloudy, due to kaolinization, while
others are remarkably fresh. luscovite and biotite are
present in small quantities. UMagnetite is present in small
opaque masses scattered throughout the rock. Zircon may be
recognized in short, prismatic crystals showing its charac-
teristiocally high birefringance. The interstitial material

is very fine and impossible to determine. (See picture 4). be

IGNEQUS ROCKS

There are four distinct igneous rocks in this re-

gion:
1 - Lherzolite, from which the serpentine *
is derived.
2 - Greenstone. |
7 - Andesite.
4 - Diabase. |
SERPENTINE

The serpentine appears in rounded hills which are
bare and strewn with rock fragments. Serpentine outcrops
are quite common. There are two distinct varieties or

facies which may be called: |

 

$7

ll

l - "Slickensided facies."
2 - "Jasgsive facies."

~~ By far the greater part of serpentine found on
Hunter Point is of the slickensided facies which is chiefly
characterized by abundant evidence of internal movement as
indicated by the presence of surfaces both plane and curved,
intersecting the rock mass in all directions along which re-
lative motion of the rock particles has taken place.

These surfaces are in reality slickensided sur-
faces, being smooth, polished and have a greasy feel, like
talc. The rock has a large variety of colors, with a pe-
culiar bluish-green tint prevailing.

In limited areas the serpentine has saoaped the
general shearing action, is quite massive and dark green in
color. This variety is generally found in spheroidal masses.

The process of serpentinization, or the change

from peridotite to serpentine may be seen in all stages.

LHERZOLITE

The rock from which the serpentine is derived is
exactly described by Palache (24), He worked on the region
known as the Potrero, in South San Francisco. The serpen-
tines of this region are continuous with, and about one
and one-half miles distant from the serpentines exposed at
Hunter Point.

At Hunter Point, with a hand lens, the rock has

a pseudoporphyritic aspect, for large grains of pyroxene

 
 

with brilliant cleavage faces. are found embedded in a

ground mass of serpentine. Grains of magnetite may be re-

cognized disseminated here and there throughout the mass.
Under the microscope the freshest specimens ob-

tainable show serpentine to be the predominating mineral.

The serpentine exhibits the very characteristic lattice struec-

ture, where the bands of parallel fibers are in a position

corresponding to the dominant cleavage of the original en-

statite, or diallage, as both the orthorhombic and mono-

clinic forms of pyroxene are present.

Chromite occurs in scattered grains embedded in
the serpentine, while magnetite may Le recognized as an
opaque mineral in small veinlets and cracks.

No trace of feldspar or olivine could be found

in this rock. (See pictures 5 and 6.)

HYPERSTHENE DIABASE
In hand specimens this rock presents a dark gray,

rather mottled appearance. It is conspicicusly diatasic in

texture, for the weathering of the ferromagnesian minerals
nave left the interlacing feldspars in relief.

The recognizable minerals are feldspar; usually
somewhat altered and milky in appearance; hornblende in
pladed orystals and grains of pyroxene.

Under the microscope the feldspar is observed
to have an extinction angle of about 16°, which would pro=-

ably place it as an acid labradorite. Iuch of the feld-

 

|
|
t
|
|
§

 

9

spar is decomposed probably to sausserite.

The hornblende is greenish-yellow, pleochroiec
and allotriomorphic to the feldspar (pictures 7 and &).
Orthorhombic pyroxene, probably hypersthene occurs in
grains of considerable size. It is slightly pleochroiec.
Extinction parallel to a distinct cleavage.

Magnetite occurs as inclusions in other consti=-
tuents.

Chlorite, as a scaly aggregate, Occurs in smell
amounts as a decomposition product, filling the interstices

among the other constituents.

LATION OF LHERZOLITE TO SANDSTONE

~ At locality #3 there is a very good exposure of
the serpentine in contact with the gandatone showing the
serpentine is intruded into the sandstone. In many regions
in the vicinity of San Francisco similar relations between
sandstone and serpentine may be seen. The contact itself
appears to be a shear zone, probably due to the adjustment
of the serpentine as it became hydrated. There is no doubt

but that the serpentine was intruded into the sandstone and

is younger than it. (See pictures 9 and 10.)

RELATION OF SERPENTINE TO DIABASE
Where the serpentine is exposed there may be
seen irregular, linear outcrops of diabase. These outcrops

are frequent but not very extensive. Their general appear

 
ance being of a group of boulders lying on the surface.

In a road-cut at locality #9 may be seen a large boulder

of diabase completely embedded in the serpentine. On the
south side of the point where the serpentine may be seen
resting on the sandstone, are several dikes of diabase com-
ing up through the sandstone but apparently sheared off at
the contact. In places, however, dikes of diabase are

seen outting the SeTpeting, The age relation then is un-
doubtedly that the diabase, in the form of dikes, was in-
truded through the! sandstone and peridotite, and was later

vroken into large blocks ty the shearing action of the pe-

ridotite in the process of serpentinization. (See pictures

11, 12, 13, and 14.)

"PILLOW BASALT" OR SPHEROIDAL GREENSTONE

This rock may be found at the extreme end of the
peninsula along the vicinity of locality #5, and outcrop-
ping from low tide level to heights varying up to 25 feet.
Above this the basalt presents a different appearence due
to weathering.

The very characteristic form of the outerop sug-
gests the name of spheroidal or pillow structure. Photo-
graphs 15 and 16 show these ourved figures are the trans-
verse sections of elongated, bale-like nasses, whose axee
are roughly parallel. A certain amount of sheared and al-

tered me terial, consisting to a large degree of chlorite

forms the matrix in which these spheroids are found. The

Br rT
Re  ——i,

 

 

=

remarkable feature of this exposure is not so much the elon-

‘gation of the spheroids in one direction, but the striking

regularity and parallelism of the matrix above and below

the spheroids. (See pictures 17 and 18.) Small folds may
be observed at various points, and in every 08.88 each in-
dividual layer of spheroids may be seen conforming to the
fold from the base of the cliff to the top of the exposure.
The folds, while wide, are always more acute at the bottom

of the cliff and die out toward the top. A decidedly strong
mpresston of the spheroids being separated by bedding planes
ie given even after a most detailed and critical insepotion
of this whole section.

The writer is of the opinion that this is an ex-
trusive rock, very probably formed according to the hypo-
thesis advanced by I. Sandis in 1912.

The spheroids are firm and sound and are more or
less impregnated with veins of calcite. With a hand lens,
little or nothing may be made out, for the rock is exceed- |
ingly dense and entirely aphanitic. At low tide specimens
may be obtained which are impregnated with pyrite. The
pyrite seems to be more highly concentrated along fissures
and smll veinlets n the rock and is probably secondary.

Under the microscope some very smll lath-1like :
miocrolites my be seen embedded in a glassy matrix. The
feldspars are too small to determine. (See picture 19.)

From the summit of the hill to within 15 to 25

 
feet of sea level, the rock, while still retaining its
spueroidal characteristics, is highly altered and decom
posed. It is almost impossible to obtain a fresh speciman,
The rock fractures in hackly pieces with sharp edges. No

matter where the rock is broken it shows the stain of iron

oxide. It is no longer firm and does not ring when hit

with a hammer.

DISCUSSION ON PILLOW BASALTS

"Pillow" or ellipsoidal lavas have been noted
and reported from time to time, and, while this structure
is uncommon, it is by no means rare.

¥. Sandis(20) cites various writers who advance
the following hypotheses to explain the development of pil-
low lava:

Due to contraction.

Concretionary.

Mechanical brecciation.

Accumulation of lava bombs.

Explosive eruptions.

Regarded as similar to aa.

Regarisd Se similar to pahoehoe.
Geikie(2l) emphasized the pronounced
association of pillow lava with marine
deposits, and concluded the pillow lava
must be 9 Syounz ing phenomenon,
Andersoni@ describes the actual forma-
tion of pillow lava in his description
of the volcano latavanu on Savii., He
states that the lava flows out into the
water and extends itself into buds or
lobes, still connected with its source
of supply and having its surface godled
tc a pasty condition. These lobes con-
tinue to accumulate to form the deposit.
He concludes by definitely stating that
at least one form of pillow Laszlt is
produced by the chilling action of water
on the surface before it has time to as-
gume the corded structure of pahoehoe.

034 OWI FA 0

\O
1

 

Saniis is in entire agreement with Anderson and ex-
plains several deposits of pillow basalt Ly this hypothesis.

Capps (83) describes in detail a series of ellip-
soidal or pillow lavas near Ellamar, Alaska, which he believes
were, without a doubt, subaqguecus flows. He points out that

the flows were interrupted by sedimentaries consisting of
o

slates and graywacke.

ANDESITE

The fourth igneous rock exposed may be seen at
low tide only at locality #6. This rock, while different
from the pillow basalt in texture, has the same peculiar
spheroidal forms separated oy a highly altered matrix. With
a lens, large plagioclase feldspar crystals may he seen em
vedded in an exceedingly dense groundmass.. The groundmass.
is conspicuously spotted with smell dark-colored amygdules.
The large feldspars may be seen to De full of dark inclu-
sions.

Under the microscope, large crystals of a medium
cligoclase may be seen embedded in a groundmass. of glass
matted with lath-shaped microlites. The large feldspars
nave been altered somewhat, and patches of chlorite may be
seen within the feldspars. Small cavities in the ground-

masse are filled with secondary silica, usually having a

rim of chalcedoney, and minute quartz crystals radiating

from the center toward this rim. (See pictures 20 and 21.)
The relation of the andesite to the adjoining

 
greenstone and sandstone is not clear, due to the very li-

mited exposure (about 25 fzet square). There are three
possible explanations for its occurrence:

1 - That it is a gradation of pillow basalt
into a pornhyry.

2 - That it was faulted into its present po-
sition from some flow or intrusion which
does not outcrop elsewhere.

3 - That it is a shallow dike following the
fault zone between the greenstone and
the sandstone.

STRUCTURE
General Features

‘The structure of this region cannot be worked to
the complete satisfaction of a critical observer, due to
the fact that reliable dips and strikes are impossible 10
obtain. As indicated on the map, the major part of this
area is covered by a highly serpentinized intrusive, the
gandstone, which cutcrops only below the serpentine is mas-
give, with a few bedding planes and shale lenses distorted
in proximity to the intrusive. The chert beds are highly
contorted where exposed, while the conglomerate Leds are

mssive and show no bedding planes or sorting action in the

distribution of the rock fragments. Approximate dips and

re EE =

strikes taken along the beach indicate two synclines sepa-

rated by a low anticline as shown in the structure section,
The axis of the folds strikes approximately north and south,
transverse to the peninsula, and plunges toward the north.

Faults

There are two main faults in this area with several

 

«9

«BO |

minor and comparatively unimportant subeidiary faults. Ly
far the most important displacement is that which forms
the line of contact between the chert and the greenstone.
Picture 22, taken at locality #7, shows this contact. Upon
casual examination this appears to be an intrusive contact,
with the greenstone cutting a sinuous and transgressive rath
through the chert. A detailed study of this outerop, however,
ghows that the sinuosity is due to small displacements along
incipient surfaces of displacement. This is indicated in
picture 23. A well defined shear zone may be observed,
which follows the contact for a considerable distance and
then passes through the chert, as shown in picture 24.
Along the vicinity of locality #8, the sandstone and chert
appear above the greenstone due to this fault,

Standing on the contact on the south side of the
quarry, locality #9, and looking across on the north side,
a typical extrusive contact may be seen with the greenstone
lying above the edges of the contorted chert. This is shown
in picture 25. The appearance, texture and extrusive type
of contact indicate that the greenstone was a basic vol=-
canic rock which flowed out upon the chert. This fault is
therefore a reverse fault having a vertical displacement of
at least 130 feet. The fault surface dips approximately
35° toward the north,

The small faults in the vicinity of locality #6

are responsible for the small exposure of the Franciscan

 
  
  

    
   
  
  
  
   
  
    
 
    
   
 
  
 
  
    
  
   
    
   

C (Marin) sandstone. 0 The sedimentaries are broadly folded with the
The second major fault is located on stratigraphic axes of the folds running approximately north and south,
evidence based upon the correlation of the conglomeratio ~The peridotite has caused local distortion of the sedimen-
sandstones of localities #1 and #2. If this correlation be . taries in the vicinity of the contact. The folds are oros-
correct, there is a decided displacement passing through the sed by a reverse fault, along which the cherts and over-
swamp between locality #2 and the inland. lying ellipsoidal greenstones are thrust south over the

greenstone at the tip of the point.

SUMMARY STATEMENT ;
The study suggests the following sequence for
The problem of the age of the Franciscan Group
the Franciscan members here exposed, from older to younger:
is still unsolved. Nothing is contributed to this problem
l - Hard, greenish-blue, arkosic sandstone

in the study of the Franciscan of Hunter Point. with shale inclusions (larin sandstone).
2 - Radiolarian chert (Sausalito chert).
Within this area the Franciscan consists of sand- 3 - Peridotite (Lherzolite) intruding the
| members above,
gtones, cherts and conglomerates, intruded by a great sill 4 - Diabase dikes intruding the members above.
: 2 5 - liageive, brown, poorly cemented, highly
( of peridotite (lherzolite) now largely altered to serpentine. conglomeratic sandstone and conglomerates.

6 - Spheroidal greenstones (Pillow Basalt).
Still later igneous action is evidenced by diabase dikes, :

which cut both sediments and intrusive.

At the extremity of the peninsula an ellipsoidal
greenstone may be seen lying upon the upturned edges of a
contorted chert bed, giving the characteristic appearance
of an extrusive contact. The texture of the rock, the pe-

culiar parallel arrangements of spheroidal greenstone em-

 

bedded in an altered matrix, and the presence of & well-
‘defined fault in the immediate vicinity, are strongly in-

dicative that the greenstone is intrusive rock, but very pos-

eibly originated as a subaqueous flow. Loose fragments of

gherts would tend to float on the fluid extrusion due to | | ©)

this chert inclusions may be seen in the greenstone.
 

10,

il,
12,

13.

18.
15.
e0.
21,
22.

23,

24,

REFERELCES

Lawson, A. C., San Francisco Folio, U, 8. G. S.
Folio #193.
Knopf, A., An alteration of Coast Range Serp.
U. C. Dept. Geol. Bull. Vol, 4, pp. 425-430.
Merriman, J. C., 20th Ann. Rep., U. 8. Geol. Surv.,
pt. 2, pp. 332-339, 1896.
Davie, Chas. H., Jour. Geol., Vol. 21, p. 453, 1913.
Louderback, G. D., Jour. Geol. Vol. 13, p. 550, 1905.
Lenses, C. A., U. 8. G. S., 15th Ann. Rept. pp. 415-35,
95. wh
Davis, E. F., U. C. Dept. Geol. Vol. II, #1.
DLijes J. S. Bull. Gecl. Soc. Amer. Vol. 19, p. 369,
QE&.
Diller, J. S. Bull. Amer. Jour. Sci. Vol. 23, 4th
Series, p. 410, 1307.
Smith, W. D., Hotes on Radiclarian Cherts in Oregen,
Amer. Jour. Sci. Vol. 42, 13916.
Smith, W. D., Philippine Jour. Sci. Vol. 5, #5, p. 327.
Martin, XK. Reisen in den Molukken, Leiden, p. ich 1902.
Whitney, J. D., Gecl. Surv. of Calif., Gecl. Vol. 1,
Philadelphia.
Hinde, App. I, 9, lblengraaf's Borneo, 1902.
Fairbanks, cited by Lawscn in 1 above.
Stanton, T. W., Jour. Geol., Vol. 3, p. 423, 1895.
Hinde, Radiolarian Rocks, etc., Jaerboek van het
Mi jnwessen in Nederl, Oost, Indid XXXVII, 1, 2, 1908.
U. S. G. S. Folio #101.
Palache, Chas., U. C. Dept. Gecl. Bull. Vel. 1, #5, p. 17G
Sandis, N., Pillow lava from the Kiruna Dist. Geclogiska
Foreningen Forhandl, Stockholm Bd. 34, p. 328, 1912.
Geikie, Archibald, The Ancient Volcanoes of Gt. Br.,
Yol. 1, London, p. 193, 1897.
Anderson, Tempest, Volcano of llatavanu in Savii,
Quart. Jour. Geol. Soc. XLVI, #264, p. 638, 1910.
Capps, 8. R., Some Ellipsoidal lavas on Prince William
Sound, Alaska, Jour. Geol., Vol, 23, p. 45, 1915.
Palache, Chas., Lherzolite. Serpentine, U. C. Dept.
Geol. Bull, Vol. 1, #5, p. 165.

 
 

 

 

 

 

 

 

-]=-
Conglomerate outcrop.
Locality #1. Curved
joint system. »

 

Conglomerate. Locality #1.

—— A ————

 
 
 

RETAKE OF PRECEDING FRAME 2

 

 

 

-]w
Conglomerate outcrop.
Locality #1. Curved
joint system. :

 

Bo
Conglomerate. Locality #1.

 
 

 

: eo
Conglomerate, Sandstone Point,
Locality #2.

 

 

Franciscan sandstone, showing
remarkable angularity of the :
grains.

 
 

-3
Conglomerate, Sandstone Point,
Locality #2.

 

Franciscan sandstone, showing
remarkartle anzularity of the
grains.

 
 

Lherzolite, showing altera-
tion of pyroxene to serpen-
tine. Development of lattice

structure.
b=

 
INIVAAINTY Ul 4 ANAUCU RJATALINT 1 ANAL VR RJ

 

Lherzeolite, showing altera-
tion of pyroxene to serpen-
tine, Development cf lattice
structure.

 
 

ri

 

Labradorite with

interstitial pyroxene.

Diabase.

 
:

1

Diabase
interst

 
-28&-

 

Contact, sandstone over
serpentine. 150 feet
’ east of Locality. #3.

 

 

=O
Sandstone inclusicns in

( ; gerpentine. 175 feet

m east of Locality #3.

 
ANA J A FRANCA JA A ANA JIN RJA AL NJ A AN AlVARJ

 

-28m

s

Contact, sandstone over
serpentine. 150 feet
east of Locality.#3.

-10=-
Sandstone inclusicns in
serpentine. 175 feet
east of Locality #3.

 
 

 

 

 

Characteristic diabase out-
crop resting on serpentine.

-12<

 
TR AS AA NJ A ANALIVYARY

 

 

-3lw
Characteristic diabase out=-
erop resting on serpentine.

 

 
 

en p————— pre ——— —- : -—

-13=
Diabase inclusion in ser-
pentine. 200 feet east of
intersection Hudson and Fol-
well Streets.

 

Diabase dike in sandstone
sheared off by serpentine.

 
 

-13=
Diabase inclusicn in ser-
pentine. 200 feet east of
intersection Hu.son and Fol-
well Streets.

 
le

 

 

Spheroidal forums of green-
stone near tip of point. Pil-
low structure.

 

 

 

-16~
Parallel arrangement of sphe-
roidal forms.

 
27
o

 

$n, oy - Cn my ”
Spheroidal forus of
stone near tip Cf p
low structure.

 

el arrangement of sphe-
forms.

 
 

 

 

-17-
Parallel arrangement of
greenstone spheroids near
tip of point.

-18-

 

 

 
 

“

greenstone sphzrc
tip of »oint.
-18-

 

 
=F

 

 

-1Gm

Microlites

in glassy groundumass
cut by seam of calcite.

Greenstone.

 
wy wy

 

-10-
Grzenstone. llicrolites
in zlassy groundrass
cut wy seam cf calcite.

 
 

> -20-
- Andesite. Large cryst of
r oligoclase in amygduloidal
groundnass containing glass
and microlites.

 

Ciera

 
—_
- 5

 
 

 

Apparent intrusive con-
tact, greenstone and chert.

 

-23-
Incipient faults below

fault contact, chert and
greenstone. Locality #7.

 
AT At ae Jl JOO J. MTR —"dl “Ama DTA

7

 

Arparsnt intrusive con-
tact, greenstone and chert.

 

-23-
Incipient faults bel
fault contact, chert and
greenstone. Localit

 
 

A pe ei sini

 

 

-2Y~

Shear zone fault con-
tact between chert and
greenstone. Locality #7.

 

North sid
feet west

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a

~25=
e of quarry 150
of Locality #7.

  

 
       
 
    

Shear zone fault con-
tact between chert and

I]. =

greenstone. Locality #7.

 

-25-
orth side of juarry 150
feet west of Locality #7.
 

 

 

 

 

-26-
Stringer of chert in-
cluded in greenstone on
veach 250 feet southwest
of Locality #7.

 
 

Pr. _y

 

 

-26~
Stringer of chert in-
cluded in greenstone cn
seach 250 feet scuthwest
of Locality #7.

 
 

 

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BASIN

J CONTOUR MAP

SHOWING PROPERTY OF

ANCISCO, CALIFORNLA.

iD C. HOLMES 6. CONSULTING ENGINEERS.

e. 450 feet to kinch.

HT TREN

Sout SAN Francisco Dock Co.

JAS.J. WALSH C
RETAKE OF PRECEDING FRAME

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762 [© Ye SAN FRANCISCO. CALIFORNLA.
HOWARD C. HOLMES @.CONSULTING ENGINEERS.
Te Scale. 450 feet to Linch.
PROM CITY BASE AS AZ ERS DLYERY IOFOOT ELEVATION Jan. 3. 1922. JAS.J. WALSH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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END OF TITLE