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Suman, John R.
A contribution to the

petrography of the
Rosamond Series

1912

 
 

 

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040 CUScCU

090 SbDISS.SUMAN.GEOL 1912

100 1 Suman, John R.

245 12 A contribution to the petrography of the Rosamond Series.

260 $c1912,

300 i, 26 leaves :8¢29 om.

502 Thesis (B.S. in Geology) -University of California, Berkeley, 1912.

610 20 University of California, Berkeley.S$SbDept. of Geology and

GeophysicsS$SxDissertations.
690 0 Dissertations, Academic$xUCB$xGeology$y1911-1920.

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A CONTRIBUTION TO THE PETROGRAPHY OF THE

 

ROSAMOND SERIES

by

 

John R. Suman. aes”
LIBRARY COPY

Thesis for B.S. Degree.
1912.

 
A CONTRIBUTION TO THE PETROGRAPHY
OF THE

ROSAMOND SERIES

by
CONTENTS.
page
Introduction Cnr a are ieee ese ves sss seven sieve]

General Description of the Rosamond Series cccesceececcvcseel
The Rocks of the Rosamond Series ececsececcesscescscascseseeed
The Granitic Basement ROCK estcevtesecresessosssssssssnnened
The Tuff=Breccia of the Rosamond Series eeecsesccecccessnsal
Tuff from Near the Base of the Rosamond Series essee«.9
Charecter of Tuff Found Outcropping at the Base of
Hills South of First Stamp Mill We. of Rosamondesess«.10
Opelized Tuff from the Rosamond Series seccceeccvceces os 11
Altered Tuff from the Rosamond S€ries eceeecee: :oveeeeld
Tuff Collected at First Stamp Mill We. of Rosamonde.«ss 12
Tuff Occuring in Angular Fragments and Bedded eeesc..eel3
Nature of the Volcanic Glass from the Tuff of Rosamond.l4
Rhyolitic Laves from the Rosamond Series sesceccceccececceedld
Rhyolitic Perlite ¢.vcceeeciesecrcntnrecsancccnnccneaesl?
White Rhyolite from Soleded Pe&k «..eeesecessacececeeeel8
Amypgdeloidal Augitetendesite from the Rosamond Series ee.<e.l9
Note on the Rocks from Black Mountain ececcevvcecinncececeeell
Olivene=busalt from Black Mountain eccecccecccncscccaeeesld
Tuff from Black Mountain eccececececcccscccsssecseneeeed
CONCIUBLONS sev tsnrsesvevsovrsnsrennsstvesssesrssssanerns eee

 

   

INTRODUCTION,

In the latter pa rt of May 1911 the writer was , for a short

time, a mamber of an expedition in the western part of the Mohave Desert
vhose primary object was the collection of verterbrate remains from the
Rosamond Series, for the Department of Palaeontology of the University of
California. Mr. Charles Laurence Baker was at the head of this expedition
and he has summarized the geological and physiogreaphical features of the
region in his paper:- "Notes on the Later Cenozoic History of the Mohave
Desert Region in Southeastern Celifornia”. ( Univ. of Cal. pub. vole VI
#I5 pp 333 - 383 ) The writer spent one wesk with Mr. Beker at the type
locality in the vicinity of Rosamond Cel., and during that time collected
rock specimens and noted the relations of the various members of the
series to one another. It is the purpose of ‘this paper to set forth the
petrographicel features of the series that may be of ingerest. It is not
the intention to deal with the geology in this paper as that has been
covered to a certain extent by Mr. Baker andnéthers. Only a short general
resume’ is here given to show what previous authors have said regarding

the series at the type locality.

GENERAL DESCRIPTION OF THE ROSAMOND SERIES 470M

 

 

The Rosamond Series is of upper Miocene age end the following
is the description given in Mr. Baker's paper above referred to:-

"Hershey gave the name Rosamond Series in 1902 (Ame Geol.
vol. 29, pp. 365 = 370) to a succession of rocks which heCharacterized as
being mainly rhyolitic in composition. The type locality is north and

northwest of Rosamond station on the Southern Pacific rpailway, near the

  
    

  
  

Rosamond

   

feet of

 

north side of Antelope Velley, and mainly to the west of the railroad between

station and the town of Mohave. This type section comprising 1650
strata, is given as follows, beginning with the base:

Type Section of the Rosamond Series near Rosamond Station.
Thickness.

Granite.

I. Coarse and fine white sandstone composed of granite

debris and rhyolitic tuff, thin-bedded, rsgularly stratified

and dipping westerly at angles of 10° to 20° =~----=v- ==== 500 ft.

2. Bright, light red, stratified sandstone containing granite (o
debris, some cobbles and boulders (water worn) of granite and
many angular and subangular fragments of white tuff -=----- - 50 ft.

3 Light yellow tuff mainly of rhyolite with an occasional
pebble of granite; roughly stratified and dipping southerly---200 ft.

4, Massive dark red lava (apparently rhyolite); varies much
in thickness, averaging about -=-=====eemcesccccacencas --=-=-= J00 ft.

5¢ Light greenish and yellow rhyolite tuff, coarse in layers
contains ebundant and large angular fragments of the under-

lying red lava and an occasional pebble and small boulder of

granite; roughly stratified end dips southerly 100 to 309 --- 400 ft.
6 White rhyolite, breccieted in layers ==-==-==== ==vee=-.- 300 ft.

7. Light brown, coarse sandstone; much granite debris and
rhyolite; occurs in limited patches cepping knolls ===----= 7100 ft.

Hershey finds the series in a low mountain four miles west and

‘one mile north of Rosamond, and at Willow Springs Mountain, two miles

farther
the rhy

Ro samond

northwest. The latter mountain he regards as the sight of one of
olitic volcanoes: ======~-cccccac~ea- Ete bE The
according to Hershey, is #ound in the low mountain three miles

south of Mohave, where the Exposed Treasure Mine is located; in Soledad

Peak, four miles south of Mohave; in many of the hills near the Santa Fe

railway,

Buttes,

five to seven miles southeast of Mohave; in Castle and Desert

several miles notth of Rogers Dry Lake; and in a narrow belt, rarely

over several miles wide, trending eastwardly across the Mohave Desert north

   

of the line of the Santa Fe railway to beyond the town of Daggett. He
suggests that the series also outcrops near Randsburg and for unknown
distances east and south of Daggett, =====~e=--aeaa memmmee—— cmm————— N

Later on in his paper Baker gives a brief description of
the Rosamond Series dn the type locality at Rosamond as follows:- (p 353)
"Strate composed mainly of volcanic tuff-breccia, with intercalated flows of
both acid and basic lavas, and a subordinate amount of granitic breccis
end bedded chert, rest on a granitic rock with a thin bassl breccia along
the track ofthe Southern P acific railroad about two and one-half miles north
of Rosamond station. In the vicinity of the railroad and eastward as far
as the west side of Rosamond Dry Lake the beds dip about I5% to the south=
ward. But at the western end of the exposure the strike gradually changes
from east-west to north-south ant the dip from south to west, ==-c=--vaa-

The thinness of the granitic basal breccia, which is wholly
absent in places, apparently indicates thet locally at the time of deposition
of the basal beds the granitic land mass was of low relief and even surface.
The matrix of the basal breccia is volcanic ashe Immediately next to the
granite in places in the Rosamond locality, as well as locally in the Barstow
syncline and east of Red Rock Canyon, is a pure white, fine grained, compact,
porous rock which is probably a leached volcanic rock.

Some four hundred feet above the base is an interbedded flow of
red acidic lava exibiting flow structure and containing obsidian, felsite,
and dark gray perlite. Mammillary and botryoidal structures are very
common in the interbedded lava. Below this lava the tuff-breccia is locally
cemented by an opeline cement and in places contains seams and botryoideal
masses of chalcedony, incrusted by opal. The lava of the breccia is appe~
rently identical in®omposition, texture, and structure with that in the

flows. On the hill north of and sgbove the first stamp mill west of the

 

  
 

 

Southern Pacific tracks, and northwest of Rosamond station, is a red porphy-
ritic lava with phenocrysts of quartz or sanadine, and subordinately feldspar.
In places this lava is also felsitic with flow structure and contains some
light gray perlite.

At the northeastern end of that portion of the Rosamond exposure
west of the Southern Pacific tracks is a fault with splendid examples of
slickensiding. The beds here contain agate, chalcedony, opal, silicified wood
and considerable amounts of bedded chert. About one eighth of a mile farther
west 1s a thin bed of highly vesicular basalt probably of not very great
horizontal extent. Immediately over this basalt is a thin layer of green, rose,
end brown agate, in which the green is interwoven with the other calors in
very complex pattems. Amygdules of quartz, with crystals sometimes forming

a comb=structure in the centers of the cavities, occur in vesicles and cavi-

ties of the basalt," =~ --

In the short time spent at Rosamond it was difficult to check
Mr. Hershey's section as well as it should have been; but the writer was able
to make out most of the members here given. On account of the great amount
of faulting in some parts of the field it is very difficult to make deter-
minations as to the thickness of the members. 1t might also be said that
somewhere hetween the fifth end sixth members of Hershey's section, or
possibly within the fifth member itself, there is a flow of emygdaloidel
endesite (Referred to as & basalt by Beker) which he has failed to include.

This will be described Rater.

IHE ROCKS OF THE ROSAMOND SERIES.

The rocks will be teken up here in petrographicel groups as
they occur in the section , as near as possible. The tuffs however will
be described in one group, but it is to be remembered that they occur both

gbove and below the lavas.

THE GRANITIC BASEMENT ROCK.

This rock in the field is everywhere ruch decomposed and is
usually very soft. Much difficulty was encountered in procuring e& specinen
from which a slide could be made. The Rosamond series rests on the eroded
surface of this granites A great intervel of time nust have elgpsed between
the intrusion of the granite and the laying down of the Rosamond series for
no trace is found of the metamorphic rocks that must have once covered the
granite. A long cycle of erosion is represented heres It is possible that
the granites are of the sare age as those of the Sierra Nevadas to the
northe The topogrephy wherever: determined by the granite is very smooth and ¥
low=lying hills with broad, shallow velleys between zre to be seen. This is
whet one would expect from the soft nature of the granite and it is to be
seen in the hills about three miles northeast of Rosamond. Dikes of eplite
and pegmatite are to be found cutting the granite end they eppear hard and
fresh. Hence they form prominent outcrops and excellent opportunity is
afforded for studying them. The gplite in the hand specimen eppears to be a
close intergrowth of e pink feldspar and quartz. The pegnatite is charac-

terized by massive crystals of pink, or flesh-colored orthoclase, sometines

near & foot in diameter. Carlsbad twinning is evident. Small quartz grains

are included in the orthoclase and as one turns the crystal so as to get the

 
 

 

at
n Th A
A

 

 

 

  

reflection of & cleavage face the characteristic "lustre mottling" of the
poikilitic structure is to be seen. Angular fragments of the granite and
pegmatite are to be found scattered throughout the lower part of the tuf f=
breccia, and even in the member that overlies the lava. The {fragments
range in size from one-half inch to a foot or so in dianéter.

Under the microscope the granite is seen to be made up of
quertz end orthoclase es essential minerals with subordinate amounts of &
plagioclase feldsper corresponding to slbite-oligoclase, microcline, and

accessory mineresls being magnetite end biotite only. The quartz made up

about 70% of the rock and while for the most part xenomorphic towards the . ~

¥

feldspar a few instances were noted in which it was developed idiomorphi-
cally. The orthoclase meade up about 20% of the rock sand could be recog-
nized by its low refraction (below balsam), and double refraction, negative
sign, and the presence of carlsbad twinning. An extinction angle of 7° wes
measured on 0I0 and 0° in the zone perpendicular to 010, on the cleavage.
The mineral sppeered to have & high optic axial angles. The plagioclase
showed very well developed &lbite twimming lamellese with a meximum.
symmetricel extinction angle of &bout 7° on the lamellae.The ndinersl had
a refraction about equel to balsam end a positive sign. It was not very
abundant in the rock and hence could not be studied as thoroughly es would
be desired, but from the above properties it is evidently albite~oldgoclase.
Microcline made up about 5% of the rock and was recognized casily between c=
crossed nicols by the grated structure produced by & combination of twinning
after the albite and pericline laws.

In some of the granitic fregments found in ths tuif-brecciea a

peculiar green coloring matter was found, and it was found in the tuffs slso in

some places giving a greenish calor to the rocks. Some sections of the

  
 

greenish-colored granite were made in order to determine the nature of this

    

mineral.

In the thin section the green coloring matter was seen to be

   

 

3

————_

shot through the rock in smell stresks and patches. In one place a feirly
good piece wes found for study. The mineral was pleochroic, being green
parsllel tol’ and yellowish parallel to’ of the section. The section
showed a positive elongation with OAc equal to 0° Refraction low. The
nineral wes evidently a chlorite and from the sbove properties it would be
penninite. Penninite is however more commonly found with CAC equal to zero
degrees. The above properties preclude the possibility of the minersl being

clinochlore. The coloring metter was of course of a secondary nature.

IHE TUFF-BRECCIA OF THE ROSAMOND SERIES.
This tuff-brecciz makes up the most prominent members of

the Rosamond Series. It is found both above and below the leaves, end wherever
observed in the Rosamond locelity it was found to rest on the eroded surface
of the granite. As observed in the field tuff~breccia resting on the granite
was fund to contain no fragments of lave but it did contain anguler fragments
of grenite and pegmatite up to one foot in diameter. The surface of the
ground was commonly strewn with large blocks of this pegmatite, which being
more resistent, had weathered out from the tuff. The tuff in immediate con=
tact with the granite was, in places, of 2 soft powdery nature, being pure
white and fine grained. Beker suggests that it is a leached volcanic rock.
A petrographicel description is made later. The tuff=breccia overlying the
lave contained commonly engular fragments of both granite and lava. These
were of all sizes up to a foot in diameter. This, of course, indicates quite

a time period between the pouring out of the lave and the deposition of the

tuff, Evidences of unconformity within the tuff-breccia itself were found

\ in quite =a few places. Large fragments of & coarse grained tuff were found

    
 

 

contained within a fine grained tuff and vice-versa.

In the Rosamond locality the tuff-breccia itself presents a
varied aspect. While so far only those characteristics of the members have
been mentioned which would indicate =@ littoral deposit under land conditions
there ere in some places evidences of water action and deposition under
waters In the hills which lie zbout three miles to the north of the first
stamp mill west of Rosamond the tuff is in many places very fine grained, is
thin bedded, and the small grains present a rounded appearance. This Herehey
has referred to &s a white sandstone. The evidences here point to a deposi=
tion under water but it is the belief of the writer that this is a local
phase of the tufif-breccia deposition and probably represents m= local
lacustrine or stream~delta conditions. In other places rounded pebbles have
~ been noted by many but they are ascribed to the same cause.

There are quite a few small mines in the Rosamond district
and prospect holes are found everywhere. From the dump heaps & study of the
tuff-breccia in its unweathered aspect could be made. All of the mining it
seems is done along fissures in the tuff=-breccia. The tuff=breccia as
examined in its fresh condition is very hard and has been firmly cemented
with secondary silica. On one mine dump along the railroad north of Rosamond
the tuff was much silicified and veins of opal traversed the rock.
Descriptions of the tuff from various specimens collected follow. It is to
be understood that the petrographicel descriptions are of the tuff which is

found in the tufi=breccia, and as such they will be referred to as "tuffs".

Tuff from Near the Base of the Rosamond Series:-

This rock is made up of small angular fragments of volcanic
ejectimenta. Rather large phenocrysts are to be found in the masse Two dis=
tinct growths of minerals are to be noted. Some of the angular fragments
have numerous smell microlites included in a n isotropic ground-mass of
colorless acid glass with refraction much lower than Canada balsam. This
glass will be described in more detail later. The phenocrysts consists of
quartz, orthoclase, plagioclase, end some biotite. The quartz makes up about
75% of the phenocrysts and is easily recognized by its conchoidel fracture,
médiuk-bierbfringence and uniaxial character with positive signe The ortho-
clase makes up approximately 20% of the phenocrysts and is recognized by its
low refractioncandidouble refresction, and biaxiel charecter and negative
signe The plagioclase had a well developed twinning on the albite law with
& maximum symmetrical extinction angle on the trace of the twinning planesdf
9°, The refraction was low, botho< and 8 being below balsame The mineral
according to the maximum extinction angle would be somewhere between albite
end oligoclase so it will be referred to as elbite-oligoclase. The sign was
determined to be positive which would place it a little nearer the albite.

The biotite was both brown and green colored, pleochroism being noticed
and being from green to black or brown to black. Good cleavage was no=-
ticed in some sections with straight extinction on the cleavage. In one
section of the green colored mica showing no cleavege or pleachroism a good
optical figure was obtained in convergent light. The minersl was seen to be
biexiel, but had such a small optic axial angle that it appeared almost
uniaxiele The sign was negetive. The mineral was not so highly double
refractive as ordinary biotite and the light green color is not characteristic

The greater part of the biotites here shown are therefore rather sltered and

 
 

 

 

  

in this way have lost much of their color. This minersl may represent a
transition period between biotite and chlorite.

The microlites included in some of the angular gragments were
very smell end difficult to work withe They showed a low double refraction,
were for the most part elongated, end showed a negative elongation. The
extinction angle measured on the sides of the microlites was sbout 0° in
those measureds Twinning was evidentx in a few. Both << and ¥* seemed to be
below Cenade belsem in refraction. The microlites were evidently of oligo-
clase end orthoclase. The extinction angle here and the acid character of the
-glassy ground-mass mekes it rather hard to distinguish between orthoclase
and ean oligoclase of the composition Angg:Abgg since both have straight
extihction and both hawe refractive indices above the glass. Where distinct
ly twinned of course the mineral was evidently oligoclase and where the
width and breadth of the microlites was nesrly equal the mineral could be
assumed to be orthoclase. But there were so many microlites of an indeter-
minate nature in this respect that one would hesitate to say whether or not
the rock was a dacite or a rhyolite tuff. Judging from the phenocrysts
however the rock is distinctly ea rhyolite tuff.

Cheracter of Tuff Found Outcropping at the Base of Hille South
of First Stenp Mill Encountered West of Rosamonds

This tuff evidently occupied & position above the acid lava. It
hes & white color and appears to be reel fine grained in the hand specimen.
Under the microscope it is found to be made up of numerous angular frag
ments of volcanic glass. No large phenocrysts were to be found in the
section examined but the microlites included in the glass appeared to be uu
made up feldspar and biotite. These were imbedded in a colorless acid glass
They were evidently of the same nature as these in the slide examined

above. Quartz was not found to be present in this sectione

 
 

 

II

A peculiar phenomenon was observed in one small fragment found in

this section. Between crossed nicols it gave & peculier lattice-work effect
the structure resembling somewhet that of & diatom test. This was not very
evident in ordinary light. The writer was not able to determine whether it was
of organic origin or simply a phenomenon of super-position or intergrowth.
The gbove was specimen # I410 - 8 .
Opelized Tuff from the Rosamond Series:-
The specimen studied was collected from a dump neer

& shaft by the side of the road sbout one mile north of Rosamond and only &
short distance west of the railroad track. In the hand specimen the rock
appears to be shot through with veins of opsline material and the tuff is of a
rather fine grained natures Under the microscope the materiel appears to be
rather kaolinized making determinations difficult, but, in general, the
rock presents the same appearance as the first section examined. There were
quite a few phenocrysts of quartz anda feldspar twinned on the slbite law
Biotite was fund sparingly. The microlites were found imbedded in a colom
less acid glass.

The opel veins , so called, were rather interssting under the
high power of the microscope. Between the tuff and the isotropic opaline

materiel there is a fringe of minute anisotropic minerel, evidently of a chal=

cedomiecnature. Secondary chalcedony is to be found distributed throughout

the rocks It is evidenced by its fibrous radiate sgructure giving a centeral
cross as the slide is rotated on the stage between crossed nicols, end by the
numergus patches of closely grouped minute anisotropic crystals.

In some of the frag ments in the tuff the microlites show good
evidence of flow stwucture by their parallel orientation.

This tuff lies near the contact with the granite heres cr Locallyn
the opel veins are quite numerous and in some places the opal is of such a

quality as to be nined for gem materiale

  

 
 
Altered Tuff from the Rosamond Series:-

Specimen collected on the dump at the mouth of tunnel at base
of the first outlier, about three miles west of Rosamond. (#1410 = 7)
The tunnel was run in the tuff-breccia and gold ore was being taken out. The
rock from which the slide was made was rather heavier than the ordinary tuff.
Upon examination under the microscope it was found to consist for the most part
of very minute anisotropic crystels with anhedral boundaries. The rock was
evidently of & chalcedonic nature. It contained however & few large crystals
of quartz thet sppeared to have been of & primarv natures Much secondary
quertz was scattered throughout. This secondary quartz hed extremely irregu=
lar boundaries and occured as evidently filling cavities. It was distingui-
shed by its uniaxial, positive, character; refraction higher than Canade
balsem, end medium bi-refringence.
Tuff Collected at First Stamp Mill West of Rosamond:-

This stemp mill lies sbout two and one-half miles west of
Rosamonde The tuff-breccia here contained fragments of & greenish colored tuff
within itself. This is sizgestive of an erosional unconformity. The tuff
breccia here occurs ebove the lave and contains fragments of the lava. The
rhyolitic fregments are for the most part angular end range in size up to
six inches in diemetere The fragments show the flow structure and evidently
belong to the member on which they rest. Under the microscope this roch
(#1410 = 2I) is seen to have a fer larger percentage of phenocrysts than
any specimen yet examinede These were well preserved and could be worked
with while the properties of the felsitic ground mass were more or less ob=
scured by its being decomposede The phenocrysts consisted of rather large

angular fragments of quartz, orthoclase, and subordinate amounts of albite=-

cligoclase. A few crystals of bictite and magnetite were to be noted also.

  
  

the basal member of the Rosamond Series described by Hershey on page 2.

  

I3
Another specimen (#I410 = 37) of a hard, greenish appearing
rock showed under the microscope that the tuff had been replaced by chelcedony
The green color was evidently due to chloritized biotite. Quartz snd ortho=
clase were noted as phenocrysts.

TUFF OCCURING IN ANGULAR FRAGMENTS AND BEDDED:-

One specimen (I4I0 = 4) was collected near the railroad about
one half mile north of Rosamond. The tuff here dips to the south at an
angle of about I59 Under the microscope this roch is seen to be made up of
more or less angular fragments of igneous material. Most of the fragments
heve a glassy ground mass in which are imbedded numerous elongated feldspar
microlites arranged in parallel order in some fregments, but irregularly dis-
tributed in others. All of those examined showed straight extinction. Some
of the microlites were thinned on the albite law but none showed symmetrical
extinction. There was in the section examined a fragment of a holocrystalline
igneous ck naving a hypidiomorphic gramular structure, and consisting of the
following minerals in order of relative abundence:- a plagioclase feldspar
twinned on the albite law with low refraction and positive sign; quartz, and
& greenish colored biotite. Therefore this fragment would correspond to a
sode=aplite on th: basis of the minerals determined. The rock section showed
the rock to be considerably decomposed as was shown by the dirty brown ap-
pearance which the rock took on due to the kaolinization of the feldspars;
and chalcedony was evideng in many places lining the borders of the fragment se
There were several fragments of perlite in the section examined and they c=
could easily be distinguished by their perlitic structure. Phenocrysts of
orthoclase were found distributed throughout .

Near the middle of the hills lying sbout three miles west and
four miles north of Rosamond an outcrop of thinly stratified and sorted tuff

is found. (#I410 - 30) This rock seems to correspond in many respects with

 
   

 
 
 

 

14

He describes the rock as a white sandstone. In a hand specimen this rock
does resemble a sandstone somewhat but under the microscope it is seen to
differ from the ordinary sandstome quite a good deal. There is a fine-grained
sort of a ground mass to the rock that is isotropic, much kaolinized and hard
to work withe But at least half of the orck consists of small angular fragments
of anisotropic grains of remarkably even size. These grains were for the most
part feldspars and while orthoclase is probably in predominance there are
many plagioclase grains to be seen. A few quartz grains were also to be noted
but they are very scerce. Some biotite is also present. From 211 of the
evidences this rock represents nothing more than a tuff that has been deposis
ted under water and hence the material became very well sorted. Finer and

coarser material were to be noted above and below this.

Nature of the VolcaniccGless from the Tuff of the Rosamond Series:-

The ground-mess of the tuff of the Rosamond Series is for the
most part hard to work with in the ordinary slide due to the relatively large
amount of decomposition and silicification that have taken place. But in a
sample of fine, white powder (#1410 - I ) collected along the contact between
the tuff=-breccia and the granite about one-fourth mile east of the rail=-
road about one half mile north of Rosamond, a rather careful study of the
material could be made. The materisl was sorted under water and ea medium
sized material studied under the microscope mounted in preparations of
different refractive index such as water, clove 0il, ethyl benzoate, carbon
bisulphide, xylol, and mixtures of xylol and carbon bisulphides The mater
ial was found to be for the most part an isotropic volcanic glass showing a
very good scoriaceous structure such as gives rise to pumices Its refraction
was determined to be slightly less than 1.4832 showing it to be a very acid

glass corresponding to obsidian but lower than any glass listed by Iddings.

I5

Microscopic anisotropic inclusions of feldspar were to be seen in the glass an
e few grains of anisotropic, uniaxial, quartz were noted. This is the rock

that Baker refers to as being probably =z leached volcanic rocke

These rhyolites, described by Hershey as making up the fourth
and sixth members of the series form the most prominent feature of the
Rosamond series at the type locality. The fourth member is characteristically
ef a red color in the outcrop, and this red color is imparted, to a certain
extent, to the overlying tuff-breccia, where the lava occurs in angular
fragments of varying size. It varies in texture from perlite up to a porphyry
with phenocrysts of quartz up to an eighth of an inch in diameter. In the
medium grained lava flow structure is everywhere noticed, it showing parti-
cularly in the weathered specimen. Sections were prepared of each kind
of lava and descriptions follow.

The section showing the best developed porphyritic structure
(#1410 = 23) was collected just up the hill from the first stemp mill encoun=
tered wpstef Rosamond. The range of hills seems to end here and the whole
western end is covered with this red porphyritic lava. No flow structure is
to be noticed in it but it apparently grades up into a finer grained lava
showing good flow structure. The phenocrysts are rather small and make up
about hall of the rock. The ground-mass is cryptocrystalline and colored a
brownish red due to iron. The minerals noted under the microscope were
quartz, orthoclase, oligoclase, biotite, and subordinate amounts of sanadine.
The quartz made up approximately 60% of the phenocrysts . Some of the pheno=
crysts of quartz have a suggestion of hexagonal outline, but the greater
majority show resorption phenomena, being eaten into by the ground=-me ss.

The orthoclase mede up about 20% of the phenocrysts and was distinguished by

 
 
 
 

16

the twinning on the Carlsbad law, the good two-directional cleavage, biaxial
character, negative sign, and large optic axial angle. In order of relative
abundance the next important mineral was biotite which made up about 10% of
the phenocrysts. It was pleochroic, brownish red to black, with euhedral
forms often but rather altered so that some of the properties were obscured.
A greenish chloritic materizl surrounded many of the hiotites.

The oligoclase made up about 5% of the phenocrysts and showed polysynthetic
twinning combined with Carlsbad in one case but the illumination was unequal.
The sign was negative and a maximum extinction angle on the trace of the
twinning plene of II° was obtained. The sanadine was distinguished by its
poorly developed cleavage, small optic axial angle, and negative sign. The
mineral was almost uniaxial. The ground-mass was so fine grained that the
mineralogical composition was indeterminate. It is stained reddish and
greenish throughout and the coloring mtter was evidently derived from the
biotite. There was a slight indication of kaolinization of the feldspars in
some cases. This rock is without doubt a rhyolite.

In sections of the fine grained lava studied (#I4I0 = 35a =35b)
the lava was seen to have been of a vesicular nature with vesicles drawn
out in the direction of flow. These vesicles are vow filled with secondary
silica. Section #35a was very fine grained but showed this amygdaloidal
structure. The amygdules wewe made up of concentric layers of quartz and
chalcedony, the quartz layers being noticed particularly on the outside and
inside of the amygdule. A layer of quartz grzins in the center of ohne any g=
dule was stained with a dark brown isotropic mineral resembling limonite.
Determinations of the charzcter of the ground mass were very difficult for
the reasons before stated An attempt was made to rid the section of the
iron stain by placing it in hydrochloric acid, but the acid seemed to have
little effect. Quite a few small lath -shaped mi¢rolitess were to be made
out in one section using the high power. For the most part these micro

kites: had a negative elongation, but & few were measured which had a positiw

 

 

 

the perlitic structure.

 

   
 
   

elongation. One showed a biaxial figure and negative sign. It was evidently
orthoclase. Some of these microlites were evidently plagiocleses however for
twinning was evident in some.

In enother slide studied the amygdules were very prominent
filling elongated vesicles arranged in the section elongated parallel to the
evident direction of flow. These vesicles were very prominent and made up
spproximately 25% of the rock. The secondary matter filling the cavaties
consisted for the most part of very minute quartz grains of irregular outline.
Much biotite could be seen in rather large pieces in this slide, and in the
hand specimen it was the onle mineral that could be made out. It was
commonly in rectangular crystals showing good cleavage and straight extinc-
tion on the cleavage. The pleochroism was brown to elmost black.

Rhyolitic Perlite:~

This glassy facies of the lava occurs interbedded with the fine
grained and vesicular rhyolites. In the hand specimen the rock has a dull
gray, vitreous lustre, and conchoidel fracture. In the outcrop it weathers out
in a very characteristic manner, the perlite breaking down into rounded pebbles
varying in size from the size of a grein of sand to three-fourths of an inch
in diameter. These little pebbles have a tendency to scale off in concentric
layers, which leaves all ofthe fragments much roundede Under the microscope
the rock is seen to be felsitic or hemicrystalline and determination of the
mineral constituents was not attenpted. The most striking thing under the
microscope is the development of the curving perlitic fissures characteristic
of the natural glasses. Under the high power the rock is seen to contain a
great number of longulites, hakulites, kumulites, and trichites, with no
definite order of arrangement or orientation. The small trichites, resembling
small hairs were twisted into various shapes, some being wound spirally like
small springs. Very small black grains, triangular in shape, and resembling

magnetite were to be seen. The section (/I410 = 6) is a very fine example of

         

 

  
White Rhyolite from Soledad Peak:=-

Rock specimen (;I4I0 - I5) collected from Soledad Peak, a knob
rising from the desert sbout three miles south of Mohave. There are numerous
mines on this hill among which are the Queen Esther and Lost Treasure. The
specimen collected seemed to be representative of the rock which made up the
hill. The rock had a porphyritic structure with phenocrysts of quartz
and orthoclase. Most all of the quartz phenocrysts were developed idio-
morphically and showed very good euhedral outlines. Resorption phenomena were
however, developed to a marked degree. Only a very few of the feldspar
phenocrysts were to be notede The phenocrysts made up approximately 15%
of the rocke The ground-mass was of a cryptocrystalline or ‘microfelsitic’
nature, being made up of an aggregate of small anisotropic grains on a very
minute scale. Harker says that this type of ground-mass is also found in
intrusives of the "“quartz=porphyry" type. No flow strusture was evident in
the rock, but it could easily be a thick flows The rock was of & rather

vesicular nature but no secondary products were noted.

This rock corresponds to the white rhyolite which Hershey gives

as the sixth member of the Rosamond Series. The writer was unable to deter=

mine in the field, in the short time, the relation of the white lava to the

other members of the series.

AMYGDALOIDAL AUGITE=ANDESITE FROM THE ROSAMOND SERIES.

This andesite was found outcropping on the summit of the high
range of hills about three miles to the north of the first stamp mill west of
Rosamond. It occured in a thin sheet, not more than a few feet thick, and
presented every evidence of being a flows The region has been faulted con=
siderably here and the relation of the andesite to the massive red rhyolite
was hard to determine. It was evident though that this body of andesite
occupies a position somewhere near the top of the series and that it overlies
the rhyolite.

The andesite when it solidified was evidently of a rather vesi=-
cular nature for the rock now contains numerous amygdules of varying sizes,
some being two or three inches in diameter and containing beautiful aggregates
of quartz cryatals.

Underlying the andesite is a stratum of flint of an evident
secondary origin resulting from the alteration of the rhyolite tuff. This
flint was very handsomely colored in rich colors of red and green with white.
Its occurence at the contact with the andesite and its limited thickness,
open up some rather interesting problems as to its origin and as to the oi.
origin of the coloring matter.

In the hand specimen the andesite is found to be rather heavy
and has a brownish black color. Cleavage faces of the feldspars is: the
only macroscopic evidence of crystals. Under the microscope the rock presents
the characteristic hyalopilitic structure of Rosenbusch with two growths of
feldspars, one growth forming a sort of a felsitic feltwork in an isotropic
ground masse The phenocrysts were all of feldspars some, however, being of a
much greater size than others. The feldspar here was evidently labradorite

showing fine twinning lamellae. The maximum extinction angle on the trace of

 
 

  

the twinning plane was 350 and the sign was positive. Inclusions of & peculiar
neture were found in the labradorite phenocrysts. These inclusions were , forh
the most part, of & dark colored, isotropic glass that resembled the glass of
the ground=mass very much. In some cases an anisotropic mineral resembling
eugite could be seen imbedded in the isotropic materiel. Generally the edge
of the lebradorite crystal conteined few or none of the inclusions but there
were exceptions to this rule. The inclusions, in one instance, seemed to be
arranged parallel to the cleavage (0I0) of the feldspar but in other cases
they were irregularly arranged. Inclusions of this nature are very common in
labradorite. Numerous small, elongated gas inclusions were noted in the lab-
radorite phenocrysts. The phenocrysts made up epproximsately 35% of the rocks

The feldspars of the ground-nass were almost all twinned and the
maximum extinction angle on the trace of the albite twinning plene was 31°
showing thet the feldspar was evidently & lebradorite. This is rather unu-
suel for the feldspar of the phenocrysts to be the same as that of the
ground- messy, but it has been noticed in certain amdesites.

A rhombic pyroxene was found well developed in the ground=meass.
It was of & yellowish=brown color in ordinary light and had & high refracticn
A straight extinction on a poerly developed cleavage and & conchoidel fracture
were noticede The minepel had a rather low double refraction and in conver-
gent light wes seen to have a high optic exial angle end a positive sign.
This mineral was found to be xenomorphic towards the feldspars of the ground
mass end while developed in elongated prismatic forms for the most part some
rounded forms were found. Augite was found in small patches throughout the
ground-mass. The feldspars of the ground-mass were developed Sdlomorphis
cally towards it « It was distinguished by its high refrection and double

refraction, positive sign, and light green color in ordinary light.

 

   

The gless of the ground=-mass had & refrection higher than bal sem
and usually has & very dark sppearance due to smell inclusions of an opaque
mineral, probably magnetite. The ground-mass makes up approximately 35% of
the rock.

The term augite~andesite, basaltic habit, seems to be the best
term to epply to this rock. It does not seem to possess sufficient quentity

of the basic constituents to be termed a basalt.

NOTE ON THE ROCKS FROM BLACK MOUNTAIN.

In August I911 the writer visited Black Mountain at the sugges
tion of Dr. Merriam to ascertain the character of the rocks making up the
mountain. Black Mountain is a high peek lying about seven miles northeast of
Gerlock California, near the head of Mesquite Canyon. Garlock is a station on
the Nevada and California Railway some thirty or forty miles northeast of
Mohave. The object of the trip wes to ascertein if the Rosamond Series of
tuffs extended up as far as Black Mount ain. The Rosamond Series had been
found outcropping in Red Rock Canyon to the southwest and Black Mountain
stends at the head of Last Chance Canyon which leads down into Kane Lake.

The lower part of Black Mountain is made up of a loosely con=
solideted sandstone of fairly even grain, yellowish in color and soft and

frieble. This extends up to within a few hundred feet of the summit. Overlying

   

this sandstone conformebly is a body of tuff that resembles in 2 hand specimen

      
 

the tuff from the Rosamond locality with the exception that it has a lower
specific gravity, is looser, and contaimss numerous large pumice fragments,
something that was not observed, mecroscopically, in the Rosamond tuffs.
These tuffs were poorly stratified and the beds had a strike epproximately
east end west dipping to the south at en angle of about 28 . The thickness
of these beds wes estimated at about 350 feet. They extend up to within about
fifty feet of the top ofthe mountain. Black Mountain is capped by a dark
colored volcanic rock and blocks of this lava are found covering the slopes
of the mountain from the base up; being found in place, on the south side, &t
the top only. This lave rests unconformably on the tuff and probably repre=-
sents a late flow. The evidence in the field points to the fact that it
covered the eroded edges of the outcropping tuff at first but due to wthéh::~
weathering awey of the soft tuff underneath the lava has been left on the
surface zs large blocks.

Fairbanks in the American Geologist, Vol. XVII pege 68, has the
following to say of Black Mountain:= "A seam of coal I4 inches thick and
inclosed between clay strata has been found southeast of Black Mountein oc-
cupying a position apparently below the tuffs. Impressions of leaves occur in
the clay inmediately ahove the seam of coal '====-= He seems to think the cosel
is of Eocene age, and goes on to say:- "Black Mountain seems to have been the
pivotal point of disturbance. Finely stratified tuffs are found in the car
yons and on its precipitous flanks. Andesite appears as flows between the
beds, in dikes cutting them, and also capping Black Mountain".

Hess in Bulletin #430 U.5.C.S., page 3I, gives & short account of

the geology of the Randsburg quadrangle end mentions & lava which he seems

to think flowed out from Black Mountain. He says;- "On the hnorth side of

 

   

the velley between Randsburg end Goler there is a large accumulation of debris
forming hills 350" high. They extend half way across the quadrangle to Sum=
mit Diggingss In them is a great quantity of lava boulders, remnants of =z
flow from Black Mountein, e few miles west of the north end of the quadrangle.
The lave boulders occur in patches over the summit of the El Paso Renge."

Both the lava end the tuff were examined microscopically.

Olivene-Basalt from Black Mountain:= (#1411 - 54)

This lava under the microscope is seen to be remarkebly fresh
and well preserved. It has the intersertel structure characteristic of basic
igneous rocks.

A feldspar twinned on the albite law and developed in long rec-
tangular idiomorphic crystels makes up by far the greater part of the rock.
This feldspar has a high refraction, e positive sign, end a meximum extinc=
tion angle on the trace of the twinning plane of ebout 35° showing that it is
rather basic labradorite. In one instence twinning on the Baveno law was
observed. A suggestion of the glomero=porphyritic structure of Judd was seen
in the development in one place of quite an aggregation of feldspar crystals a
in a granular mass. The feldspars were spotted with a black opaque mineral
that may have been either magnetite or ilmenite but owing to its indeterminate
nature it will be referred to as "opazite".

Surrounding the feldspar laths and filling in the interstitial
spaces were irregular grains of sugite with high refraction, high bi-refrin-
gence, positive sign, and appearing light green in ordinary plane polarized
lights This enclosed the labradorite in meny cases giving the typical

diabasic structure.

  
 

 

24

Olivene in rounded grains was also sbundant in this rock and
could be distinguished by its yellowish-brown color in ordinary light, high
refraction and double refraction, irregular-conchoidel fracture, large optic
axial angle when viewed in convergent light, end positive sign. It was found
included included in both the =augite and labredorite and evidently was one
of the first minerals to forme It showed & slight alteration in the produc-
tion in many of the crystals of a reddish-brown margin called by Weinschenk
Yhyalosiderite". In same of the crystals this covered the entire crystele

Magnetite in small patches, was also observed.

This rock, before referred to as an andesite, is without =

doubt a besalt, and the term "olivene-basalt" seems to apply best.

Tuff from Black Mountain:-

The Black Mountain tuff is in the main of the same nature as
that found at Rosamond excepting that it has not been silicified to the extent
that most of the tuff in the Rosamond Series has, snd hence the structure
of the volcanic glass is much better preserved Under the microscope, using
the high power, the flow structure of the isotropic, pumiceous material with
steam bubbles drawn out in the direction of floy, shows up very nicely. The
rock is seen to be made up of fragments of this glassy-material’mbény of which
show the pumiceous structure, for the most part isotropic but congaining
anisotropic microlites of the same nature es those of the Rosamond tuff, and
a few crystels of orthoclase end quartz were also noted An :~ isotropic
cementing materizl could be made out between many of the fragments and it
appeared to be of an opaline nature.

From the above description it can be seen that the tuff is of

a rhyolitic nature. It is evident that in the Black Mountain area we are

25

getting rather close to the source of the tuff of the Rosamond Series.
Large pieces of pumice, up to an inch in diameter, are found here and it

is evident that they could not have travelled far in the eir. Conditions are
much better for studying the tuff of the Rosamond Series at Black Mountain

since it is found in a more unconsolidated condition.

CONCLUSIONS.
The only conclusions that are justifiable in a peper of this

sort seem to be regarding the origin of the sedimentary deposits of.c the
Rosamond Series. In reading the literature regarding these deposits,
referred to as "sandstones" etc., one is 1lisble to get the idea that lacus
trine conditions must have been prevalent during their deposition. In the
microscopic study of the rocks,the slides studied that resembled sandstones
deposited under water (#1410 - 30) when examined mecroscopically, were
clearly tuffs and showed no degree of sorting that could not arise from sub=
aerial deposition.

The tuffs are in places thin-bedded but this could easily arise
from a sorting action produced by air.

It is to be remembered, of course, that it will be the rule
rather than the exception that much foriegn material, formed from decompositim
of adjacent rocks, will accumulate in sediments of this sort and one must

expect to find it.

 
 

 

 

26

Of course in certain rather rare places in the Rosamond Series
deposits of 2 clearly local lacustrine nature are found. They are to be exclu=-
ded from the shove broad generalization, but it is very clear that they are

the exceptions which prove the rule.

 
   

END OF TITLE