a4
C3
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STATE OF CAUFOBNIA
DEPARTMENT OF NATURAL RESOURCES
GEOLOGY OF
OWEB LAKE QUADRANGLE.
CAOFORNIA
BULLETIN 166
1953
DIVISION OF MINES
FERRY BUUDINa SAN FRANCISCO
smii^i^;iiu^mmii;t&mit^i:^:Ki\;t^^ii!imt:m!ij
THE LIBRARY
OF
THE UNIVERSITY
OF CALIFORNIA
DAVIS
STATE OF CALIFORNIA
EARL WARREN, Governor
DEPARTMENT OF NATURAL RESOURCES
WARREN T. HANNUM, Director
DIVISION OF MINES
FERRY BUILDING. SAN FRANCISCO
OLAF P. JENKINS. Chief
SAN FRANCISCO
BULLETIN 166
APRIL 1953
GEOLOGY OF
LOWER LAKE QUADRANGLE,
CALIFORNIA
By
JAMES C. BRICE
Containing a section on economic geology
by James C. Brice and J. Grant Goodwin
LIB.HARY
UNIVERSITY OF CAUFORNIA
DAVIS
LETTER OF TRANSMITTAL
To His Excellency
The Honorable Earl Warren
Governor of the State of California
Sir : I have the honor to transmit herewith Bulletin 166, Geology of
Lower Lake Quadrangle, California, prepared under the direction
of Olaf P. Jenkins, Chief of the Division of Mines, Department of Natural
Resources. In addition to a descriptive text, the bulletin includes also a
colored detailed geologic map and other maps, charts, and photographs.
The report represents the results of an investigation by James C. Brice
who carried on the work in fulfillment of the requirements of the doctorate
at the University of California. Supplementing the descriptive geology
is a section on economic mineral deposits, prepared in part by J. Grant
Goodwin, a member of the staff of the Division of Mines.
The Lower Lake quadrangle lies nearly wholly in Lake County, though
the southwest corner enters Sonoma County. Economic minerals de-
scribed include quicksilver, sulphur, chromite, asbestos, borax, diato-
maceous earth, manganese, copper, crushed rock for cement blocks,
vesiculated obsidian for plaster sand, and gravel for aggregate.
This bulletin represents one of the results of the Division of Mines'
many cooperative projects with the University of California.
Respectfully submitted,
Warren T. Hannum, Director
Department of Natural Resources
December 8, 1952
(3)
CONTENTS
Page
Abstract 7
Introduction 7
Geography 9
Stratigraphy and petrography 10
Franciscan-Knoxville sequence 11
Petrology of the sedimentary rocks 11
Petrology of the igneous rocks 14
Metamorphism 18
Stratigraphic relations and origin 21
Age and correlation 22
Cretaceous (undifferentiated) 23
Lithology 23
Stratigraphic relations and origin 26
Martinez (Paleocene) rocks 27
Tejon (Eocene) rocks 29
Cache beds 30
Clear Lake volcanic series 34
Quaternary deposits 50
Alluvium 50
Landslide and talus deposits 50
Terrace deposits 50
Geomorphology 51
Geologic structure 55
Folding 56
Faulting 57
Geologic history 58
Economic geology 60
Asbestos 60
Borax 61
Chromite 61
Clay 61
Copper 61
Diatomaceous earth 62
Gem materials 62
Manganese 62
Mineral springs 62
Quicksilver 62
Soda 04
Rock, sand and gravel 64
Index 68
ILLUSTRATIONS
Plate 1. Geologic map of Lower Lake quadrangle In pocket
2. Economic Map of Lower Lake quadrangle In pocket
3. Photomicrographs of Franciscan sandstone between 32 and 33
4. Photomicrographs of acidic lava between 32 and 33
5. Photomicrographs of inclusions in Clear Lake lava between 32 and 33
6. Photo of quartz inclusions in olivine basalt between 32 and 33
7. Structure sections across Lower Lake quadrangle In pocket
Figure 1. Index map showing location of Lower Lake quadrangle 8
2. Generalized columnar section 12
3. Variation diagram of Cl:ar Lake lavas 45
GEOLOGY OF LOWER LAKE QUADRANGLE, CALIFORNIA
By James C. Bkice *
ABSTRACT
The Lower Lake quadrangle comprises an area of about 240 square miles, located
in the midst of the California Coast Ranges some 70 miles directly north of San Fran-
cisco in Lake and Sonoma Counties. The topography is varied, being mountainous in
the western part and hilly elsewhere, except for several irregular flat-bottomed valleys.
Prominent mountains in the western part are built of large bulbous protrusions of
acid lava ; and flows of more basic lava extend across the quadrangle, forming rolling
highlands fringed by cliffs and talus-slopes. In the areas underlain by pre-volcanic
rocks, the broad valleys are incongruous with the otherwise early mature landscape, and
are believed to have originated by downwarping or subsidence along boundai'y faults.
A similar origin is postulated for the basin which Clear Lake occupies.
The oldest rocks exposed are gray wackes, shales, serpentine, and greenstone assigned
to the Upper Jurassic Franciscan group. These rocks are in fault contact with the
younger Upper Jurassic Knoxville group, which is similar in lithology but contains a
larger proportion of shale. The Knoxville shales grade upward without significant break
into massive yellow-brown graywackes and mudstones of Cretaceous age, which are
unconformably overlain by massive feldspathic sandstones bearing Martinez Paleocene
fossils. The Martinez rocks are overlain by white conglomeratic sandstone bearing a
fossil assemblage which suggests correlation with the restricted Tejon formation. The
record here was interrupted for the remainder of Tertiary time, to be resumed by
the deposition of fresh-water gravels and silts of the Plio-Pleistocene Cache formation.
Widespread diastrophism occurred during this Tertiary interval, as the Cache beds
unconformably overlie all older rocks. The upper part of the Cache beds includes
pyroclastic rocks and intercalated lava flows ; they mark the beginning of a volcanic
epoch which continued intermittently through the Pleistocene.
The volcanic rocks range in composition from olivine basalt to obsidian, and include
intermediate types of hybrid mineralogy. It is suggested that both mixing of magma
and contamination by sedimentary material were involved in the petrogenesis.
Plagioclase phenocrysts differing in composition are associated in many of the
dacites, and some dacites contain magnesian olivine in association with quartz. Much
of the olivine basalt and andesite is quartz-bearing, and locally contains in addition
aluminous and siliceous xenoliths.
At the present time, the only minerals being produced commercially in the Lower
Lake quadrangle are crushed rock for cement blocks, vesiculated obsidian for plaster
sand, and gravel for aggregate. Six quicksilver mines have produced approximately
126,550 flasks of mercury, most of which came from the Sulphur Bank mine just off
the northern edge of the quadrangle. Moderate reserves of low grade ore still exist.
This mine also produced 2,000,000 pounds of sulfur valued at $53,500. Other minerals
produced commercially in small quantities include chrysotile asbestos, chromite, and
borax of which small reserves exist. Diatomaceous earth, manganese, and copper
prospects are known, but no production has been recorded. Health resorts in the
vicinity of mineral springs are a major source of income in this area.
INTRODUCTION
Location and Accessibility. The Lower Lake quadrangle is located
in the midst of the California Coast Ranges, about 70 miles directly north
of San Francisco. The parallels 38°45' to 39°0' North and the meridians
122° 30' to 122 °45' West constitute its border lines, enclosing an area of
about 240 square miles. Except for a few square miles in the southwest-
ern corner, which lie within Sonoma County, the quadrangle lies entirely
within Lake County. Paved state or county roads offer easy access to
most portions of the quadrangle, and these are supplemented by numer-
ous graded roads leading to resorts or ranches ; but the almost unin-
habited region between Cache Creek and Rocky Creek must be reached
by trail.
* Washington University, St. Louis Missouri. Condensation of a thesis for the degree,
Doctor of Philosophy, University of California at Berkeley, 1950.
(7)
LOWri; LAKE QfADIJAXCIyE
[Bull. 166
Figure 1. Index map of part of northern California showing location of Lower Lake
quadrangle (heavy lines) and recently published geologic maps (light lines).
The principal towns of the quadrangle are Middletown (pop. 450)
in the Collayomi Valley, and Lower Lake (pop. 875) at the southeastern
end of Clear Lake, In addition, there are numberous year-round resorts
and summer homes at Clear Lake Park, Clear Lake Highlands, and else-
where around the lake; and the number of permanent residents is
increasing. Besides the resorts on Clear Lake, there are many resorts
in the highlands which offer the benefits of mountain scenery, pleasant
summer climate, and medicinal spring waters. The resort business, com-
mercial production of walnuts and plums, and stockraising, are the
principal occupations of people living in the region.
Methods of Investigation. A total of about 30 weeks was spent in field
mapping, during the summers of 1947, 1948, and 1949. Geologic map-
ping was done on the 1945 edition of the Lower Lake quadrangle, scale
1 :62,500, contour interval 50 foet. Complete coverage of the area by
aerial photograph, scale approximately 1 :20,000, provided further con-
venience and accuracy in mapping, and most contacts were plotted on
these photographs as well as on the quadrangle sheet.
Acknowlcih/cmcnts. I wish to (>xprcss my gratitude to faculty mem-
bers of the Department of Geology and of the De])artment of Paleontology
of the University of California, for their guidance in the carrying out
1953] INTRODUCTION 9
of this study. Especial thanks are due N. L. Taliaferro and Charles M.
Gilbert for assistance in the field, and F. J. Turner for advice on petro-
graphic problems. Howel Williams very kindly read the section on Clear
Lake volcanic rocks, and made many helpful suggestions. The graduate
student research fund of the Department of Geological Sciences of the
University of California furnished generous financial assistance, which
largely paid living expenses incurred during the field work.
Previous Geological Work. The geology of the northern part of the
Lower Lake quadrangle was mentioned by AVhitney (1865), described in
reconnaissance fashion by Becker (1888), described in part by Dickerson
(1914), and, with regard to volcanic rocks, mapped and described in
detail by Anderson (1936). The geology of the central part apparently
has been neither mapped nor described in the literature. The geology of
the southern part has been mapped in reconnaissance fashion by Forstner
(1903) ; and an area of about 8 square miles in the southwestern corner
has recently been mapped in detail by geologists of the United States
Geological Survey (Yates and Ililpert, 1946; Bailey, 1946).
Becker described the general geology of the Clear Lake region, with
special reference to the quicksilver deposits. His report includes the
petrography of the lavas (with analyses) , the petrography, stratigraphy,
origin, and metamorphism of the sediments, and the origin of Clear Lake.
Forstner, in his studies of the quicksilver resources of California, made
sketch maps which included the southern parts of the quadrangle. The
faunal lists presented in Dickerson 's w^ork are valuable, but the geologic
map is a sketch map, and the descriptions of formations are incomplete.
A part of the present study is intended to supplement Anderson 's work,
by mapping the same ground on the accurate topographic sheet now
available, and by further study of the interesting and diverse Clear Lake
volcanics. The geologic map accompanying the report on the Mayacmas
Quicksilver District, by Yates, Hilpert, and Bailey, extends into the
Lower Lake quadrangle. This overlapping ground was remapped, but
little change was made in the more detailed parts of the Mayacmas map.
Geography
Relief and Drainage. The highest point in the quadrangle is the top
of Cobb Mountain, at an elevation of 4722 feet. Other prominences are
Mt. Hannah (elevation 3978), Seigler Mountain (elevation 3681), and
Brushy Sky High (elevation 3195). The lowest measured point, on the
nearly level floor of Coyote Valley, stands at an elevation of 963 feet. The
valley floors are in general strikingly flat, and bordered by steep-sided
hills which rise abruptly at the valley margin. The crest of the northwest-
trending Mayacmas Kange crosses the southwestern corner of the quad-
rangle, forming a divide which diverts nearly all of the drainage eastward
into the Sacramento Valley. Sulphur Creek, which crosses the corner of
the quadrangle beyond the crest of the Mayacmas Eange, is the only
stream draining westward into the Russian River. All of the drainage
into the Clear Lake hydrographic basin flows northwestward through
Cache Creek; the flow is controlled by the Clear Lake Water Company
Dam, which is located at a sharp bend in the creek about 3 miles east of the
lower end of Clear Lake. Putah Creek drains the southern half of the
quadrangle, carrying the water eastward to the Sacramento Valley by a
devious route. Because the rainfall is seasonal, the tributaries of Cache
10 LOWER LAKE QUADRANGLE [BuU. 166
Creek are mostly intermittent streams ; exceptions are those tributaries
that are fed by permanent springs.
Climate and Vegetation. Climatic data recorded by the U. S. Weather
Bureau observers for a period of about 20 years ending in 1930, may be
summarized as follows: precipitation (recorded at Clear Lake), yearly
average 21.2 inches, of which 18.3 inches fell in the period November 1-
March 31; temperature (recorded at Upper Lake), average for year, 57
degrees; highest monthly average, 74 degrees in July; lowest monthly
average, 44 degrees in January; highest temperature recorded. 111 de-
grees in July; lowest temperature recorded, 13 degrees in January and
February.
The distribution of different types of vegetation is influenced by the
soil, by the character of the underlying rock, by the available water, by
altitude, and, in some localities, by chance. If one of these factors be held
constant, the vegetation will vary by influence of the others, except that
the assemblage growing on serpentine varies only slightly. The following
characteristic associations have been noted: (1) The chaparral or brush,
which consists principally of chamise (Adenostoma) , manzanita, and
buckthorn. This association appears on serpentine (where chamise and
manzanita predominate), on the lava at the lower elevations (where
chamise and buckthorn predominate), and on the hilly regions underlain
by a diversity of rock types. (2) The oak and grassland association, which
appears on the floors of the larger valleys, where large valley oaks are
nicely spaced and ground is carpeted with wild oats, perhaps accompanied
by tarweed (Hemizonia) and the star thistle. In the hilly regions, where
less moisture is available, the wild oats are accompanied by scrub oaks.
(3) The pine forests, interrupted by an occasional patch of manzanita,
appear on the highlands in the eastern part of the area.
The growth of the brush seems capricious in many places, and not
altogether dependent on either available moisture or on soil ; for a single
slope may be covered by thick brush and open grassland in random distri-
bution. In general, however, the northern slopes are the more brushy;
about one-fourth of the region is covered with thick brush.
STRATIGRAPHY AND PETROGRAPHY
The rocks of this quadrangle may be grouped according to origin into
three main kinds : marine geosynclinal rocks ranging in age from Upper
Jurassic to Paleocene ; fresh-water continental gravels and silts of Plio-
Pleistocene age which accumulated in a structural basin ; and Pleistocene
volcanic rocks. Although deposition in the marine geosyncline was essen-
tially continuous until the end of Cretaceous time, its character changed
from strongly volcanic in Franciscan time, when extensive deposits of
submarine lavas were poured out, to less volcanic in Knoxville time, to
non-volcanic in Cretaceous time. Large bodies of serpentine intruded the
Mesozoic sediments, but do not appear in the Cretaceous. The Paleocene
geosyncline was much more restricted than its predecessor, but its
deposits are similar to earlier deposits, although cleaner and better
sorted. The geosynclinal sediments are almost exclusively graywackes
and shales, which accumulated to a total estimated thickness of 25,000
to 30,000 feet. Measurement of thickness is precluded by poor exposures
and structural complications.
1953] STRATIGRAPHY AND PETROGRAPHY 11
Franciscan- Knoxville Sequence
For purposes of geologic mapping in the Lower Lake quadrangle, the
Franciscan-Knoxville sequence as defined by Taliaferro (1941), has been
here separated into two units, the Franciscan group and the Knoxville
group. The Franciscan group consists principally of graywacke, with
a moderate proportion of interbedded shale, and minor amounts of
chert and conglomerate. Greenstones and intrusive serpentine rock
are abundant, but subordinate in amount to sediment. Zones of shearing
and of hydrothermal veining are numerous, so that a considerable part
of the sediment is sheared or crumpled, and veining is common. No fossils
were found in these rocks. The lithological assemblage corresponds fairly
well with Taliaferro's first and second stages of the Franciscan-Knoxville
sequence.
The Knoxville group is distinguished from the Franciscan by its
predominance of shale, which occurs in a ratio of about 4:1 with the
interbedded graywacke. Conglomerate is prominent in the exposures
east of Lower Lake, but rare elsewhere. Extensive serpentine bodies
nearly equal the sediment in areal extent, and are more prominent than
in the Franciscan, but greenstones are relatively rare. Shear zones are
uncommon, and where present the shearing is weaker than in the Fran-
ciscan. An Upper Jurassic (Tithonian) age is indicated by specimens of
Buchia (Aucella) piochii (Gabb), which were found at several localities
in the Knoxville rocks east of Lower Lake. The lithological assemblage
corresponds fairly well with Taliaferro's third and fourth stages of
the Franciscan-Knoxville sequence.
Petrology of the Sedimentary Rocks
Rocks of the Franciscan and Knoxville groups are here described to-
gether, because they are lithologically similar; the distinction between
them lies not in difference in rock type, but rather in differences in
relative abundance of sandstone and shale.
Sandstone. Structural and textural features are not usually apparent
on outcrops of Franciscan sandstone, having been obscured by incipient
recrystallization and by complex, fine-scale fracture systems. The sand-
stone is locally laminated with dark siltstone, or interbedded with shale.
The unweathered sandstone is typically yellow gray to dark greenish
gray ; the weathered sandstone is light yellow to yellow red, and has a
distinctive greasy luster, probably caused by the weathering of micaceous
and chloritic minerals in the matrix. The microscope shows that sorting
is poor, with the grains ranging from the lower sand limit up to about
1 millimeter. Coarse-grained sandstones occur locally, especially in as-
sociation with conglomeratic zones. The grains are angular to sub-
rounded, and slivers of quartz are not uncommon. Grain boundaries
are usually indistinct, and appear to merge with the matrix, which is
typically a murky yellow-brown paste composed of silt, micas, chlorites,
and a small amount of clay. The bulk of the matrix is composed of
particles of silt size, resolvable under the microscope ; the clay fraction
has apparently been removed by sorting and deposited elsewhere. The
matrix is not so abundant as to separate entirely the larger grains, which
are commonly in contact at corners and locally along surfaces. In some
specimens, grains are locally interlocked. Chemical cement of any kind
is rare, but one of the slides examined is partly cemented with carbonate,
12
LOWER LAKE QUADRANGLE
[BuU. 166
AGE
ROCK UNIT
FEET
DESCRIPTION
Alluvium, londslides,
ond lerroce deposits
Ooi. oi». 01 -
Gravel, sand, silt, clay; lanlulldee are
inofltly volcanic debris emd/or serpentine
Quartz-, olivine-, and sanldlne-bearlng
dacitlc and andeeltlc lavas
Black, rhyolltlc
Black, siliceous, glassy
RhyoHtlc flows and tuffs
Dork gray porphyrltlc andesltlc lavas
Andesltlc flows bearing xenocrysts of quartz
and xenollths of aluminous rocks
Mostly quart 2 -bearing
White coarse- to fine-grained tuff
Fresh-water deposits of gravel, silt, and
clay, except near top of section where
tuffaceous sediments, marl, limestone,
and dlatcmlte predominate
W^.lte conglomeratic sandstone
Shale at top; conglomerate and sandstone,
yellow feldspathlc sandstone; white
feldspathic sandstone at base
Undifferentiated
Cretaceous
Sllty, yellow-brown feldspathlc sandstone,
Interbedded with about an equal asount of
mudatone
-Sequence concealed •
Knoxville
Group
Froneiscon
Group
1 0,000 ±
Mostly gray shale Interbedded with smaller
amounts of graywacke-type sandstone;
conglomerate locally prominent, little
chert, greenstone, and schist. Large
bodies of serpentine
Mostly graywacke-type sandstone, Bome
shale and conglomerate, little chert;
intruded by serpentine (sp) and
greenstone (gs). Scattered areas of
glaucopbane schist
FKiUHE 2. Generalized columnar section of Lower Lake quadrangle.
1953] STRATIGRAPHY AND PETROGRAPHY 13
A yellow-brown micaceous mineral having fairly uniform optical prop-
erties is abundant in the matrices of most of the sandstones that were
studied. The mineral was separated from two specimens by use of a
magnetic separator, and found to be faintly pleochroic from light yellow-
ish brown to dark yellowish brown, with a maximum birefringence of
about 0.028, very small 2V, optically (-), Ny about 1.61. These prop-
erties indicated that the mineral is transitional between chlorite and
biotite, and its occurrence suggests strongly that it is authigenic.
Approximate percentages of the principal mineral constituents were
estimated from thin sections of sandstones, with the following results:
8/ 19 8/2S 8/29 8/96 A 8/188 8/201 Average
Quartz 40 20 20 20 12 15 21
Feldspar 15 15 20 15 15 20 17
Rock fragments 15 45 30 15 55 30 32
Matrix (including
cement if present)— 25 20 30 50 15 35 29
Rock fragments of diverse lithologic type are characteristic of the sand-
stones. Fragments of mudstone and chert are most numerous, but
igneous porphyries are common. Typically the porphyries contain tiny
laths of feldspar in a microcrystalline groundmass, and are altered to
chlorite. Schist fragments of many kinds are found in most specimens.
Much of the quartz is dusty with small inclusions, but some is clear.
Most feldspar is somewhat cloudy, and some is much altered to sericite,
but nearly clear feldspar appears together with altered feldspar in some
of the slides. Most of the feldspar is twinned but not zoned, and is in the
oligoclase-andesine composition range ; microcline twinning was rarely
observed. The heavy minerals were not specifically studied, but biotite is
abundant in many slides, and minerals of the epidote-clinozoisite group
appear in most of the slides. Carbonized wood fragments are locally
abundant, but were not seen generally disseminated through the sand-
stone. It is noteworthy that a high proportion of rock fragments to
quartz and feldspar is characteristic of the Franciscan-Knoxville sand-
stones of this area.
Shale. Franciscan shales are typically dark gray and silty, and occur
as laminations or relatively thin layers interbedded with sandstone.
The Knoxville group is characterized by thick sections of nodular gray
to greenish-gray clay shales, commonly containing nodules or thin beds
of dark fine-grained limestone ; but one thick section consists of alter-
nating beds of clay shale and fine-grained sandstone.
Conglomerate. Scattered rounded pebbles appearing from place to
place in the soil of areas underlain by Franciscan rocks indicate that
conglomerates are present; but well-exposed Franciscan conglomerate
was found at only one place. In Bear Canyon, about three-fourths of
a mile south of Anderson Springs, the conglomerate crops out ; here it
is composed of rounded pebbles, cobbles, and a few boulders in- a sand-
stone matrix. Of the lithologic types represented, dark fine-grained
volcanic rocks are most abundant, and these are accompanied by smaller
amounts of chert, shale, greenstone, sandstone, and quartz.
In the Knoxville group, a conglomeratic zone about 1 mile wide extends
from the southern part of Soda Creek, near the middle of the eastern
border of the quadrangle, to J^xcelsior Valley, about 4 miles to the north-
14 LOWER LAKE QUADRANGLE [Bull. 166
west. The conglomerate is in general poorly sorted, interbedded with gray-
wacke (which also forms tlie matrix) , and constituted principally of chert,
dense porphyritic volcanic rocks, and quartz. A noteworthy conglomerate,
of probable intraformational origin, crops out on Soda Creek, south of
Hill 1550 ; it consists of large angular to subangular pieces of graywacke,
up to 2 feet across, cemented into a mosaic pattern by a scant matrix of
sand and rounded pebbles. To the northwest, in the same conglomeratic
zone, felsic plutonic rocks are conspicuous in the conglomerate ; these are
best seen near Hill 2155, where a thickness of about 400 feet of conglom-
erate interbedded with graywacke is well exposed. Pebble conglomerate
is most common, the pebbles consisting of the usual dark cherts and por-
phyries, but certain beds contain well-rounded boulders of felsic plutonic
rocks, embedded in a matrix of graywacke, or these boulders may appear
singly in a bed of the massive graywacke. Most of the boulders consist of
coarse-grained granitic rocks, but diorite, gabbro, and limestone are
represented.
Limestone. Limestone, occurring as thin beds or round nodules,
usually associated with shale, is a minor constituent of the Knoxville
group, but was not seen in the Franciscan. Typically it is dense and dark
gray on fresh surfaces, although weathered surfaces may be yellow or
light gray. Fossils, mostly of species of Buchia, are abundant in some
of the nodules.
Chert. Chert is not a prominent constituent of the Franciscan-Knox-
ville sequence in this quadrangle, although it appears as discontinuous
bands or as large isolated blocks throughout the area of Franciscan-Knox-
ville rocks. Similar Franciscan chert beds have been described by Davis
(1918, pp. 235-432) and by Taliaferro (1933). It is usually interbedded
with shale, but the shale partings may be very thin. The texture is micro-
crystalline, and specimens are typically traversed by innumerable
limonite-stained fracture planes and by veinlets of clear quartz. The
cherts appear in many striking hues, of which red and green are most
common.
Petrology of the Igneous Rocks
The Franciscan and Knoxville groups of this area are intruded by,
and interbedded with, various types of basic and ultrabasie igneous rocks
which are not found in the overlying Cretaceous rocks. Of the intrusive
rocks, the serpentinized ultrabasie rocks are by far the most abundant,
cropping out over some 25 percent of the total Franciscan-Knoxviile
area, as great irregular bodies only slightly elongated in the direction of
regional strike, or as long narrow sill-like bodies. A single large lenticular
body of gabbro and diabase crops out immediately west of Harbin
Springs, apparently intruded between Knoxville shale and serpentine.
A small sill of diabase intrudes Knoxville shales north of Middletown,
where it is associated with larger bodies of diabase intrusion breccia.
Locally, diabase also appears within or on the borders of serpentine
masses. Altered volcanic rocks, called greenstones in this report, occur as
flows interbedded with sediments and as small intrusive bodies.
Serpentine Rock. The serpentine rocks may be best studied along
State Highway 53, from 1 mile to about 3 miles northeast of Middletown,
where the highway cuts across the strike of the serpentine bodies. These
1953] STRATIGRAPHY AND PETROGRAPHY 15
exposures present a cross-section of the lithology and structure of ser-
pentine in this area, and they yield more information than the natural
outcrops. About a mile northeast of Middletown, a deep cut has been
made through a narrow serpentine body intrusive into Knoxville shale.
Closely spaced shear planes in the serpentine dip steeply to the north (as
do the intruded sediments) , and bands of light serpentine mark the planes
of strongest shearing. The texture is generally sugary, but is locally very
fine grained. Disseminated flecks of chromite are more or less abundant,
and may become concentrated and streaked along shear planes, forming a
banded structure. Some of the rock is penetrated by finely reticulating
veinlets of chrysotile. About a mile to the north, the highway cuts through
an extension of a large serpentine body, exposing irregularly fractured
dark olive green rock which is only locally sheared. Magnesite veins were
observed here, the largest of which was half an inch wide and about 12
feet long. The serpentine is very fine grained, and contains an abundance
of finely divided chromite, of which tiny segregations give the rock a
mottled appearance. A contact of the serpentine with the intruded Knox-
ville shales is exposed in the next road cut, adjacent to B.M. 1030. Both
shale and serpentine have been strongly sheared at the contact. Very
massive serpentine, fractured into large blocks but little sheared, appears
in the last cut of the sequence, just south of B.M. 963. Weathered surfaces
and fracture planes are white to light green, whereas the unweathered
rock is olive green. Notable here are relict textures of the original coarse-
grained ultrabasic rock, especially the conspicuous shiny yellow-green
pseudomorphs of serpentine after enstatite (bastite).
The petrology and structure of the serpentine is thus variable from
place to place. Whatever the nature of the original ultrabasic rock, it has
been everywhere replaced by serpentine. By far the most common type
consists of coarse, shiny bastite in a fine-grained matrix, probably derived
from peridotite or related coarse-grained rocks. The sugary -textured and
aphanitic types, probably derived from dunites, are less common. Dis-
tinctively different aspects of the serpentine have been produced by dif-
ferent degrees and kinds of deformation : (1) strongly sheared pale-green
or white serpentine, in which relict textures are destroyed, and which
commonly appears along contacts; (2) angular to rounded boulders in a
matrix of sheared serpentine; (3) massive serpentine, little sheared, but
fractured into large irregular blocks.
Four representative specimens were selected for microscopic study, and
in the identification of serpentine minerals, the data of Self ridge (1936)
were followed. Thus identified, none of the slides contain antigorite, but
are formed entirely of the mineral serpentine, whose mesh structure
suggests that the original rocks were rich in olivine. A common type shows
birefringent patches of fibrous bastite set in a pale brownish-yellow,
complexly veined groundmass exhibiting distinct hourglass structure and
containing relicts of augite and enstatite. Replacement relationships in-
dicate that the fibrous bastite has formed from enstatite, and that the
hourglass structure, which Selfridge relates to bastite structure, has
formed from augite. Another slide shows large birefringent patches
of fibrous bastite in a groundmass of mesh structure serpentine, and
the remaining two are almost entirely of mesh structure serpentine.
Chromite and picotite are normal accessories, and some specimens appear
black in hand specimen because of an abundance of chromite.
16 LOWER LAKE QUADRANGLE [BuU. 166
The strong shoarinp: at contacts of serpentine bodies sngrfjests that they
have been scjueezed into their present positions as mobile masses of rock
by movements following: orijrinal intrusion, coolinjr, and emplacement. No
thermal eflFects of serpentine upon wall rocks were observed, although
metasomatic effects from solutions, perhaps emitted from serpentine
bodies at depth, are locally strong. None of the rocks mapped as Cre-
taceous show the metasomatic effects which are common in Jurassic rocks.
Diabase Intrusion-Breccia. In the area of Knoxville sediments along
Iligliway 53, 1 to 2 miles northeast of Middletown, three bodies of diabase
breccia form conspicuous steep-sided hills, on and about which large
boulders of the resistant breccia are scattered. The area of the smallest
is 12 acres, and that of the largest, which is located just northwest of
B.M. 1040, is 20 acres. The roughly oval outcrops are elongated almost
at riglit angles to the strike of the enclosing sediment, suggesting that
the foi-m is that of a discordantly intrusive plug. Concordant layers of
the same breccia, however, crop out as interbeds witli shale in the nearby
i-oad cuts ; they range in thickness from a few inches up to about 25 feet,
and grade horizontally into beds of shale. The fragments forming the
intrusive bodies are entirely unsorted, varying in size from small pebbles
to angular blocks 5 feet in diameter, and are embedded in a scant fine-
grained diabasic matrix. Every variation in shape and roundness is
represented, although most fragments are angular to subangular; the
smaller fragments are generally more rounded than are the larger. The
texture, also, varies widely, from very fine diabasic to very coarse
gabbroic, but tlie fine textures predominate. The evidence indicates that
tlie breccias resulted from intrusion of pluglike bodies through un-
consolidated sediments, onto the sea floor, and that the viscous breccia
extruded from the vent flowed for a short distance, forming concordant
beds which were covered by later sediment. Similar breccias intrusive
into Franciscan rocks have been described by Iluey (1048) in the Tesla
quadrangle, which is about 80 miles to tlie south.
Diabase and Oabbro. In addition to the diabase described above,
which is older than the serpentine, bodies of basic igneous rock are
associated with tlie serpentine, as large independent intrusive masses,
and as border phases and differentiates grading into the serpentine.
A large body of diabase and gabbro. over 2 miles long by about half a
mile w^ide, crops out in the area immediately west of Harbin Springs,
where it forms a ridge surmounted by a number of prominent knobs.
Textures of the rock vary from fine-grained diabasic to very coarse-
grained pegmatitic ; and the relative proportion of pyroxene to feldspar
varies considerably from place to place. T'nder the microscope, an average
specimen was seen to be a medium-grained diabase composed entirely
of intergrown diallage and calcic plagioclase (Aus.-) in about equal
amounts. This gabbro-diabasic body lies between a large sill-like body
of .serpentine and Knoxville shale. The shale is not metamorphosed
beyond induration in the immediate vicinity of the contact, and silicifi-
cation in narrow vein-like extensions reaching several hundred feet
from the contact. The contact between the gabbro-diabase body and the
serpentine is not well exposed, but local jiegmatitic phases of gabbro
appear to extend into the serjientine, suggesting emplacement of gabbro
and diabase between the shale and serpentine.
1953] STRATIGRAPHY AND PETROGRAPHY 17
Greenstone. The general term p;reenstone is applied to a variety of
greenish to dark greenish-gray rocks of megaseopically indeterminate
mineral composition, which are derived from volcanic rocks and which
are generally similar in color, specific gravity, and hardness. Some
Franciscan areas are mainly composed of such greenstones, but poor
exposures do not permit determination of the relation of greenstones to
surrounding sediments. An area of about 5 square miles northwest of
Burns Valley consists of greenstones and sediments in an estimated
ratio of 9 :1, and is shown on the geologic map as greenstone, and similar
rocks were mapped in the mountainous country east of Middletown.
Innumerable smaller bodies which appear throughout the Franciscan-
Knoxville sequence M^ere not shown on the geologic map. The intrusive
nature of many of the smaller bodies is indicated by a plug or dikelike
form and by autobrecciated structure. Flows ranging in thickness from
a few inches to several feet are interbedded with shales in a gully about
1000 yards west of B.M. 1040, 2 miles north of Middletown. In detail,
the greenstones vary in appearance : the color ranges from pale greenish
gray to nearly black, and the texture ranges from entirely aphanitic
to distinctly porphyritic. Variable features include spots of various
colors, veining, autobrecciated structure, or mottling. Pillow structure,
common elsewhere in Franciscan greenstone, was observed in one locality
only ; such features are here mostly concealed by rock mantle.
Of twelve specimens from various localities selected for microscopic
study, most show porphyritic texture, with large euhedral to subhedral
phenocrysts of albite or augite, or both, in a fine-grained groundmass. A
diabasic texture appears in six of the specimens, and a granoblastic
texture in two. One specimen is composed almost entirely of large altered
feldspar phenocrysts, with scant interstitial material. Patches of calcite
or of chlorite, which may represent amygdule fillings, are common.
Most of the specimens are cut by veins or quartz or albite, or of both.
All of the feldspar sufficiently unaltered to be determined showed
refractive indices less than balsam, and extinction angles appropriate
to albite. More accurate determinations of indices by oils were not
possible, because the clouded conditions of the fragments confused the
Becke line. The Rittman zone method, and extinction angles of albite
twins, were used in the determinations. No unaltered feldspar was seen,
and the alteration ranges from almost complete to slight. Chlorite is the
common recognizable alteration product. Augite, commonly titaniferous
and unaltered but uralitized in one slide, occurs in most of the slides.
Yellow acmite was noted as phenocrysts in one slide. Green or greenish
yellow chlorite is an abundant and ubiquitous mineral. Epidote and
clinozoisite are present in many slides, and one slide is composed almost
entirely of epidote and quartz. Sphene, finely granular, is an abundant
constituent of most of the slides. It is commonly accompanied by
leucoxene.
Certain features of the greenstones have genetic significance : (1) The
feldspar is albitic, and is commonly associated with nearly fresh augite,
chlorite, and sphene, indicating alteration with introduction of soda.
(2) The usual intricate veining indicates tliat solutions have moved
freely through the rocks. (3) The diabasic texture of many greenstone
bodies suggests that they are intrusive. Because of the prevalence of
sodic feldspar, these greenstones are classed as spilites. Turner has
18 LOWER LAKE QUADRANGLE [Bull. 166
recently (1948) snmmarizod current opinion on the orip:in of the spilitic
rocks as follows: "Development of albite in spilites is usually essentially
a metasomatic process, involving addition of Na20 and SiOo and com-
plimentary removal of CaO and AloO.i. Crystallization of albite and
aupite from a spilitic magma is also admitted as a possible mode of origin
for the albite-pyroxene association of some ophitic albite diabases." An
epidote-quartz rock from Bald Mountain (I/120D) was probably formed
from a basic flow by the action of lime-rich solutions originating from
adjacent cooling spilites. The occurrence of such rocks, and the presence
of the almost ubiquitous veining, suggests that these Franciscan-Knox-
ville spilites are metasomatized volcanics.
Metamorphism
Regional metamorphism of rocks of the Franciscan-Knoxville groups
has not advanced beyond an incipient stage which is evidenced in slight
recrystallization of the matrices of sandstones, slight shearing which
tends to cause boundaries of individual grains to become a little indis-
tinct, and elongation of plastic fragments such as mudstones. In addition,
quartz fragments commonly show strain shadows, and the twin lamellae
of feldspars are commonly distorted. However, in local zones of shear
or intrusion, more advanced stages of metamorphism are reached, as
phyllonites have been found in some shear zones, and completely re-
crystallized schists occur locally in the vicinity of intrusions.
Local Dislocation Metamorphism. The Franciscan area southeast of
Cobb Mt., comprising about 18 square miles, is crossed by a large number
of northwestward-trending shear zones, marked by moderately deformed
rocks. A number of these shear zones are well exposed along Bear Canyon,
where they vary in width from a few feet to a few tens of feet, and in
intensity from degrees marked by strong crumpling, to others marked
by slight shearing. Because of the heavy cover of soil and vegetation, the
shear zones cannot be traced into the interstream areas, but much of the
float there is schistose sandstone. It appears that well over half of the
Franciscan sediments in this local area have undergone some degree of
deformation. In rocks of the Knoxville group, dislocation metamorphism
was observed in only a few places, and these are in the vicinity of large
faults. The deformation in the Knoxville is restricted to slight shearing,
commonly along widely spaced slip planes, and no crumpling was
observed.
Since this deformation of the Franciscan is confined to narrow and
irregularly spaced shear zones, true metamorphic zones in the regional
sense cannot be mapped. However, products of different degrees of this
local dislocation metamorphism correspond with products of low grade
regional metamorphism of grayAvacke, as described by Ilutton and
Turner (1936) and as discussed further by Turner (1948). In the rocks
of South Westland, New Zealand, the index minerals in order of increas-
ing grade for metamorphic derivatives of graywacke are chlorite, biotite,
and oligoclase. The chlorite zone occupies a much greater area than the
other two zones combined.
Metamorphism Principally hy Metasomatism. Many prominent knobs
of resistant sedimentary rock rise above the deeply weathered Franciscan-
Knoxville terrain, and the question arises as to the cause of their rela-
1953] STRATIGRAPHY AND PETROGRAPHY 19
tively greater resistance to weathering. Most outcrops of such resistant
sediments were found in the vicinity of exposed serpentine bodies, and
microscopic examination reveals that all are metasomatized or otherwise
altered by the intrusions. Examples of replacement by carbonate, by
silica, and by feldspar have been found.
In the bed of Palmer Creek, a zone of white crystalline carbonate rock
up to 20 feet in width appears in the midst of a bed of pebble conglomer-
ate. Ghosts of replaced conglomerate pebbles appear throughout the
carbonate rock; and locally, unreplaced or partially replaced pebbles
are brought into relief by differential weathering. In the vicinity, a dike
of greenstone a few feet wide terminates in a calcite vein of about the
same width, and the vein tapers out gradually along a length of some
25 feet. There are a number of small greenstone bodies nearby. Micro-
scopic examination of the conglomerate shows partially replaced pebbles
of chert and mudstone set in a murky carbonate matrix, which contains
patches and euhedral crystals of clear carbonate. Similar but much more
extensive zones of carbonate replacing conglomerate occur a few hundred
yards to the west. There are many examples of carbonate replacing
sandstone throughout the Franciscan-Knoxville sequence.
An example of large scale replacement of sandstone by silica may be
seen about a mile northwest of Anderson Springs ; the silicified sandstone
appears as a steep knob about 100 feet high, located about 300 yards
from a large serpentine intrusion. The microscope shows that the gray-
wacke sandstone is intricately veined with quartz in interlocking crystals.
These veins merge into the matrix, forming clear patches of small inter-
locking quartz grains.
Chlorite has replaced sandstone to form a very tough, resistant,
greenish-gray rock which crops out prominently in the region a mile
north of Brushy Sky High. Microscopic examination shows unsorted
coarse fragments of rocks, quartz, and feldspar in a silty matrix. Pale
greenish-yellow veins and patches of antigorite and finely crystalline
chlorite have replaced matrix and detrital grains, so that chlorite com-
prises about one-third of the rock.
Large scale replacement of diabase by pectolite may be seen about 1^
miles north of Middletown, just beyond the northernmost corner of the
CoUayomi Grant. Here a diabase sill forms a cliff along Putah Creek;
a 40-foot thickness of the sill is exposed, to its upper contact with the
intruded shales. The fine-grained diabase is strongly and intricately
veined with white pectolite, which locally forms large irregular masses.
To the east, a dike of pure white pectolite, which extends several hundred
feet and reaches a maximum width of about 30 feet, crosses a body
of diabase breccia.
Induration of sediments in the vicinity of serpentine is especially
striking in the SE^ of sec. 32, along the Big Canyon Creek road. Here
the indurated sediments form a number of very steep, prominent knobs
along the road. Among the specimens selected for microscopic study,
one strongly veined gray rock showed an indistinct diabasic texture in
plain light, although nearly isotropic under crossed nicols. The diabasic
appearance is attributed to abundant feldspar microlites, a few of which
are well-formed and clearly birefringent. Small angular quartz frag-
ments attest the original sedimentary nature of the rock, which is also
borne out by scattered foraminiferal remains, too poorly preserved for
20 LOWER I.AKE QUADRANGLE [BuU. 166
specific identification. The rock is intimately penetrated by an intricate
network of finely crystalline qnartz-albite veinlets a few millimeters or
less in width, near which recrystallization of the groundmass indicates
diffusion of the hydrothermal solutions. These albitized raudstones may
be classed as adinoles.
(ilaucophanc Schist. Schistose structure and the presence of glauco-
phanc arc connnon but by no means universal features of a varied group
of rocks called the glaucophane schists, Avhich are formed by local me-
tasomatic action on Franciscan-Knoxville rocks. The term has been
extended to include an assemblage of metamorphic rocks of diverse
mineralogical and textural character, but apparently of similar origin.
In the Lower Lake quadrangle, the glaucophane scbists occur as large
blocks, up to hundreds of feet in length, or as smaller isolated outcrops,
tliat are surrounded by a deep mantle of weathered rock or terminate
abruptly against other rocks. They usually crop out prominently as dark
irregular masses, commonly deep blue or green. The schists occur mostly
in the Franciscan area, in the vicinity of serpentine or greenstone. In
the Knoxville group, a zone of schists borders the large serpentine body
at the south end of Coyote Valley, and a few isolated outcrops were noted
in the region south of the Knoxville road. Glaucophane schists are not
prominent in this quadrangle, the total area of outcrop being on the
order of a square mile. Two specimens which appeared to be representa-
tive of the major types were selected for microscopic examination. One
specimen is from Lincoln Kock, a deeply fissured crag 200 feet in height.
The rock is dense and dark bluish gray in color ; the microscope shows it
to be formed principally of irregular blades of glaucophane, elongated in
the direction of schistosity, and intergrown with clinozoisite. A repre-
sentative specimen of dark green schist is formed principally of coarsely
crystalline green actinolite and associated granular albite; chlorite is
abundant, and sphene, clinozoisite, and epidote are accessory minerals.
Silica-carhonate Rock. The term siliea-earbonate rock is applied to a
rock composed essentially of silica minerals and mineraloids associated
with carbonate minerals, usually of the calcite group. In the field, the rock
may be easily recognized by the distinctive green opal that is commonly
one of its constituents ; or by the conspicuous appearance of the weathered
rock, which stands out as ridges and knobs because of its resistance, and
typically is colored in vivid hues of yellow and red brown. The outcrops
arc usually cellular box-works of silica, formed by the leaching of asso-
ciated carbonate. Silica carbonate rock is significant in the search for
quicksilver deposits in tliis region, because it is closely associated with
most of the known ore bodies. In detail, the texture and composition of the
rock is decidedly variable. Some glassy varieties are composed almost
entirely of silica mineraloids ; other varieties are coarsely crystalline, com-
posed mostly of carbonates. Massive, intricately veined, and schistose
structures are all represented in various specimens. From outcrops west of
the Wcipcr IMinc, a representative specimen was selected for study. The
rock is of complicated structure, mottled Avith irregular bands of bluish
gray, masses of light-gray crystalline carbonate, and thin seams and
masses of green opal. The rock is composed principally of the carbonjite
siderite, intergrown with a fibrous mineral in scaly aggregates, which was
identified as okcnite, a zeolite. Carbonates in the ankerite range are
1953] STRATIGRAPHY AND PETROGRAPHY 21
abundant, as is opal. Knopf (1907) showed that silica carbonate rock
may be formed by hydrothermal alteration of serpentines, and subsequent
observations in the Coast Ranges have shown this mode of origin to be
general. The rock is commonly found in known fault zones, where it
apparently formed by alteration of serpentine previously squeezed into
the fault.
Stratigraphic Relations and Origin
As the structure of the Franciscan rocks is complex and obscure, the
thickness can only be surmised ; it is probably not less than 5000 feet,
and it may be much greater. The base of the Franciscan was not observed ;
indeed, there is no record of its having been observed anywhere. Further-
more, the relations between the Franciscan and the overlying Knoxville
northwest of Middletown are obscured by a large elongated body of ser-
pentine, whose nearly straight southern boundary suggests faulting. The
boundary between Franciscan and Knoxville is arbitrarily placed along
this supposed fault. According to the structural interpretation shown in
the geologic sections, the Knoxville group has a thickness of about 10,000
feet in this map area. This does not include thickness of the serpentine
bodies. The relationship of the Knoxville to the overlying Cretaceous is
obscure, because of poor exposures and similarity of lithology. If an un-
conformity exists, it is of low angularity.
The Franciscan-Knoxville rocks exposed in this quadrangle constitute
so small a portion of the total extent in California that no attempt will
be made to discuss the regional paleogeographic conditions that prevailed
during deposition. This has been done by Taliaferro in several publica-
tions (1941, 1943), and recently summarized by Eardley (1951).
Instead, an attempt will be made to deduce the local tectonic environment
from the lithologic associations, in the light of recent developments in
sedimentary tectonics, as set forth by Krynine (1941), Petti John (1949),
and especially Krumbein and Sloss (1951). The characteristics of a sedi-
mentary rock depend upon the environment of deposition (e.g. conti-
nental, shallow or deep marine) and the tectonic setting of deposition (e.g.
shelf or geosynclinal). The concept of lithologic associations was derived
from the principle that sedimentary properties are related to the tectonic
intensity which prevailed during their deposition. Tectono-environmental
classifications represent integrations of tectonic elements, lithologic asso-
ciations, and sedimentary environments. For the sedimentary environ-
ment within each tectonic setting, the probable lithologic association may
be predicted.
The following lithologic features of the Franciscan-Knoxville group in
this quadrangle have especial significance for tectonic analysis : Thickness
is great, in excess of 5,000 feet ; clastic sediments are poorly sorted ; sand-
stones and shales are composed of angular grains, but components of
conglomerates are generally moderately rounded; in the Franciscan,
sandstone is locally laminated with thin layers of black shale, while in
the Knoxville shales predominate ; sandstones are graywackes, composed
of rock fragments, quartz, and feldspar set In a silty matrix which con-
stitutes about one-third of the rock; chert, greenstones, and ultrabasic
rocks are associated with the clastic sediments.
The Franciscan lithologic association is typically geosynclinal; the
high ratio of graywacke to shale suggests that deposition tended to be
22 LOWER LAKE QUADRANGLE [BuU. 166
more rapid than subsidence, causing transitional or perhaps continental
conditions to prevail in the geosyneline. The conglomerates of rounded,
unsorted components are also suggestive of such conditions. On the other
hand, the cherts (which contain fossil radiolarian skeletons) are sug-
gestive of marine environment at depths exceeding 600 feet. Evidently
the rate of subsidence of the geosyneline was irregular both in space and
time. Such irregularities have been noted elsewhere in geosynclines of
this type (eugeosynclines, or geosynclines which are orogenically and
volcanically active) . The source area furnished large quantities of quartz,
mudstones, cherts, schists, and dark, fine-grained volcanics, and lesser
quantities of both fresh and partly weathered sodic feldspar. The
abundance of mudstones and chert fragments suggests that these ma-
terials may have been derived by erosion of uplifted earlier sediments
witliin the geosyneline, the "cannibalism" suggested by Krynine. The
earlier sediments may have been supplied from a volcanic archipelago
lying to the west of the present coastline, and this archipelago may have
continued to supply a part of the clastic material during the existence of
the geos^Ticline.
The Knoxville differs from the Franciscan in its greater ratio of shale
to graywacke, and in the character of the shale, which is typically not
silty, but clay shale. That these shales are at least partly marine is shown
by presence of the marine pelecypod Buchia which is locally abundant in
limestone nodules enclosed in the shales. The lithology suggests that sub-
sidence was more rapid than deposition, thus implying that the island
arcs projected only slightly above sea level. The conglomeratic belt in the
Knoxville, which locally contains boulders, would reflect a period of
uplift in the source area.
Age and Correlation
The rocks mapped as Franciscan were distinguished entirely on the
basis of lithologic association, which consists of graywacke with little
shale, and no fossils were found in them. On the basis of ichthyosaur
remains found in chert boulders identified as Franciscan, and on struc-
tural and stratigraphic relationships observed throughout the Coast
Range, Taliaferro (1941) dates the Franciscan as post-Nevadan and
pre-Cretaceous, and believes it to be confined to the Tithonian stage of
the Jurassic.
The following fossils, identified by the writer, were found in rocks
mapped as Knoxville: Buchia piochii, Buchia aff. Buchia stantoni, and
the belemnite Oxyteuthis tehamaensis. They indicate an Upper Jurassic
age, although piochii may range into the Lower Cretaceous. The Knox-
ville group as used in this report corresponds to the third and fourth
stages of Taliaferro 's Franciscan-Knoxville sequence. A further correla-
tion is made, in that the lithology of the Lower Lake Knoxville group
corresponds closely to that of the type section, as mentioned by White
(1885), and defined by Becker (1888) as "The group especially
characterized by the presence of Aucella in the Coast Ranges will be
referred to as the Knoxville series, because they are typically developed
and have been especially studied in tlie neighborliood of the mining town
of tliat name." A more complete description of tlie Knoxville was pub-
lished by Diller and Stanton (1894). F. M. Anderson (1945) divided
his Knoxville series into three groups of which the Elder Creek and the
Grindstone are Portlandian, the Newville Tithonian. The Knoxville
1953] STRATIGRAPHY AND PETROGRAPHY 23
group as used in the present paper corresponds, at least in part, with
Anderson 's Newville.
Cretaceous (Undifferentiated)
A thick and monotonous succession of massive yellowish-brown sand-
stones and gray sliales, interbedded in about equal proportions, overlies
the Knoxville rocks. Stratigraphic relations east of Lower Lake indicate
that the sandstone and shale beds are confined to the Cretaceous system :
they are overlain by fossiliferous beds of Paleocene age, and underlain
by beds containing Buchia piochii Gabb, which is believed to denote an
Upper Jurassic or possibly Lower Cretaceous age. No fossils of strati-
graphic significance were found within these supposed Cretaceous rocks
cropping out in the Lower Lake quadrangle, but fossils were found
within similar rocks cropping out in the adjoining Morgan Valley
quadrangle by B. L. Conrey (unpublished Master's Thesis, University
of California, 1947). Conrey mapped under the designation "Shasta
Group" a sequence of sandstones and shales of similar lithologic type
and proportions to the Lower Lake rocks, except that conglomerate
members, very minor in the Lower Lake rocks, are prominent in three
zones. A coarse and persistent basal conglomerate contains large heavy
ribbed Buchia crassicolis and Buchia crassicolis var. graciles, which indi-
cate a Lower Cretaceous age.
The rocks mapped as undifferentiated Cretaceous in the Lower Lake
quadrangle are those overlying the Knoxville group, and locally overlain
by the Martinez formation. Distinction between Knoxville and Cre-i
taceous was based on the following lithologic criteria: the Knoxville
rocks contain a larger proportion of shale, are generally slightly more
indurated and commonly veined, and are associated with a variety of
basic igneous rocks; none of the sediments mapped as Cretaceous are
intruded by igneous rocks. The Cretaceous rocks are exposed over an
area of some 25 square miles of the country between Middletown and
Lower Lake, and overlapping lava flows cover unknown additional areas.
East of Lower Lake, similar sandstones and shales crop out over an
area of some 5 square miles, and are overlapped by younger rocks.
Lithology
Massive yellowish-brown feldspathic sandstone is the most conspicuous
component of the Shasta group here, but this sandstone is interbedded
with an approximately equal proportion of pelitic rocks. The sandstone
is typically fine- to medium-grained and has an abundant silty matrix.
The pelites are typically micaceous and brownish gray in color. The main
associations of pelite and sandstone are (1) massive sandstone in beds
up to 15 feet in thickness with minor shale partings (2) shale with little
interbedded sandstones and (3) interbedded sandstone, shale, and mud-
stone, the individual beds ranging in thickness from a few inches to a
few feet. Conglomerate and limestone are sparingly represented, con-
stituting only a fraction of a percent of the total volume.
Sandstone. Megascopically, the Cretaceous sandstones show some
characteristics which would relate them to the graywacke group as
defined by Pettijohn*. The massive sandstone beds reach a maximum
thickness of about 15 feet, and commonly show no internal structural
features. Cross-bedding is rare. Lamination is common, but the laminae
• Pettijohn, F. J., Sedimentary rocks : Harper and Brothers, New York, 1949.
24 LOWER LAKE QUADRANGLE [BuU. 166
show little change in texture from top to bottom. Isolated flat pieces of
sliale embedded in massive sandstone are common in some localities.
Tou-^hness is moderate to friable, except tliat carbonate-cemented varie-
ties, which are uncommon, are very tou<,'h. The j)redominating color is
a medium yellowish brown, but yellowish-gray and medium-gray varie-
ties are common. Dusky to dark shades are decidedly unusual in these
sandstones. Gray varieties weather to yellowish brown; and many appar-
ently fresh specimens, now uniformly yellowish brown, may have origi-
nally been gray. The generally poor sorting of the sandstones is apparent
in the hand specimen: rock fragments and large flakes of biotite are
conspicuous, and the silty nature of the matrix is easily seen.
Microscopic study of 12 representative thin sections reveals a mineral
composition consistent with the graywacke group of Pettijohn, yet these
sandstones lack other characteristics which Pettijohn considers definitive
of graywacke, such as dark color, and toughness of the matrix. Possibly
these Cretaceous sandstones, though mineralogically of graywacke com-
position, have not undergone those diagenetic changes which produce
the characteristic graywacke appearance. The sorting is poor, although
the grain size rarely exceeds 1 millimeter and the clay fraction is only
sparingly present. Coarse-grained varieties are common, but a more
common variety is composed of scattered coarse fragments in a silty
matrix. The grains are generally angular to subangular, and slivers of
quartz are not uncommon. The matrix is mostly of silt which contains
abundant shreds of mica and chlorite. Patches of yellowish-brown biotite
with indistinct borders are abundant in the matrix of many specimens.
Textural relations with surrounding grains suggest that some of this
biotite is regenerated, but this is difficult to establish. This yellow-brown
biotite, together with brownish clay, seems to be responsible for a yellow-
ish brown color of the rock. Gray sandstones have a larger amount of
chlorite in the matrix and less of the brown clay. Most of the finely
divided chlorite in the matrix appears to be authigenic, but other larger
flakes appear to be altered from biotite. A small proportion of the
sandstone is cemented with carbonate, which, in the specimens examined,
appears to have replaced the original matrix, and to have partially
replaced detrital grains. Packing of these sandstones is loose to moder-
ately tight; that is, in most specimens, the detrital grains are not entirely
separated by the silty matrix, but touch at corners and locally along
surfaces.
The percentage of the principal minerals in 12 thin sections averaged :
quartz, 28% ; feldspar, 31% ; rock fragments, 14% ; matrix, 23% ;
biotite, 2%, If the specimens studied are representative, as they appear
to be, this composition presents a decided contrast with the average
Franciscan-Knoxville sandstone, in which the amount of rock fragments
nearly equals the (.-ombined amounts of quartz and feldspar. Most of
the quartz is clear, but shows strain shadows under crossed nicols. Clear,
unaltered feldspar occurs together with cloudy, altered feldspar in most
of the slides. The altered feldspars are partially changed to sericite or
to clay. Composition of both altered and unaltered feldspar is generally
in the oligoclase-andesine range and zoning is rare; feldspars showing
microcline twinning are fairly common. Of the rock fragments, dark
chert and mudstone are the most common, but dark volcanic rocks
consisting of tiny laths of feldspar in a microcrystalline base, and usually
1953] STRATIGRAPHY AND PETROGRAPHY 25
altered, are present in most of the slides. Fragments of various kinds
of schist are well represented. Of the heavy minerals, epidote is by far
the most abundant, occurring in large grains, and also in fine aggregates
which maj' be schist fragments. Sphene is next in abundance, followed
by clinozoisite. Biotite in detrital flakes is so commonly present as to
be almost characteristic of these sandstones. Glauconitic sandstone was
observed at one locality (center, sec. 10, T.llN., R.7W.) where it appears
in massive dark-green beds cropping out along the creek for a distance
of about 100 feet. The glauconite occurs as angular to subangular detrital
grains, up to 0.4 millimeter in diameter, and comprises some 30 percent
of the rock.
Pelitic Bocks. Most of the pelitic rocks show definite lamination or
fissility and may therefore be classified as shales. The lamination is gen-
erally formed by bands of gray silty clay alternating with bands of
grayish-brown silt. Some 20 percent of the pelitic rocks are massive, and
exhibit little fissility ; they are called mudstones. Brownish-gray silty
mudstones and grayish clayey mudstones are the most common varieties.
Fissile but unlaminated gray clay shale is more common than laminated
shale. Most of these pelitic rocks appear micaceous and silty when ex-
amined with the unaided eye. No thin sections were made, but crushed
specimens examined in oil indicate that the silty fraction is prominent
throughout.
Conglomerate. Only discontinuous outcrops and scattered gravel
from conglomerate were found in the Cretaceous here. The traceable
zones and beds of conglomerate which elsewhere mark the base of the
Cretaceous and appear locally throughout the system are absent in the
Lower Lake region. Coarse conglomerate interbedded with sandstone
and shale appears on a hill along the Big Canyon Eoad. where it consists
of angular to rounded pebbles and cobbles, largely of shale and feld-
spathic sandstone, in a matrix of carbonate cemented sand. Another minor
zone of conglomerate crops out along Highway 53, about 2 miles south of
Lower Lake, where it consists of rounded pebbles of quartz and resistant
crystalline rocks in a friable sandstone matrix.
Detrital Serpentine. About 2 miles north of Lliddletown, a wide
northwest-trending zone of loose serpentinous material appears in the
midst of sandstones and shales of the Cretaceous. The serpentine occurs as
rounded boulders and unsorted fragments of all sizes embedded in a
scant matrix of fine detrital serpentine and black shale. It also occurs
as "muck," a term applied to soft greenish-white mud and sand derived
from serpentine, and interbedded with black shale. Outcrops of serpen-
tine boulder gravel are superficially identical with some outcrops of
intrusive serpentine. The differences are in the nature of the matrix,
which is detrital in the gravel and sheared in the intrusive serpentine.
Fragments of black shale in the matrix are the most diagnostic feature
of detrital serpentine. Such detrital serpentine zones were apparently
interbedded with normal sediments by the mechanism of submarine
landslides.
Limestone. Limestone in the Cretaceous occurs as beds (which are
generally thin but which reach a maximum observed thickness of 12 feet)
and as nodules in shale ; but the proportion of limestone is small. The
26 LOWER LAKE QUADRANGLE [Bull. 166
texture is characteristically dense and the color is a medium gray, which
weathers to lighter tones of gray or yellow. Fossils were found in the
limestone at one locality, near the nose of a northwest-trending ridge,
half a mile directly east of Rivcrview Lodge. The assemblage consists of
cylindrical worm burrows, casts and molds of a small pelecypod, and rare
fragments of oj^ster shells, none of which proved to be of stratigraphic
value.
Stratigraphic Relations and Origin
The contact between the Cretaceous rocks and the overlying Martinez
is not well defined, but the distribution of the formations as mapped shows
that the relationship is non-conformable. The Cache formation lies with
distinct angular unconformity upon the Cretaceous rocks. On the basis of
the structural interpretation presented in the geologic sections, tlie thick-
ness of the Cretaceous rocks in the area immediately north of Middletown
is about 5000 feet, the maximum thickness along Highway 53 is about
10,000 feet, and the maximum thickness in the area east of Lower Lake is
about 2000 feet. Because of the lack of units within the Cretaceous, the
relationships between the three areas of outcrop are unknown.
The following lithologic features of the Cretaceous rocks in this quad-
rangle have especial significance for tectonic analysis : Sorting is poor,
but clay is mostly separated from sand; clastic grains are generally
angular to subangular ; lamination of mudstone is common, cross-bedding
is rare and on small scale ; intraf ormational shale pebble conglomerates
are not uncommon; detrital quartz and feldspar constitute more than
half of the average sandstone, rock fragments are present but minor,
typically argillaceous matrix forms less than a fourth of average sand-
stone, is accompanied locally by carbonate cement; sandstone is inter-
mediate between graywacke and arkose as defined by Krumbein and Sloss
(1951), and occurs in about equal quantities with mudstones, conglom-
erate and limestone are minor, fossils are rare, but a few marine pele-
cypods were found, and carbonized plant fragments are locally abundant.
This lithologic association is characteristic of the miogeosyncline,
which is a less active, linear or connected ovate geosynclinal zone charac-
terized by a lack of active volcanism ; it may border the eugeosyncline
formed by subsidence accompanied by active volcanism, or develop
adjacent to it in later stages of geosynclinal history. The high ratio of
shale to sandstone suggests that water depths may generally have ex-
ceeded 120 feet, and the laminated shales may record deposition in quiet
waters below wave base. The relatively small amounts of chert and mud-
stone fragments in Cretaceous clastic l3eds, as compared with Pranciscan-
Knoxville clastic beds, suggests that earlier geosynclinal sediments had
been stripped from the uplifted furrows within or on the edge of the
geosyncline, exposing the granitic basement. The belts of thick massive
sandstone probably record intervals of more shallow water, during which
deposition tended to be more rapid than subsidence. The virtual absence
of megafossils is enigmatic, but may be partly a result of turbid seas with
muddy bottoms. Taliaferro (1943) wrote that the Coast Range Cretaceous
sediments "were deposited in very shallow water in sinking basins. The
greatest accumulation in both the north and central Coast Ranges took
place in a long, probably continuous but far from uniform trough which
lay along the west bordor of the Great Valley."
1953] STRATIGRAPHY AND PETROGRAPHY 27
Martinez (Paleocene) Rocks
Massive light gray sandstone beds crop out prominently in the region
directly east of the town of Lower Lake. Intercalated with the sandstone
is a section composed mostly of shale, and another composed mostly of
conglomerate. Fossils characteristic of the Paleocene Martinez are found
in scattered localities through the sandstone, and the lithologic assemblage
is similar to that of the type Martinez formation. A friable white sand-
stone bed is here considered as the basal member of the Martinez, because
it is the stratigraphically lowest rock, identified as Martinez by fossils,
which can be distinguished lithologically from the underlying Cretaceous
sandstone. The petrographic variation among Martinez and Cretaceous
sandstone beds is such that some types are common to both. Sandstone
beds with the following features were mapped as Martinez: (1) Very
massive sandstone which shows little or no change in lithology through
thicknesses of 25 feet or more; (2) sandstone more quartzose and less
silty than the typical Cretaceous sandstone. Martinez rocks crop out over
an elliptical area of about 4 square miles east of Lower Lake. In addition,
a smaller area of sandstone located about 2 miles south of Lower Lake,
was identified lithologically as Martinez, although no fossils were found
in it.
Petrology. In order to facilitate mapping and the interpretation of
geologic structure, the Martinez was separated into four members. Each
of these members is given a descriptive name which indicates the principal
lithologic type by which it was distinguished, but all members contain
feldspathic sandstones of somewhat similar appearance, so that a single
outcrop or a single specimen is inadequate for recognition of a member.
A white sandstone member forms the basal unit. About half a mile east
of Lower Lake, the road to Knoxville crosses Copsey Creek over a metal
bridge ; an outcrop of this member appears about 900 feet south of the
bridge, where it dips steeply to the south. The white sandstone crops out
for about 300 feet along the creek, until it comes in contact with beds of
brown biotitic sandstone not distinguishable from Cretaceous sandstones.
The white sandstone contains a thin but richly f ossilif erous layer of Mar-
tinez fossils, and no fossils were found in the underlying brown sandstone.
Outcrops of the white sandstone are characteristically stained with
limonite, and surrounded by loose white sand produced by weathering
of the poorly cemented rock. Microscopic study shows the sandstone to
be composed of fairly well sorted angular to subangular grains (average
diameter about 0.3 millimeter) in a scant matrix of clay and silt. The
mineral composition of a representative specimen is estimated as quartz,
45 percent ; feldspar, 30 percent ; rock fragments, mostly chert, 10 per-
cent; matrix 10 percent. Heavy mineral analysis showed tourmaline,
zircon, and epidote most prominent, with traces of hypersthene, garnet,
rutile, and brookeite.
Overlying the white sandstone member is a yellow sandstone member,
which crops out at the Copsey Creek bridge. Here it dips gently to the
north at about 15 degrees, and appears to be in fault contact with the
white sandstone. From the bridge, the yellow sandstone may be traced
along the Knoxville road for a distance of about 1^ miles, as it forms
steep cliffs some 150 feet high along the north side of the road. The yellow
sandstone is notably massive, and characteristically cliff -forming ; shale
28 LOWER LAKE QUADRANGLE [BuU. 166
partings are rare, and bedding planes or other structural features are in-
distinct or lackill^^ Tlie liand specimen is yellow to yellow-gray, distinctly
speckled with black, and only moderately hard, except where locally
cemented with calcite. The microscope shows angular to subangular
grains, average size about 0.15 millimeter, in a scant clay matrix. Mineral
composition as estimated from a representative specimen is as follows:
quartz, 50% ; feldspar, 20% ; rock chips, mostly chert, 10% ; clay
matrix, 15%. Heavy mineral analysis showed zircon, epidote, elino-
zoisite, and tourmaline, with traces of hypersthene, rutile, garnet, and
staurolite.
Overlying the yellow sandstone is a conglomerate and sandstone
member, which is best exposed along the Clear Lake Water Company
dam road, near the dam. Although the most distinctive and most easily
traced of all members of the Martinez, it does not appear in the southern
limb of the synclinal structure in which the Martinez is preserved. A
typical outcrop is composed of massive light brown f eldspathic sandstone
interbedded with thick lensing layers and stringers of conglomerate.
The conglomerate is poorly sorted, but some of the beds are mostly of
cobbles and others are mostly of pebbles; these components are sub-
rounded to rounded, and reach a maximum diameter of about 6 inches.
Dark cherts, quartz, and dark, fine-grained igneous porphyries are the
most common rock types found in the conglomerate. A noteworthy
feature is the common occurence of an isolated cobble or pebble embedded
in massive sandstone.
The uppermost member, composed principally of shale and sandstone,
is well exposed in a gully which extends about north-south through the
center of sec. 36, and crosses the Clear Lake Water Company dam road.
The upper 900 feet of strata consist principally of well-bedded light-gray
silty shale, conspicuously micaceous ; a few beds of yellow sandstone are
interbedded with these shales. A single 2-foot bed of yellowish limestone,
which shows distinct cone-in-cone structure, appears about midway in
this upper section. The lower 1250 feet of strata consist of well-bedded
silty shale and clay shale. A search disclosed no microfossils in the entire
section. The shales are not unlike the Cretaceous shales, but strata above
and below contain Tertiary fossils.
Stratigraphic Relations and Origin. The Martinez rests with angular
unconformity upon Knoxville and Cretaceous rocks. The relations
between the Martinez and the overlying Tejon rocks are not clear, because
no well-exposed contact between these units was seen, and both exhibit
attitudes ranging from nearly horizontal to nearly vertical. A study of
attitudes at several points near the contact indicates an angular uncon-
formity of 20 degrees or less between Martinez and Tejon rocks.
The thickness of the Martinez here can be determined only approxi-
mately, because the structure is complicated and the distinction from
the underlying Cretaceous rocks is not certain everywhere. The fol-
lowing thicknesses, computed from data shown on the geologic map, are
maximum, and some of the members lens out very sharply from these
thicknesses: shale and sandstone member, 2150 feet; sandstone and
conglomerate member, 700 feet; yellow sandstone member, 900 feet;
white sandstone member, 500 feet ; total maximum thickness of Martinez,
4250 feet.
1953] STRATIGRAPHY AND PETROGRAPHY 29
A shallow marine geosynclinal environment is suggested by the litho-
logic and faimal associations of the IMartinez. The sandstone differs
from that of the underlying Cretaceous in being more quartzose, less
silty, and occurring in thicker and more massive beds. The removal of
silt suggests a higher degree of sorting, presumably by currents, whereas
the absence of cross-bedding suggests that no direct current action was
involved in deposition. These difficulties may be resolved if it be assumed
that Martinez sandstones were derived from uplifted Cretaceous sand-
stones, the material being somewhat reworked by stream action, and
deposited in quiet marine waters of the transitional or epineritic (less
than 120 feet) zones. Lithology of the sandstone and conglomerate
member suggests deposition under more shallow conditions, perhaps
transitional or even continental. Because tlie fossils occur in local zones
rather than throughout the formation, the faunal evidence cannot be
taken as indicative of environment for the whole formation. Dickerson
(1914) considered that the character of the fauna indicated inshore con-
ditions : ' ' The ratio of gastropods to pelecypods is about 2 : 4-| in the
fauna. Not only is this true in the number of different species but a census
shows that pelecypods flourished in the waters of the shallow Martinez
sea of this time better than the gastropods."
Age and Correlation. From a locality 1 mile southeast of Lower
Lake, Gabb (1866) found fossils which in his opinion showed a com-
mingling of the Chico and the Tejon fossils in a single thin stratum.
Stanton (1895) made further collections from this original locality on
Herudon Creek (named Copsey Creek on the 1945 edition of the Lower
Lake Quadrangle Sheet), and also from material taken from a well at an
old brickyard a quarter of a mile nearer town, but in the same zone. In
1912, Packard and Dickerson (Dickerson, 1914) made a stratigraphic
study of the Tertiary rocks near Lower Lake, which included a sketch
map of the geology. Dickerson 's description of the section on Herndon
(now Copsey) Creek includes the following statement: "... the basal
Martinez consists of 1200 feet of medium gray sandstone. This is overlain
by 500 feet of fine, massive, tan-colored sandstone with gray shale . . .
[which] unconformably overlie the Martinez." Dickerson 's "1200 feet
of medium gray sandstone" includes the fossiliferous white sandstone
member, considered basal Martinez in the present report, together with
the unfossilif erous brown biotitic sandstone, considered as Cretaceous in
the present report. His ' ' 500 feet of fine, massive tan-colored sandstone
with gray shale ' ' makes up part of the yellow sandstone member of the
present report. A striking resemblance between the fauna found in the
white sandstone member, and the fauna of the basal beds north of Mt.
Diablo is noted by Dickerson. The reader is referred to Dickerson 's
paper for an extensive list of fossils from the Martinez at Lower Lake.
The index fossil Turritella pachecoensis appears in both upper and
lower beds,
Tejon (Eocene) Rocks
White conglomeratic sandstone containing fossils which indicate a
Tejon (restricted) age, crops out over an oval area about 1^- square
miles in extent in the region east of Lower Lake. The conglomerate
occurs as beds, lenses, or stringers of generally unsorted granules, peb-
bles, or cobbles ; these are composed principally of quartz, but dark chert
30 LOWER LAKE QUADRANGLE [Bull. 166
and volcanic rocks are represented. The sandstone is light gray, hard,
and medium-grained, speckled with dark rock chips. The microscope
shows a representative sandstone to consist of fairly Avell-sorted angular
to subangular grains, average size about 0.3 millimeter, in a scant clay
matrix, partially cemented with carbonate. Mineral composition was
estimated as follows (in approximate percentages) : quartz, 40% ; feld-
spar (oligoclase to andesine), 20% ; rock fragments, mostly chert, 20% ;
matrix and cement, 15%. Much of the feldspar is partly altered to seri-
cite. Heavy mineral analysis (based on only two samples from the same
locality) showed tourmaline, zircon, staurolite, and rutile as the principal
nonopaque minerals, accompanied by traces of hornblende, epidote,
garnet, and anatase. The presence of significant quantities of staurolite
might be a distinctive feature of the Tejon here, but no such generaliza-
tion can be based upon a small number of analyses. Dickerson estimated
the thickness of the Tejon here at 1100 to 1200 feet, but this is approxi-
mate, as the structure is complicated and no complete sections are
exposed.
The lithology of the Tejon is indicative of geosynclinal conditions,
which varied from shallow water marine through transitional to conti-
nental. The conglomerate stringers are especially suggestive of stream or
beach deposition. Dickerson found marine fauna at two localities, show-
ing that the formation is at least in part marine. The mineral composition
of the Tejon rocks suggests that they were derived from the reworking
of Mesozoic graywackes and conglomerates, from which the silt fraction
had largely been removed by stream action. The reader is referred to
Dickerson 's (1914) paper for a list of Tejon fauna. According to Dr.
J. W. Durham (oral communication, 1950), the presence of Whitneya
ficus Gabb indicates that the Lower Lake Tejon may be correlated with
the now restricted Tejon.
Cache Beds
A thick fresh-water deposit of gravel and silt blankets the northeastern
corner of the map-area, and appears in other localities at the edges of
protecting lava caps. This deposit was described by Becker (1888)
under the name Cache Lake beds; but Anderson (1936) thought the
name unsuitable because there is no " Cache Lake ' ' in the vicinity, and
because the term might be interpreted as ''Cache" lake beds, thus imply-
ing an origin for the sediments that might be questioned in part. Ander-
son suggested as a substitute the name Cache formation, which is adopted
for the present report. The main area of Cache sediments lies east of
Burns Valley. At Quackenbush Mountain, the belt of Cache narrows,
swings around the southern end of Clear Lake, and reappears beneath
the lavas north of Seigler Canyon. Deposits which are correlated with
the Cache formation on the basis of lithology crop out over a small area
north of Coyote Valley, and also east of Hells Half Acre. In addition,
water-laid rhyolitic tuffs, cropping out at the edges of lava flows in the
vicinity of Seigler Springs, are believed to be generally contemporaneous
with the Cache formation.
Petrology. Outcrops of the Cache formation are characterized by
light hues of gray or yellow-brown and by erosional features of the
badland type, which reveal the unconsolidated nature of the deposit.
Through most of its thickness, the Cache is composed of gravel, silt.
1953] STRATIGRAPHY AND PETROGRAPHY 31
and sand, but near the top of the section, which is exposed in the vicinity
of Burns Valley and Clear Lake, water-laid tuffs and tuffaceous sands
become dominant, and these upper sediments are intercalated with clay,
marl, pebbly limestone, and diatomite.
The dominant constituent of the Cache beds is light-gray silt, which
gives the formation its generally light color. Individual beds are formed
by mixture of this silt with varying proportions of clay, of sand, or of
gravel. In the coarser unsorted layers, bedding is indistinct unless em-
phasized by some unusual feature or cementation. Beds of coarse unsorted
pebble and cobble gravel a few feet or tens of feet in thickness, having
a silty matrix, and intercalated with silt or pebbly silt, are characteristic
of the formation. Cross-bedding was rarely observed, and an individual
layer, although thin, can usually be traced through the length of an
outcrop.
Basal beds of the Cache overKe serpentine and sediments of the Knox-
ville group near the intersection of Deadman Canyon with Cache Creek.
These basal beds are poorly sorted pebble and cobble gravels, made up
of subangular to subrounded cobbles in a matrix of pebbles, sand, and
silt. The gravel is mostly of grayivacke, but chert, dark volcanic rocks,
and other crystalline rocks are represented. Sediments higher in the
section are best exposed along Dry Creek, which cuts across the strike
of thick succession of interbedded silts and gravels. The typically un-
sorted gravels have a maximum size in the cobble range, and an average
diameter of about 2 inches. A few beds are composed mostly of boulders,
up to 3 feet in diameter, of chert, schist, and feldspathic sandstone.
The Cache beds cropping out on ' ' The Peninsula, ' ' a topographic feature
located just west of the intersection of Cache Creek and the North Fork
of Cache Creek, are composed largely of light-gray silt. The silt
contains granules which are locally concentrated to form distinct laminae
or beds. The granules are angular to subrounded, and composed of sand-
stone, quartz, or chert.
The lithology and fossils of the upper Cache beds indicate a lacustrine
origin. Marl and pebbly limestone are interbedded with the coarser
elastics, and diatom tests are a common constituent of marls and tuf-
faceous sediments. These tuffaceous sediments, which are variable in
composition, are interbedded with gray silt and clay. Detailed study
of a representative specimen revealed gray, well-bedded granules of
fine-grained basic volcanic rock, accompanied by fragments of basic
feldspar, pyroxenes, and quartz. A more acid variety is made up of sub-
angular granules and pebbles of acidic volcanic rock, pumice shards,
and fragments of quartz and chert in a silty matrix; diatom tests are
also present.
Locally, diatom remains are sufficiently abundant to form diatomites.
Anderson (1936) described such a diatomite bed, located 1 mile east of
the outlet of Clear Lake, which contained the following flora (determined
by G. D. Hanna, of the California Academy of Sciences) : Fragilana
sp., Navicula cf. major, Rhopalodia, sp., Cymhell^i cf. gastroides, Coc-
coneis sp., and Cyclotella small sp. Diatomite crops out over an area of
about half a mile by half a mile near the intersection of Thurston Creek
with the Lakeport road, in the northwest part of the quadrangle. At this
locality, a drilled well is said to have passed through 180 feet of the
diatomite interbedded with light-gray clay. Cuttings from the well are
scattered, and this information could not be verified.
32 LOWER LAKE QUADRANGLE [BuU. 166
Along the north edge of Coyote Valley, about 9 miles directly south of
the main area of Caclie sediments, the lava flows are associated with beds
of silt, cobble gravel, and tuffaceous sediments. The lithology of the
sediments is like that of the upper Cache near Clear Lake, except for the
composition of tlie cobble gravels, whicli are composed largely of rounded
cobbles of white rhyolite. The rhyolite is absent from Cache beds east of
Clear Ijake, and it may liave been derived from the Sonoma volcanics.
Light-gray water-laid tuffs underlie the quartz-bearing basalt flows east
and northeast of Hells Half Acre, and these tuffs also are correlated with
the Cache formation. The tuffs and tuffaceous sands of the Coyote Valley
region are varied in composition ; both acidic and basic types are repre-
sented, as well as mixtures of the two. Textures of the fragments range
from vitric to lithic to crystal. One representative specimen of volcanic
sand is composed of grains of quartz and fresh pyroxene in a silty matrix ;
another is composed entirely of subangular fragments of basalt in a clay
matrix. A specimen of fine-grained vitric-crystal tuff from the base of
Hill 1300 is composed mostly of pumice shards, accompanied by angular
grains of hornblend and quartz.
Some flows of basalt are intercalated Avith the Cache formation. On the
surface of lava-capped tablelands just north of the Lower Lake quad-
rangle, Anderson (1936) found scattered water-worn chert and quartz
pebbles. This evidence, together with the observation that the mesas have
a westerly dip conformable to that of the underljang gravels and sands,
led him to conclude that this basalt was intercalated with Cache sediments.
This was further established by the discovery of a 10-foot bed of limestone
overlying basalt, west of Anderson Flat, at the southeast end of Clear
Lake. In the present study, rounded cobbles and pebbles were found on
the surface of basalt flows in the region immediately north of Coyote
Valley, and similar gravels also underlie the flows.
All clastic sediments interbedded with lavas may not be correlative
with Cache sediments, but they have been correlated here on the assump-
tion that the lavas were erupted within a short span of time, and that the
conditions under which the sediments accumulated were similar to condi-
tions of Cache deposition. On the north rim of the canyon at Hells Half
Acre, the 500 feet of lava exposed in the canyon is overlain by 35 feet of
lithic tuff which is capped by a few feet of cobble gravel ; the tuff, com-
posed of rhyolite fragments in a clay matrix, is similar to many Cache
tuffs, but the gravel contains rounded cobbles of basalt, which do not
appear in the Cache. Adjacent lava flows are at a higher elevation than
these clastic deposits, and apparently overlie them. There is evidence that
some of the basalts underlying this tuff cooled under water, which may
mean that the basin of Cache deposition had not yet been filled by basalt.
About 75 feet below the previously described tuff, a few thin beds of
granular basaltic tuff are interbedded with basalt; the tuff is poorly
sorted, but the bedding is thin and distinct. In addition, the appearance of
the basalt associated with this tuff suggests that it may have cooled under
water : it is porous and slaggy, and stained a deep red-brown.
Deposits of rhyolite tuff which appear beneath basaltic or andesitic
lava flows at various localities in the quadrangle are correlated with the
Cache formation because of resemblance to known Cache deposits and
a comparable degree of deformation. Well-bedded deposits of rhyolite
tuff appear beneath the cliffed, eroded edges of lava flows south and west
DIVISION OF MINES
ir.
il^#r/v"
mm.
A
BULLETIN 166, PLATE 3
I mm.
B
1 mm.
mm.
C D
PHOTOMICROGRAPHS OF FRANCISCAN SANDSTONE
Showing stages in deformation. A. Unsheared. Rock chips are most abundant constitu-
ent. B. Slightly sheared. Shear planes marked by dark micaceous streaks. C. Further
metamorposed. Schistosity is marked by light-colored quartz-rich bands and dark argil-
laceous bands containing sericite. D. Dark argillaceous bands in rock showing greater
metamorphism contain well-developed muscovite. Rock is sheared and crumpled across
banding.
DIVISION OF MINES
,'v
0
BULLETIN 166, PLATE 4
09
►
f
1
X
J^:..
I mm.
mm.
B
PHOTOMICROGRAPHS OF ACIDIC LAVA
A. Quartz- and olivine-bearing dacite from lava field east of
Mount Konocti. Olivine (ol) is surrounded by prisma of
hypersthene ; large .tabular phenocryst is oligoclase. No
quartz in thin-section, but cluster of augite rods probably
replaced quartz. B. Banded rhyolite from Cobb Mountain.
Lighter patches in groundmas.s — which form pink bands in
hand specimen — contain.s tridymite, hematite, and car-
bonate mineral. Large phenocrysts are sanidine (s) and
oligoclase (og).
DIVISION OF MINES
BULLETIN 166, PLATE 5
MwW
•<*:• >
I I
( mm.
C D
PHOTOMICROGRAPHS OF INCLUSIONS IN CLEAR LAKE LAVA
A. Garnet inclusion in andesite from Perini Hill. Feldspar is enclosed in garnet, sur-
rounding reaction rim. B. Schistose xenolith in olivine basalt (basalt is dark band across
bottom). Granular quartz (qz) and augite (au) ; large garnet (ga) in corner. C. Inclu-
sion-loaded hyersthene crystals (by) in andesite from Perini Hill. Round inclusions are
quartz, with a little tridymite. D. Xenolith in andesite from Perini Hill. Dark patches
are biotite, spinel, and inagnetite, surrounded by lighter labradorite.
DIVISION OF MINES
BULLETIN 166, PLATE 6
\
■\
I inch
QUARTZ INCLUSIONS IN OLIVINE BASALT
I'pper left, milky quartz ; upper right, amethystine quartz ; bottom, clear quartz.
Note sharp, irregular boundaries of inclusions.
1953] STRATIGRAPHY AND PETROGRAPHY 33
of Mount Hannah. This tuff reaches a maximum thickness of about 200
feet near Bonanza Springs, and gradually thins to the east and west.
Near Bonanza Springs, tlie tuff consists of angular to sub-angular pebbles
of rhyolite and rare fragments of pumice or obsidian, scattered through a
white ashy matrix. Lensing beds indicate stream deposition. Along High-
way 29, just south of Loch Lomond, the tuff rests upon eroded Cretaceous
sandstone, and contains in its upper part small angular boulders of black
obsidian. North of Seigler Springs, where the tuff is exposed over an area
of several acres, it varies from fine-grained types to types consisting
mainly of coarse pumice fragments. The distribution and lithology of the
tuff suggest derivation from a local vent and deposition in a lake or river
basin continuous with the basin of Cache deposition. It may be contem-
poraneous with rhyolitic pyroclastic rocks described by Anderson (1936),
which crop out southeast of Kelseyville and underlie the lavas forming
Mount Konocti.
The appearance of Cache sediments in some localities suggests that
they have been hydrothermally altered. Over an area of several acres in
the northern half of sec. 2, just north of Lower Lake, Cache silts and sands
are very limonitic, and the soil is dark red-brown. The limonite occurs as a
network of hollow concretionary structures which apparently replace the
sediment. Dense mottled gray-brown or gray-green rocks, composed prin-
cipally of opal, are associated with the limonitic sediments. Such opaline
rocks crop out over several acres at Clear Lake Highlands, at a locality 1
mile east of Lower Lake and another just north of Lower Lake. The
opalized and limonite-rich rocks are believed to be products of alteration
of Cache sediments by hot volcanic solutions or gases, which opalized
the sediments and deposited metallic sulfides ; subsequent decomposition
of the sufides resulted in the formation of limonite. However, no un-
weathered metallic sulfides were found.
Thickness and Origin. The computed thickness of the Cache forma-
tion is so great for a continental deposit that a description of the compu-
tation is included. Basal beds of the Cache formation overlie Knoxville
rocks near Deadman Canyon. The Cache beds dip away from this ex-
posure of older rocks, which is near the axis of a well-defined northwest-
trending anticline. The thickness of the Cache beds forming the south-
western limb may be computed, as these beds dip consistently westward.
Attitudes of the beds, as observed along Phipps and Blackeye Canyons,
are reasonably consistent, with an average dip of about 25 degrees which
flattens in the immediate vicinity of the lake, but does not change direc-
tion. From these considerations, the maximum thickness of the Cache
beds in this quadrangle is computed as 6,500 feet. It is of course possible
that this thickness includes beds repeated by faulting, since the lithology
is so uniform that faulting is not easily detected. The minimum thickness
of the Cache, which is found near the edges of the basin of deposition, is
only a few hundred feet.
The origin of the Cache formation was first considered by Becker
(1888), who was baffled by the great thickness of the deposit, but sug-
gested that it might have accumulated in a lake "of vast dimensions".
Anderson (1936) proposed that the Cache sediments are largely of flu-
vial origin, deposited in a subsiding basin ; except that the calcareous and
diatom-bearing upper beds are lacustrine. The present study indicates
that the Cache formation is thicker and more widespread than has been
2 — e8207
34 LOWER LAKE QUADRANGLE [Bull. 166
previously recorded, but no new evidence bearing on the conditions of
sedimentation has been found. Significant petrographic features include
the following: The finer elastics are commonly well bedded, and cross-
bedding is rare; some gravel beds several tens of feet in thickness are
poorly sorted, and show no structural features; a few beds of boulder
conglomerate appear ; a few fossils of land animals have been found, but
no marine fossils ; the Cache is formed of material which could have been
derived from the local Mesozoic rocks. It seems probable that the Cache
formation was deposited in one or more large tectonic basins which sub-
sided by downwarping or, more likely, along major boundary faults.
Streams from the surrounding highlands carried debris into this subsid-
ing basin, the surface of which was covered from time to time with lakes
or swamps. Periods of increased rainfall or increased stream gradient
caused by rapid subsidence of the basin would permit accumulation of
gravels. The clay and silt deposits might accumulate in shallow lakes or
swamps during times of less rapid deposition. Toward the end of Cache
time a large lake was formed in which chemical and organic sediments
accumulated, and Cache time closed with deposition of these and the
associated tuffaceous sediments, and the occasional outpouring of a
basalt flow.
Age and Correlation. No fossils of stratigraphie significance were
found in the Cache during the present study. Anderson (1936) has sum-
marized the known fossil evidence bearing on the age of the Cache. A few
fragmentary vertebrate remains were found b}" Becker, and Marsh ( 1888)
suggested that they might indicate very late Pliocene age. The fragment
of a lower jaw found by the late AV. M. Davis was examined by V. L.
VanderHoof, who suggested that it is probably the ramus of the lower
jaw of Elephas sp., indicating a Pleistocene ajie. Anderson suggested that
the lithologic similarity of the Cache to the upper Pliocene Tehama for-
mation, which is exposed on the western margin of the Sacramento Val-
ley, might indicate a relationship. He regards the Cache formation tena-
tively as lower Pleistocene, but considers that it may be upper Pliocene.
Clear Lake Volcanic Series
Volcanic rocks in the Clear Lake area were first described by Becker
(1888), who presented petrographic descriptions of some of the lavas
and three chemical analyses. Anderson (1936) discussed in detail the vol-
canic history of the area, mapped and described the petrography of the
lavas around Clear Lake, and presented ten additional chemical analyses.
The present study embraces most of the volcanic rocks in the Clear Lake
region, including much of the ground described by Anderson, as well as
previously undeseribed volcanic rocks to the south. Anderson's report
and map have been constantly referred to, but a more detailed study of
the petrography has been made in an effort to solve the problem of petro-
genesis.
Rhyolitie flows and tuffs, basaltic lavas, and lavas of dacitic and ande-
sitic composition are represented in the Clear T^ake area. The dacitic
lavas contain crystals of sanidine, quartz, and magnesian olivine which
are probably xenocrysts, as well as plagioclase phenocrysts of distinctly
different composition in the same rock. One andesite flow contains abun-
dant inclusions, among which the assemblages cordierite-hypersthene-
biotite, hypersthene-quartz-oligoclase, and labradorite-hypersthene-bio-
tite-spinel-garnet, are well represented. The olivine basalts are nearly all
1953] STRATIGRAPHY AND PETROGRAPHY 35
quartz bearing. It is suggested that both contamination by sedimentary
material and mixing of magmas were involved in the petrogenesis, and
that many of the lavas are consequently hybrid.
The different volcanic rocks of the Clear Lake region belong to a single
volcanic group, herein called the Clear Lake volcanic scries. Volcanism
apparently began in early Pleistocene time, bringing Cache deposition to
a close, as evidenced by rhyolite tuffs and olivine basalt flows intercalated
Avith uppermost Cache beds. Most of the volcanic activit.v was confined to
the Pleistocene, but the appearance of Roundtop Mountain, a cinder cone,
suggests that it is Recent, and solf ataric activity associated with obsidian
at Borax Lake suggests that it, too, may be Recent. The Clear Lake Vol-
canic series is apparently younger than the extensive Sonoma volcanics
(of middle or upper Pliocene age), as quartz-bearing basalts overlie
eroded remnants of the Sonoma to the south, in the Calistoga quadrangle.
(Yates and Hilpert, 1946). Furthermore, the Clear Lake volcanic rocks
are less disturbed and less eroded than the Sonoma. Age relations of in-
dividual units of the Clear Lake Series are imperfectly known because
some units are isolated ; moreover, the contacts of contiguous flows are
commonly obscured by sliding. The relations are in fact clear at only one
locality (near Manning Flat), where rhyodacite overlies obsidian, which
in turn overlies Cache cliatomite two miles to the northwest. The olivine
basalt is probably oldest, as it is in part intercalated with the Cache for-
mation, which is overlain, locally with slight angular unconformity, by
other flows.
No volcanic vents or intrusive dikes which might represent feeders
were observed. The lavas were apparently extruded from a system of
northwest-trending fissures, the different flows no doubt rising along sep-
arate fissures. That local explosion vents were present is shown by the
pyroclastics at Cobb Mountain and the rhyolite tuff bearing blocks of
obsidian, located near Loch Lomond.
Olivine Basalt. Flows of olivine basalt which were apparently inter-
calated with the Cache formation appear as small mesas and as scattered
patches east of Clear Lake. Flows northeast of Middletown form an
extensive plateau-like highland, surmounted by large rounded hills and
bordered by cliffs and talus slopes. Olivine basalt covers an area of some
21 square miles in this quadrangle and extends beyond for several miles
to the southwest. Thickness of the basalt varies considerably because the
lava flowed over an uneven surface, and furthermore built large rounded
hills, presumably above the centers of extrusion. In the main basalt area,
near the center of sec. 16, the measured thickness is 550 feet, but this may
be exceeded in other places. Judging from the height of basalt cliff's, the
thickness of individual flows ranges from a few feet to a maximum of
about 100 feet. As for the number of flows, the terraced contour of the
main basalt area suggests three principal ones, the first and thinnest
being intercalated with the Cache formation, whereas the later flows
partly cover the first and elsewhere overlap onto rocks older than tlie
Cache beds.
The olivine basalts are medium to light gray and inconspicuously
porphyritic, having small crystals of olivine, usually accompanied by
pyroxene and more rarely by plagioclase, set in an aphanitic groundmass.
Vesicular structure is not common, but where present the vesicles are
usually lined with tridymite or crystobalite. A UDteworthy feature of
these otherwise ordinary-appearing basalts is the presence of inclusions
36 LOWER LAKE QUADRANCtLE [BllU. l(J(i
of clear shattered quartz, ranging in size from tiny grains to irregular
masses 15 centimeters in length, and averaging about 2 millimeters. Some
flows apparently contain no quartz, but it is so universally distributed
through most flows that a random liand specimen will contain a few
grains. In a few localities, described later, scattered xenolitlis accompany
tlie quartz.
Of the mineral composition, plagioclase constitutes 50 to 70 percent,
varying little from medium labradorite, and occurring mainly as lathlike
niicroplu'uocrysts or as microlites. Faintly pleochroic hypersthenc (1-10
percent, rarely 25 percent) appears as a minor constituent in about half
of the slides, usually in the groundmass as euhedral to subhedral grains,
less commonly as phenocrysts. Diopsidic augite (10 to 20 percent) is
ubiquitous as a phenocrystic mineral, and is usually present in the
groundmass. Subhedral to anhedral olivine (usually 10 to 20 percent,
rarely up to 30 percent) is by far the most abundant phenocryst, but is
not prominent in the groundmass. The microscope shows a wide variation
in groundmass textures; the most common is microporphyritic, having
small laths of labradorite and grains of augite, accompanied by small
prisms of hypersthene and rarely by grains of olivine. Clear patches
of tridymite, cristobalite or both are common in the groundmass of
some lavas, constituting up to 5 percent of the rock.
Andesite. Perini Hill, located about 3 miles southwest of Lower Lake,
is the central and highest knob of an upland formed by flows of dark gray
lava overlying Cretaceous and Knoxville rocks. This upland, which is sur-
mounted by a number of prominent knobs, is a roughly circular area
of about -1 square miles, fringed on all sides by lava cliffs up to 100 feet
in height. Streams have cut into the upland, and if the knobs represent
accumulations over the vents, they are much modified by erosion. The
lava is mantled with a deep soil which supports a heavy growth of
vegetation.
The dark gray aphanitic groundmass of the Perini Hill lava is
sprinkled with small phenocrysts of plagioclase, and with a striking
abundance of inclusions that range in size from less than 1 millimeter
to about 20 millimeters across. These inclusions are of three main types :
(1) Subhedral to anhedral grains of hypersthene, generally prismatic,
crowded with rounded inclusions of quartz, plagioclase, and rarely, of
vioiet curdieritc. (2) Rounded grains of clear sliattered quartz. (3) Fine-
grained noritic fragments. Thin sections of the lava show cloudy zoned
phen.)crysts of plagioclase (15-20 percent), moderately pleochroic hypers-
tlient' (5 percent), accomi)anied by small amounts of colorless augite
and brown hornblende, in a microporphyritic groundmass composed of
smaller laths of clear plagioclase and prisms of hyperthene in a murky
microcrA'stalline base. Most of the larger plagioclase phenocrysts
(Ano,-,--.-,) have centei-s full of glass inclusions, and clear, slightly more
sodic rims; the ground mass plagioclase is also sodic (Auso-io)- The
euhedral to subhedral hypersthene phenocrysts reach a maximum size
of 2.5 millimeters, and zoning, emphasized by differences in pleochroism,
was seen in a few grains. Tridymite appears as scattered patches in the
groundmass.
Bogiis Mountain is an elongate, rolling highland capped with ande-
sitic la\as which cover an area of about 5 square miles. The lavas have a
maximum thickness of about 500 feet, but the cliffs which border the
lava cap arc rarely more than 75 feet in height. Abundant small tablets
1953] STRATIGRAPHY AND PETROGRAPHY 37
of feldspar set in a black aphanitic groundmass give the basalt a distinc-
tive speckled appearance, which is notabl}' uniform throughout, although
the basalt is locally vesicular, or alternately composed of a few scattered
phenocrysts set in an aphanitic groundmass. Microscopic study of six
thin sections and of the powdered rock indicates that the texture and
mineralogy is fairly uniform. Tabular crystals of labradorite, and sub-
hedral crystals of hypersthene and augite are set in a murky groundmass
which contains small, nearly square plagioclase crystals, granules of
augite, and abundant metallic opaques. Composition of the plagioclase
general^ varies within narrow limits (Anco-Ts), but some lavas contain
small amounts of sodic plagioclase (An3o.4o)- Refractive indices and
optic angle measurements indicate that the augite is diopsidic, and that
the hypersthene contains 23 molecular percent of ferrous silicate.
Rhyolite. Cobb Mountain is formed largely of rhyolite, which is super-
posed on the Franciscan rocks forming the principal ridge of the Mayac-
mas Range, building the mountain some 800 feet above the ridge. Tuffs
crop out only on the precipitous southeastern flank of the mountain, where
they form the base of the volcanic series, and reach a thickness of several
hundred feet. The typical rhyolite is characterized by abundant glassy
or white phenocrj^sts set in an gray aphanitic groundmass, marked with
irregular and discontinuous streaks of pale pink, which become concen-
trated in a single layer to form a pronounced banding. Near the top of
the mountain, pale-pink or yellow aphanitic rhyolites are found, and
these are locally associated with agglomerates. The pale pinkish-gray
tuffs have a pumiceous groundmass, formed by twisted plates and fibres
of opaque glass, through which are scattered abundant white or glassy
phenocrysts, mostly euhedral and little broken. The phenocrysts of the
Cobb Mountain volcanics are of fairly uniform size, averaging about
3 or 4 millimeters in length, but reaching a maximum length of about
10 millimeters.
The microscope shows the graundmass of the banded lavas to be formed
of scattered tiny laths of plagioclase, accompanied by grains and rods
of pyroxene, set in a murky cryptocrystalline base. The lighter streaks
(which appear pink in the hand specimen) consist of flaky hematite and
of reddish or black iron oxide pseudomorphs of ferromagnesian minerals
in an otherwise colorless fabric of feldspar microlites, patches of well-
crystallized tridymite, and cryptocrystalline material. Locally, abundant
finely granular carbonate accompanies this assemblage. In some of the
lighter streaks, groundmass feldspars are better developed in size and
number than in the rest of the groundmass. The pink streaks were
probably formed by fumarolic vapors which, acting along irregular
planes of flow banding, oxidized ferromagnesian minerals and at the
same time deposited hematite, tridymite and /or carbonate.
The conspicuous white or glassy phenocrysts are of sanidine, plagio-
clase, or quartz. The sanidine phenocrysts are fractured and generally
rounded, and their optical properties indicate, from data of Larsen et al.
(1938), a content of 25 percent of the albite molecule. Sanidine could
not be detected in the groundmass of these lavas. Most of the plagioclase
occurs as clear, twinned euhedral crystals in the composition range
Anis-30- However, a small proportion of very calcic plagioclase (Augd-ss),
mostly dusty and strongly zoned, was revealed by study of electromag-
neticall}' separated feldspar in oil immersion. A single such calcic feld-
38 LOWER LAKE QUADRANGLE [Bull. 166
spar appeared in thin section as a tabular phenoerj-st about 3 millimeters
lenpth, havinj; a ealeic core (An,;:,) honeycombed Avith brown jrlass, and
a clear sodic rinu The quartz usual I}' occurs as cracked rounded f?rains,
but some jjjrains show euhedral outlines ; no reaction rims around (juartz
were seen. Ilypersthene, biotite, and brown hornblende are minor ac-
cessories, and are usually much oxidized.
Pyroxine Dacite. Flows of glossy black to gray quartz-bearing pyrox-
ene dacite crop out over an area of about 3 square miles northeast of Mount
Hannah. An aphanitic groundmass, sprinkled with medium-sized white
phenocrysts, distinguishes these pyroxene dacites from the associated
obsidian, which they superficially resemble. A small area of olivine-bear-
ing pyroxene dacite is associated with the obsidian south of Borax Lake ;
nvQch of this dacite is vesicular and not megascopically porpliyritic.
The olivine-bearing dacite is composed of abundant small phenocrysts
of colorless granular augite (10 percent), small laths of basic andesine
to acid labradorite (5 percent), small prisms of hypersthene (3 percent),
and scattered anhedral olivine (2 percent) set in a hyalopilitic ground-
mass. Much of the plagioclase appears in holocrystalline clots with
pyroxene and olivine, and these clots may be xenoliths.
The quartz-bearing dacite has colorless euhedral to subhedral augite
(2 percent) and hypersthene (3 percent) as the most abundant pheno-
crysts, although quartz grains and andesine laths are most conspicuous
in the hand specimen. The groundmass is typically micro- to crypto-
crystalline, formed principally of feldspar microlites in a felty or a
pilotaxitic arrangement, accompanied in some rocks by granules of
augite and needles of hypersthene.
The olivine-bearing dacites were so classified on the basis of chemical
analysis, which corresponds fairly well to analyses of dacites presented
by Johannsen (1932). The modal composition is not considered reliable
for classification, because there are indications that much of the pheno-
crystic material is foreign. Thus, the magnesian character of the olivine
(16 percent fayalite) as shown by refractive indices, is incompatible
Avith the high silica content of the rock, as is the calcic nature of the
plagioclase (Auso-ct)- As suggested by Anderson, the holocrystalline
aggregates are probably inclusions of a more basic rock, and the isolated
crystals of olivine probably result from disintegration of olivine-bearing
inclusions. Some of the pyroxene and plagioclase described as pheno-
crysts may in reality be xenocrysts.
The chemical analysis of the quartz-bearing pyroxene dacite corres-
ponds satisfactorily with that of other dacites, except that the potash
content is somewhat high.
Obsidian. Black obsidian, typically banded with gray, crops out over
an area of about 5 square miles in this quadrangle south of Clear Lake,
and over an additional area of one square mile southeast of Borax Lake,
where it is locally so vesicular as to be almost pumiceous.
Little can be added to Anderson's description of the microscopic
features. The gray bands are formed by swarms of microlites in a parallel
arrangement. Inclusions and phenocrysts are a notable feature. A chemi-
cally analyzed specimen from south of Borax Lake contains inclusions
up to 3 millimeters in diameter that consist largely of greenish-brown
acicular hfjrnblende and andesine with accessory augite and hypersthene.
These minerals also appear as clots and as scattered isolated crystals.
1953] STRATIGRAPHY AND PETROGRAPHY 39
A second obsidian in the same locality contains rounded and embayed
plagioelase (An35-r.o) and liypersthene phenocrysts, which reach a maxi-
mum length of 1.3 millimeters and constitute nearly 2 percent of the
rock. Refractive index of the glass is about 1.489. These obsidians are
rhyolitic, as shown by the chemical analyses and refractive indices of
the glasses. The holocrystalline aggregates of plagioelase and pyroxene,
as well as the individual phenocrysts, may represent material torn from
the walls of the reservoir or conduit. The size of some of the individual
crystals and the calcic composition of some plagioelase crystals suggests
that these are xenocrysts derived by breaking up of larger inclusions,
or perhaps by addition of foreign magma.
Dacite. Flows of conspicuously porphyritic gray to pink lavas cover
about 15 square miles south of Clear Lake within the Lower Lake quad-
rangle, and extend beyond the quadrangle boundary where they build
Mount Konoeti, which covers an area of some 15 square miles and rises
2800 feet above the level of Clear Lake. Five miles south of Clear Lake,
similar flows form Mount Hannah, Seigler Mountain, and a portion of
the adjacent highland, covering a total area of about 7 sc^uare miles. The
lavas of Mount Konoeti and the adjoining lava field were called rliyoda-
cites by Anderson (1936), who suggested that the sanidine crystals which
they contain were possibly xenocrysts rather than phenocrysts. Further
petrographic study has revealed that plagioelase phenocrysts of distinctly
different composition are present together in most of these lavas, and that
magnesium olivine and quartz are present together in some. These associa-
tions suggest a hj^brid origin.
Most of the lavas on and around Mount Konoeti are dacitic. Dark gray
lavas outcropping on Fraser Point, which projects into Clear Lake, are
megascopically similar to the daeites, but the microscope shows that both
phenocrystic and groundmass plagioelase is mostly labradorite. On and
around Mount Hannah, andesitic lavas are conspicuous, and although
in general they are intimately associated with and even pass gradually
into dacite, some andesitic flows are sufficiently distinct to be mapped
separately. The large phenocrysts characteristic of the dacitic lavas are
not conspicuous in the andesites, which exhibit abundant small tablets of
labradorite and prisms of hypersthene set in an aphanitic base.
According to Anderson, most of the flows on Mount Konoeti are massive
and from 50 to 60 feet in thickness. Within the Lower Lake quadrangle,
exposures are too i)oor for measurements, but the landforms indicate
that the flows were thick and viscous. Erosion has apparently little altered
the original form of the flows, which retain their steep fronts and a
number of initial closed depressions.
Plagioelase phenocrysts of markedly different composition are asso-
ciated together in most of these daeites and andesites. This difference in
composition is strikingly demonstrated when the powdered feldspar,
electromagnetically separated from the crushed rock, is immersed in oil.
The mutual association of feldspar phenocrysts of distinctly different
composition was also repeatedly demonstrated by optical measurements
on thin sections. In the dacitic lavas, the typical association is a small
proportion of calcic plagioelase (An.-r.-To) together with the principal
sodic plagioelase (An^s-g.-,). Groundmass feldspars are generally more
calcic than are the phenocrysts. Most of the conspicuous milky phenocrysts
of the dacitic lavas are sodic (An25_35). Zoning appears in most of the
40 LOWKll LAKE QUADRANGLE [BuU. IGG
plagioclase, made apparent by differences in extinction or by the differ-
ences in clarity of the zones. Normal zoning predominates, and the maxi-
mum observed difference in anorthite content from core to rim was 10
percent. Many of the large phenocrysts from the dacitie lavas show no
zoning, but jiraetically all of those in the andesitic lavas show conspicuous
zoning. Besides the plagioclase, rounded glassy phenocrysts of sanidine
are present in most of the dacites. Optical properties of the sanidine are
nearly constant, and indicate a content of 25 percent of the albite
molecule.
Other important phenocrystic minerals include hypersthene, pyroxene
and quartz. Although most of the colorless to faintly pleochroic hypers-
thene occurs as euhedral to subhedral phenocrysts, grains or needles of
hypersthene are not uncommon in the groundmass. Refractive index and
optic angle measurements on hypersthene from three lavas showed prop-
erties which indicate 18 to 24 molecular percent of ferrous silicate. The
augite is colorless to pale green, and occurs as anhedral grains, or, in a
few lavas, as jackets around or cores within, hypersthene. Optical deter-
minations show that the augite is not far from diopside in composition.
Shattered grains of clear glassy quartz reach a maximum diameter of
about 5 millimeters, and none show euhedral outlines. Many of the quartz
grains are surrounded by a rim of slender augite rods, and all degrees of
replacement were observed, judging by the width of this rim. It is note-
worthy that many specimens contain grains of quartz with no reaction
rim, associated with other grains in an advanced stage of replacement.
Tridymite appears in many of the lavas as patches in the groundmass or
as well-formed plates on the walls of vesicles. Tridymite also is present in
the very dusty, nearly opaque feldspar phenocrysts which are abundant
in the dacites.
Accessory minerals are biotite, hornblende, and, in some lavas, olivine.
The biotite is strongly pleochroic from yellow brown to deep brown, and
occurs in about half of the dacitie lavas. The pleochroic formula for the
hornblende is X, pale yellow ; Y, light yellow brown ; Z, dark yellow
brown. Olivine in scattered anhedral grains appears as a minor acces-
sory in about 20 percent of the dacites and andesites. Refractive indices
of the olivine in 1/168 are alpha — 1.669 and gamma — 1.707 ; these indi-
cate about 18 percent of the fayalite molecule. Three of the four olivine-
bearing dacites studied also contain grains of quartz.
Inclusions composed of plagioclase and ferromagnesian minerals hav-
ing a diabasic or rarely a gabbroic texture are common in the Konocti
volcanic rocks. Some of these inclusions are formed of the same minerals
as those occurring as phenocrysts in the lava, but the plagioclase of others
is labradorite, i.e. more calcic than the main lava phenocrysts. The largest
observed inclusion was an irregular clot about 10 centimeters in diameter,
formed principally of labradorite and hypersthene.
Cinder Cone. Roundtop Mountain, the single cinder cone in this quad-
rangle, is. described by Anderson: "The most southerly of the recent
cinder cones is perched upon the rhyodacite flows, less than a mile south-
east of Thurston Lake, and resembles the Sulphur Banks cones in that it.
also, is breached, opening to the northeast. It is composed of reddisli
basaltic cinders, lapilli, and bombs, up to several feet long." A small flow
of basalt extends to the east from the cone, which is thought to post-date
the building of the cone, as no cinders were observed on its surface.
1953] STRATIGRAPHY AND PETROGRAPHY 41
Microscopically, the basalt flow is similar to other olivine basalts in the
region, but it contains no modal quartz.
Inclusions. Cognate inclusions are well distributed through the Perini
Hill lava, but are rare in the olivine basalt ; accidental inclusions are lo-
cally abundant in the Perini Hill lava, and locally present in the olivine
basalt. Nearly everywhere xenocrysts of quartz are sporadically distrib-
uted through both lavas, although the olivine basalt seems devoid of in-
cluded material at Quackenbush Mountain, and at a number of localities
in the main area of outcrop. The andesites which cap Boggs Mountain are
not quartz-bearing, and other inclusions are decidedly rare.
Of the basic lavas, only the Perini Hill flows carry cognate xenoliths in
abundance. The xenoliths appear usually as tabular chips, averaging
some 2 centimeters in length, of fine-grained light-gray feldspar, flecked
with magnetite. The microscope shows a representative specimen to con-
sist of a hypidiomorphic granular aggregate of basic labradorite (70 per-
cent and colorless hypersthene (10 percent), both of which enclose con-
spicuous anhedral grains of magnetite (5 percent). Patches of well-
crystallized tridymite locally replace the feldspar, which is much pitted
and veined with colorless glass. The border of the inclusion is sharp
against the enclosing rock, with no suggestion of reaction. These noritic
fragments were probably torn from the basic margins of reservoirs from
which the lavas were erupted, and they suggest a magma rich in alumina.
Accidental xenoliths may be classified as siliceous xenoliths, which rep-
resent sandstone, chert, and quartz schists in various stages of alteration ;
and aluminous xenoliths, which are probably derived from schists rich in
alumina, or from pelitic sediments. The described zenoliths are from one
locality in the olivine basalt, and from a number of localities in the ande-
sites of Perini Hill.
Most of the siliceous xenoliths have a more or less distinct schistose
structure. The largest and most significant of these xenoliths was found
embedded in an andesite boulder at the southeastern edge of the Perini
Hill lava cap. It was a rounded mass about 6 inches in length, formed of
wide lensing bands of .quartz up to 1 inch in width, separated by fine-
grained bands composed of narrow red and gray laminae. The boundary
of the xenolith with the surrounding pink andesite was indistinct, and it
was not possible to remove the xenolith intact. The quartz has the same
limpid clearness and the tendency to break into elongated fragments,
which characterize the quartz xenocrysts of the Clear Lake lavas. The
microscope shows the fine-grained material between quartz bands to be
composed of narrow irregular bands of granular quartz alternating with
bands of basic andesine. Anhedral grains of hypersthene and strongly
oxidized brownish-yellow biotite are intergrown with the feldspar and
to a lesser extent with the quartz, and a few anhedral grains of garnet
are scattered about. This mineral assemblage would place the xenolith
in the pyroxene hornfels facies. A similar xenolith was found in the oli-
vine basalt near Hill 1812, about 7 miles from the Perini Hill locality;
although only 1 inch in length, it is schistose in appearance, and the
microscope shows veinlike mosaics of coarsely crystalline quartz and basic
andesine (in small amount) separated by irregular bands composed of
granular quartz intergrown with pale green augite. A single large (3
millimeters in diameter), anhedral grain of pale pink garnet, full of
augite and quartz inclusions, appears within the coarse quartz-andesine
3— 6S207
42 LOWER LAKE QUADRANGLE [Bull. 166
mosaic. Whether the quartz of the xenoliths became segregated into the
wide bands and pods before or after inclusion in the lava is not clear, but
the segregation is attributed to some process of metamorphic differentia-
tion within the schist.
The aluminous xenoliths, which occur in tlie lava as rounded inclusions
with a maximum observed length of 5 centimeters, are holoerystalline
aggregates of intermediate plagioclase, hypersthene, or both, with cordi-
erite, biotite, spinel, and sillimanite as accessories. Though their bounda-
ries are not always sharp, the xenoliths show no perceptible effect on the
enclosing rock. An aluminous xenolith (I/274B) from the olivine basalt
near Hill 1812 is of flat oval shape, about 5 centimeters in length, and
mottled bluish gray and light gray in color. The microscope shows a
hypidiomorphic granular aggregate of plagioclase (An45.5o) apparently
replacing a mosaic of pale violet cordierite, which encloses abundant
felted needles of sillimanite, scattered subhedral to anhedral grains of
green spinel up to 1 millimeter in length, and anhedral grains of mag-
netite. A cordierite-sillimanite-spinel assemblage from the classical Mull
localities has been described by Thomas (1922), who believed that the
assemblage formed by reaction of a tholeiite magma with the still fluid
matrix of a sillimanite-buchite, which gives rise to cordierite and spinel
or cordierite and corundum, according to the amount of available mag-
matic magnesia. The calcic andesine of the Clear Lake xenolith represents
the phase with which the magma was saturated at the time of reaction
with the xenolith. (c/. Bowen, 1928).
Large inclusion-loaded crystals of hyersthene are the most abundant
form of included material in the Perini Hill flows. Most of these hypers-
thene prisms contain no cordierite ; some contain only rounded grains of
quartz, others only plagioclase (oligoclase to andesine) , still others contain
both quartz and plagioclase. Garnet, biotite, hornblende, and tridymite
occur separately or together as accessory included minerals. In one such
hypersthene prism, glassy violet inclusions were tentatively identified as
cordierite, and further study in oil immersion showed optical properties
consistent with some varieties of cordierite (Ny — 1.536, 2V — nearly 90°).
Regarding the origin of these inclusion-loaded hypersthene crystals, the
enclosed quartz and sodic plagioclase suggest derivation from a mure acid
rock, but no such hypersthene crystals were seen in the acid rocks of the
region ; moreover, the included garnet and cordierite suggest a relation-
ship with the xenoliths. Many of the xenoliths studied contain hypers-
thene crystals in process of including other minerals. Upon fragmenta-
tion of such xenoliths, the inclusion-bearing hypersthene crystals would
continue to grow in the magma. Growth of hypersthene w^ould be favored
in the magma, already saturated with respect to hypersthene.
The aluminous material which constituted some xenoliths has appar-
ently been largely or wholly resorbed, leaving only a hypidiomorphic
granular clot of labradorite and hypersthene, usually with accessory
biotite and magnetite, to mark its former presence. That such clots do
represent resorbed aluminous material is borne out by the presence in
many similar clots of a few grains of green spinel, commonly accom-
panied by colorless anhedral garnet. A single section of Perini Hill lava
contains two such inclusions, both round and about 1.5 millimeters in
diameter. The first is composed of two irregular grains of garnet about
0.2 millimeter in diameter, accompanied by a few grains of green spinel
1953] STRATIGRAPHY AND PETROGRAPHY 43
and magnetite, and surrounded by a hypidiomorphic granular cluster of
hypersthene and glass-charged labradorite ( Auco) . The second is similar,
but contains biotite and no garnet. In another slide of the same flow, a
rounded isolated grain of colorless garnet is surrounded by a wide
reaction rim formed of colorless radiating fibers, possibly of wollastonite.
Most of the quartz occurs as small irregular grains, with an average
diameter of some 2 millimeters, but many clots reach 3 centimeters
across, and one irregular mass measured 15 centimeters. Some of these
larger inclusions are sharply angular, but most are rounded, commonly
egg-shaped. The limpid clarity and unusual brilliance of the quartz,
emphasized by its setting of dark stony basalt, are so striking that it
has been called "Lake County Diamond." Other less common varieties
are milky or amythestine. The grains are invariably shattered, usually
into elongated fragments bounded by smooth curved surfaces ; the shat-
tering may result from volume changes attending inversion from high
to low quartz. Although without crystal outlines, many of the xenocrysts,
including some up to 5 centimeters in length, possess the optical continuity
of a single crystal. Generally a narrow reaction rim of radiating augite
rods separates the quartz from the surrounding basalt, but this rim is
lacking around some grains, and elsewhere it widens to fill or nearly fill
the space formerly occupied by quartz. The edges of some grains have
been dissolved to form a corona of pale brown glass, in which microlites
of augite may be embedded. Needles and wedge-shaped twins of tridy-
mite, usually adjacent to the quartz, are present in some reaction rims.
Quartz-bearing basalts have been described from many localities, but
the origin of the quartz has rarely been established. Diller (1891)
described quartz inclusions in basalts erupted from Cinder Cone near
Lassen Peak, and concluded that the quartz crystallized early in the
history of the basalt, before the olivine. Iddings (1890) described inclu-
sions in basalts from the San Juan region which are nearly identical
with the Lake County inclusions, and suggested that the inclusions are
primary ''secretions" of the basalt. Harker (1909) suggested that the
quartz basalt is in reality a hybrid rock, derived from admixture of
basalt and acidic rock. The idea of a hybrid origin has recently received
strong support in the conclusions of Larsen et al. (1938) from their
intensive study of the San Juan lavas, which include quartz-bearing
basalts. Lacroix (1893), from studies of the quartz-bearing basalts of
the Central Plateau of France, concluded that the quartz had been picked
up from quartz-bearing rocks at depth.
In deciding the origin of the quartz in the Clear Lake basalts, three
possibilities may be considered: (1) original crystallization of quartz
from basaltic magma; (2) derivation of the quartz from accidental
inclusions of material forming the walls of magma chamber or conduit ;
(3) mixing of quartz-bearing acid magma with basaltic magma.
Experimental evidence for the system MgO-FeO-SiOo (Bowen and
Schairer, 1935) indicates that quartz and olivine are not in equilibrium,
and therefore that quartz would not crystallize from a magma from which
olivine was crystallizing. Larsen (1936) felt that the complexity of the
natural system, perhaps in conjunction with increased pressures, might
permit the crystallization of quartz from olivine basalt magma. This
possibility cannot be disregarded, but the sporadic distribution of the
quartz inclusions in the Clear Lake basalts, the common angularity, the
44 LOWER LAKE QUADRANGLE [Bull. 166
great range in size, and the ubiquitous reaction rims are not suggestive
of original phenocrystic material, but of some foreign source.
Derivation of the quartz from accidental inclusions is supported by
the abundant occurrence of xenolithic material in the Perini Hill lavas;
but such material, though locally abundant, is generally rare in the
quartz-bearing basalts. The large schistose xenolith (1/279) found in
Perini Hill lava gives a clear answer to the puzzling question as to what
type of xenolith could give rise to the quartz xenocrysts : it contains thick
lensing bands of clear shattered quartz, identical to the quartz in the
xenocrysts ; its metamorpliic nature is indicated by its schistose structure
and the scattered crystals of garnet. Could the material other than quartz
in such a xenolith — the feldspar, garnet, and perhaps other minerals —
be entirel}^ assimilated by the olivine basalt magma, leaving only clots
and grains of clear quartz? Such assimilation is entirely possible, but
proof that it has happened was not obtained. Partially digested bits of
xenolithic material, altliough abundant in the Perini Hill lava, are rare
in the olivine basalt. Perhaps the xenolithic material remained longer
in the olivine basalt magma, so that everj^thing was digested but the
quartz. H. H. Read (1923, p. 452) has shown that quartzite appears to
float on basic magma, and that quartz tends to be rejected and segre-
gated in the contamination process.
The possibility that the quartz xenocrysts were derived by mixing
with acidic magma is not supported by the presence of other xenoliths or
xenocrysts from acid magma. However, the presence of basic minerals
(calcic feldspar and olivine) in the acidic magmas of the series indicates
that mixing has occurred; furthermore, most minerals of acid magmas
are more readily absorbed by basic magma than the reverse. Bowen
(1928) showed that members late in the reaction series are dissolved in
basic magma, the required heat being furnished by crystallization of the
phase with which the magma is saturated, and the amount of liquid
being increased. Quartz is the most refractory of the late members, and
would consequently remain after resorption of the others. There remains
the difficulty that some of the quartz xenocrysts are larger than any
normal quartz phenocryst ; however, this might be explained by rejection
and segregation of the quartz in the contamination process.
To conclude, the presence of xenoliths of quartz-bearing schist and
the partly assimilated fragments of such xenoliths in the Perini Hill
lavas constitutes strong evidence that the quartz xenocrj'sts in these lavas
were derived from the schist. The local presence of similar xenoliths
in the quartz-bearing olivine basalt suggests that these quartz xenocrysts
were also derived from the schist; but the general scarcity of schist
xenoliths and the virtual absence of partly assimilated xenolithic frag-
ments, casts doubt on this possibility. The derivation of a greater or
lesser part of the quartz xenocrysts in all of the basic lavas by
mixing with acidic magmas of the series is considered a distinct
but unproved possibility.
Chemical Composition of the Lavas. No chemical analyses were made
for the present study. Tlic following analyses, quoted from Anderson's
(1936) paper, are of rocks collected within oi- just outside the Lower
Lake quadrangle, and are considered representative of their respective
lava types in the series. Lava types for which analj'ses are not avail-
able are the andesitic lavas of Perini Hill and Boggs Mountain, and
1953]
STRATIGRAPHY AND PETROGRAPHY
165"" ' ' ' [70"" ' ^
45
AI203
^ Fe203
■ OlMgO
Figure 3. Variation diagram of Clear Lake lavas. (From Anderson, 1936, p. 660.)
the rhyolitic lava of Cobb Mountain; however, the composition of the
biotite rhyolite pumice, analysis of which is shown below, is probably
very similar to the rhyolite of Cobb Mountain.
The olivine basalts (nos. 1 and 2), apparently intercalated with the
Cache formation and therefore the oldest lava type in the series, have a
composition very similar to the quartz-basalts listed by Johannsen (p.
416, V. II, 1932), except that the Lower Lake rocks are a little high in
MgO and a little low in AI2O3. When compared with ordinary basalts
(as listed in Johannsen, p. 261, v. Ill, 1932), the Lower Lake olivine
basalts are high in silica (7-8 percent), low in FeO plus Fe203 (about
5 percent) , and a little low in CaO. Anderson notes that the olivine basalt
collected by him (no. 1) is typical of the locality, and that olivine was
the only visible phenocrystic mineral, no visible quartz grains being
present.
As noted by Anderson, soda generally exceeds the potash even among
the more silicic rocks, so that the k value (molecular ratio of potash to
total alkalies) ranges from 0.23 to 0.40, excepting the biotite rhyolite
pumice, which has a k value of 0.50. According to Peacock's (1931) classi-
fication, the Clear Lake rocks are calcic, with an alkali-lime index of
about 62 ; and in this respect they are like the High Cascade volcanoes
of Lassen Peak, Mount Shasta, and Crater Lake.
A comparison of the Clear Lake variation diagram with variation
diagrams for some of the Cascade volcanoes (Williams, 1935, p. 296-298)
shows that the AI2O3 curve is much lower, and the MgO curve is higher,
in the Clear Lake variation diagram. No evidence of the assimilation of
foreign material has been reported from the Cascade volcanoes, and
it seems likely that such assimilation by the Clear Lake lavas may
46
LOWER LAKE QUADRANGLE [Bull. 166
Mineral composition * of Clear Lake volcanic series.
Mode
Sample
number
1
2
3
4
5
6
Plagioclase _ . .
50
25
17
11
12
< 1
Hypersthene
5
5
5
3
1
< 1
15
5
1
1
1
< 1
1
2
< 1
Biotite
2
1
Olivine
15
Quartz _
<1
<1
<1
10
Sanidine __
3
12
Percent An in plagioclase
55-65
50-70
65-75
30-40
25-35
55-70
10-30
10-30
1. Olivine basalt (av. of 16).
2. Andesitic lavas of Boggs Mountain (av. of 6).
3. Andesitic lavas of Perinl Hill (av. of 2).
4. Dacites of Mount Konocti and Mount Hannah (av. of 15).
5. Rhyolitlc flows and tuffs of Cobb Mountain (av. of 6).
6. Rhyolltic obsidian (av. of 2).
* Estimated from thin section, material too small for identiflcation excluded, mineral identification by oil immer-
sion of crushed and magnetically separated fractions.
account for these differences. Addition of the mineral assemblage quartz-
andesine-hypersthene-biotite — which is the composition of the large xeno-
lith found in the andesite of Perini Hill — to the Clear Lake lavas, could
account for their chemical differences from Cascade volcanoes.
Petrogenesis. Chemical and petrographic data for the Clear Lake
Volcanic series are not sufficiently full to warrant any extended specula-
tion regarding petrogenesis. The variation diagram must be interpreted
with caution, as there are no analyzed rocks having silica content in the
57-66 percent range, and variation diagrams plotted according to various
schemes all show considerable gaps between rock types. However, the
widespread occurrence of xenocrysts and xenoliths in many of the lava
types as well as certain abnormalities in chemical composition, require
explanation.
Differentiation of the lava types by the process of fractional crystal-
lization of basaltic magma is suggested by the variation diagram. Bowen
(1928) showed that such curves are consistent with the course of frac-
tional crystallization. Furthermore, the average feldspars of the acidic
lavas are consistently more sodic than those of the basic lavas, indicating
the influence of the experimentally established reaction series of feld-
spars ; and evidence of the reaction series olivine-pyroxene-hornblende-
biotite is demonstrated by rims of one mineral around another, and by
the appearance and disappearance of these minerals in the experimentally
determined order.
Some persistent mineralogical features of the Clear Lake lavas cannot
be explained by fractional crystallization, however. Two distinct kinds of
plagioclase phenocrysts (commonly An25.35 and Anyo) are mutually asso-
1953] STRATIGRAPHY AND PETROGRAPHY 47
dated in many of the dacites and andesites; the rhj'olites of Cobb
Mountain contain feldspar phenocrysts as calcic as Anss ; and the rhyo-
litic obsidian commonly contains phenocrysts of plagioclase as sodic as
Anso. The olivine basalt is typically quartz bearing, the quartz inclu-
sions being distributed in varying amount throughout an extensive
outcrop area. Andesitic lavas of Periui Hill also contain abundant quartz
inclusions. Distinctly magnesian olivine (Faie-is) occurs in some of the
andesitic and dacitic lavas, and three of the four olivine-bearing dacites
studied are also quartz bearing. Sanidine occurs only as phenocrysts in
the dacite, being absent from the groundmass, suggesting that the sani-
dine may be xenocrystic rather than phenocrystic. All these abnormal
mineralogical features suggest that minerals belonging to two or more
stages of magmatic evolution have been brought into association. More-
over, the fact that phenocrysts are mainly concerned indicates that this
association took place at considerable depth.
That contamination played a part in the petrogenesis of the andesitic
flows of Perini Hill is suggested by the abundance of siliceous and alumi-
nous xenoliths which these flows contain. Xenoliths are abundant locally
in the olivine basalt ; and if the widespread quartz inclusions are indeed
foreign, a significant amount of contamination is indicated.
Present volumes of the different lavas, including the extensions of these
lavas beyond the limits of this quadrangle, are estimated as follows, in
cubic miles : olivine basalt, 1.5 ; andesites of Perini Hill and Boggs Mt.,
0.55 ; dacites of Mt. Konocti, Mt. Hannah, and Seigler Mt., 3.0 ; rhyolite
of Cobb Mt., 0.25 ; rhyolitic obsidian, 0.45.
The characteristics of the Clear Lake lavas could be explained by
several different schemes, or combination of schemes, or magmatic evo-
lution. Two such general schemes will be considered: (1) An original
basaltic magma differentiated into acidic and intermediate magmas by
fractional crystallization, the course of differentiation being somewhat
influenced by assimilation of sedimentary material ; and portions of the
different magmas so formed mixed to produce hybrid magmas. This
scheme, while theoretically competent to produce the lava types, would
produce a relatively small proportion of acidic to basic lava ; whereas the
total volume of acidic lavas in fact greatly exceeds the volume of basic
lavas. It might of course be argued that the requisite volume of basic
lava from which the extruded acidic lavas were differentiated, remains
at depth.
(2) Basic and acidic magma, formed independently by differential
fusion at the roots of the continental platform, became mixed in rising
and thus produced hybrid magmas ; composition of some magmas was
altered by assimilation of sedimentary material. This genetic scheme,
adopted from suggestions by Turner and Verhoogen (1951, p. 358-367),
satisfactorily explains all of the obeserved features of the Clear Lake
volcanic series. The nearly straight lines of the variation diagram —
which may, to be sure, be only apparent because of an insufficient number
of chemical analyses — are thus explained by the mixing of two end
members in various proportions. The olivine basalt, whose silica content
is decidedly above that of the average basalt, is not considered to be the
basic end member, but is considered to be contaminated, probably with
quartz schists of the basement complex.
48
LOWER LAKE QUADRANGLE
[Bull. 166
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1953]
STRATIGRAPHY AND PETROGRAPHY
49
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50 LOWER LAKE QUADRANGLE [Bull. 166
Quaternary Deposits
Alluvium
Although the floors of the larger valleys are formed of a thick deposit
of transported alluvial material, the mantle of soil and broken rock which
covers the uplands is sedentary, and its composition serves to identify
the underlying rock. The character of the valley fill varies from place to
place, depending on the rock types composing the neighboring uplands.
The soil in those parts of the valleys adjoining lava flows is reddish,
porous, and generally productive. Near large serpentine bodies, con-
stituents derived from the serpentine cause the soil to be sticky and
plastic when wet, and to dry rapidly with much shrinkage : during the
dry season, the surface of some parts of Long Valley is broken by a net-
work of large cracks up to 6 inches across and several feet deep. The
thickness of the alluvial fill in the valleys around Middletown is about
100 feet, according to reports from well drillers. For a technical descrip-
tion of the soils of the Clear Lake area, which includes this quadrangle,
see Carpenter, Storie, and Cosby (1931).
Landslide and Talus Deposits
Landslides are common everywhere in the quadrangle, although they
are especially numerous in the serpentine areas. They are recognized by
topographic expression — hummocky surfaces, distorted drainage, or cir-
quelike scars — or by the broken rock masses, commonly of unrelated
lithology, which compose them. Factors which combine to cause the land-
slides include general steepness of slopes, depth of the rock mantle,
seasonal rainfall, and sparseness or absence of trees and shrubs. The
serpentine, being slippery and usually strongly fractured, slides very
readily.
No attempt was made to show all of the slides on the geologic map, but
the more important ones are shown. By far the largest single slide, both
in distance traveled and amount of rock material involved, originated on
the southern flank of Cobb Mountain and extends about 2 miles to the
southeast. Great volumes of rhyolitic debris, including blocks up to 30
feet in length, have moved down the steep flank of Cobb Mountain,
covering the Franciscan rocks in place there. Farther down slope, the
rhyolitic debris becomes mixed with increasing amounts of Franciscan
rocks, mostly schist and sandstone ; and north of Anderson Springs, near
the end of the slide, andesitic debris from Boggs Mountain joins the
confused assemblage. Another large slide originates at the southeastern
end of Boggs Mountain, where it is composed entirely of andesitic debris ;
farther down slope this is joined by serpentine, schist, and shale debris to
form a shallow but extensive landslide. Smaller slides composed entirely
of sedimentary rocks are numerous along the Knoxville road, east of-
Lower Lake, and also along Soda Creek. The lava-capped uplands are
nearly everywhere skirted by talus slopes, or by a mantle of weathered
lava and scattered boulders.
Terrace Deposits
The branching intermittent streams in Burns Valley have cut into an
alluvial deposit whose surface is about 15 feet above the present level of
the valley floor, thus forming a terrace of irregular outline. The surface
of the terrace is not flat but slopes upvalley and merges almost imper-
ceptibly with the Cache terrain. It is not apparent from the composition
and structure of the terrace, which is almost entirely clastic debris from
1953] GEOMORPHOLOGY 51
the Cache formation, whether the terrace material was deposited in the
lake or subaerially ; but it is improbable that the higher, up valley portion
of the terrace is lacustrine. Cutting of the terrace suggests that the lake
level has been lowered. This lowering could have been caused by the
breaking of an hypothetical barrier, as suggested by Anderson ; but other
equally probable causes may be suggested, such as change of climate or
warping of the basin.
GEOMORPHOLOGY
The evolution of much of the present landscape begins with the out-
break of a volcanic epoch early in Pleistocene time : lava flows then cov-
ered an older landscape of moderate relief, and subsequent erosion has
both dissected the lava flows and sharpened the relief on older rocks.
The lava originally covered at least one-fourth of the quadrangle. In
addition to the flows, bulbous protrusions of acid lava, several hundred
feet in height, were formed ; the protrusions now appear as rounded,
forest- or brush-covered mountains, and the more basic flows appear as
rolling highlands fringed by cliffs and talus slopes. Elevations of the
bases of lava flows indicate that the flows were poured out upon a surface
having a maximum relief of some 750 feet, which is considerably less than
the present relief. In post-volcanic time, erosion has cliffed the edges of
the flows, causing their retreat by an unknown amount, and streams
have cut deep gorges through some flows. The main volcanic areas, how-
ever, have remained relatively unaffected, while the relief of surrounding
areas, underlain by the less resistant pre-volcanic sediments, has been
sharply increased. Thus the present landscape features may be logicallj-
described under two main headings : landforms evolved on pre-volcanic
rocks, and landforms of volcanic origin.
Landforms Evolved on Pre-Yolcanic Bocks. The Mesozoic and Eocene
rocks of the quadrangle have been subject to erosion continuously since
Eocene time, except for those areas which were covered by Cache sedi-
ments. The beginning and history of the present erosion cycle cannot
be determined, because there are no remnants of uplifted land surfaces
or other necessary evidence, but the region and most of the stream valleys
are now in the stage of early maturitj^ Slopes are generally graded,
and divides are ridges or rounded hills with convex slopes. That some
adjustment of drainage to structure exists is shown by the general north-
westerly trend of the larger streams ; but the stream pattern is complex,
partly because of the random distribution of igneous intrusions in the
Mesozoic sediments, and the influence of the lava flows. Streams in the
uplands have steep valley sides, fairly high gradients, and little or no
flood plains. In addition, there are innumerable insequent gullies which
finely dissect the topography.
Streams from the uplands descend into wide, flat valleys which are
bordered by the sharply dissected uplands. A group of such valleys
occupies the southeast corner of the quadrangle, where, although inter-
connecting, they are separated by irregular hilly masses which rise from
their floors like islands in a sea. The cover of alluvium in Long Valley
is not much greater than 100 feet, and serpentine underlies much of the
valley, according to oral information from well-drillers.
These large valleys are plainly anomalous, and they require explana-
tion. They could be attributed to blocking of former drainage by lava
52 LOWER LAKE QUADRANGLE [Bull. 166
flows, and there is indeed evidence that such did occur. However, that
similar valleys were already in existence at the time of eruption of the
lava is indicated by positions of the flows : east and south of Middletown,
a floAv of quartz-bearin<2: basalt descends from an elevation of about
1800 feet to the valley floor at an elevation of 1100 feet, where it is
apparently buried by alluvium. There is a similar occurrence north of
Coyote Valley, whore the lava descends to the valley floor from an eleva-
tion of 1500 feet. Furthermore, the distribution of remnants of the
flows indicates that the thickest part of the flow descended to the south-
west, directly across the former drainap:e paths of the valleys. The pre-
volcanic valleys may have been in harmony with their surrounding?
topography, but the presence of faults alonj? the borders of some of the
valleys suj^pests that they are, at least in part, of tectonic origin. The
evidence indicates, therefore, that the present wide, flat valleys were
formed by a combination of geologic circumstances : downfaulting and/or
downwarping led to the formation of the original topographic depres-
sions, which were followed by streams. When lava flows blocked the
eastern outlet, drainage was deranged and probably ponded for a time.
Moderate uplift followed the volcanic epoch, renewing dissection of
the uplands. Putah Creek at length cut a gorge through the flow at Hells
Half Acre, and the present rapids there show that the downcutting is
still in progress. In the meantime, the flat valleys remain as oversized
graded reaches.
Landforms of Volcanic Origin. Several massive volcanic mountains
lie along the western border of the quadrangle, rising conspicuously
above the surrounding highlands. The crest of Mount Konocti (elevation
4200 feet), largest and best known of these, is located about 1 mile
beyond the western boundary of this quadrangle, which includes only
the eastern flank of the mountain. Mount Konocti has been described
by Anderson (1936), and termed an eroded multiple volcano. The
highest part is south of the center of the mountain, and includes three
prominent peaks, one of which appears to be a remnant of a secondary
cone. Several crater remnants appear in the summit area, and a number
of small parasitic cones on the flanks. Although the mountain is built
principally of lava flows, local exposures of pyroclastic material require
its classification as a composite cone. Mount Konocti is believed to be
middle or late Pleistocene in age, judging from the degree of erosion
and the fact that its lavas are similar to those overlying the Plio-Pleisto-
cene Cache formation.
Five miles to the south of Mount Konocti, the steep flanks of Mount
Hannah rise to an elevation of some 1400 feet above the adjoining
flat, forming a nearly symmetrical rounded cone. A thick pine forest
covering the mountain obscures many of the details of its form; but the
presence of soil cover sufficient to support such a forest testifies to a
considerable age. However, water courses down the flanks are broad and
shallow, and the work of erosion is proceeding mostly by gravity trans-
port of blocks down the steep slope. The top of the mountain is' formed
of thick lava flows, and there is no evidence of a crater depression or of
pyroclastic rocks. Mount Hannah may therefore be described as a
bulbous protrusion of dacitic and andesitic lavas. Judging from the
relative degrees of erosion, Mount Hannah is somewhat more recent
than Mount Konocti. Seigler Mountain is similar to Mount Hannah in
1953] GEOMORPHOLOGY 53
structure and composition, although less symmetrical, and appears to be
formed of coalescing protrusions of the viscous lavas.
Cobb Mountain rises prominently along the crest of the northwestward-
trending Mayacmas Range, reaching the highest altitude in the map
area (4722 feet) and an elevation of some 2350 feet above the adjoining
valley. The top of the mountain is broad, and its flanks, especially to
the south, are exceedingly steep and gashed by deep, narrow canyons.
The semi-circular configuration of the top of the mountain is suggestive
of the remnants of a crater rim, breached to the south by erosion, and the
presence of an explosive vent is further suggested by the rhyolite tuff
cropping out on its southern slopes. However, the mountain is built
mostly of banded rhyolite, which probably piled up as thick viscous flows
around a central vent. Cobb Mountain is more deeply eroded and there-
fore probably older than either Mount Konocti or Mount Hannah.
Boggs Mountain is formed of a thick, elongated cap of andesite over-
lying older rocks. The andesite appears to have been extruded from the
crest of a low irregular ridge and to have flowed down the sides of this
ridge. The cap is bordered by cliffs up to 150 feet high, below which
there is a great deal of lava slide debris.
Thurston Lake occupies a flat-bottomed basin whose steep sides
are formed of dacitic lavas. The water is generally shallow, not
covering the entire bottom in the summer, and stands at an elevation
about 100 feet above the level of Clear Lake, from which it is separated
by a ridge of lava. It is probable that the lake bottom is not underlain by
lava, but by Cache sediments, for an outcrop of Cache diatomite appears
just beyond the western end of the lake, along the creek which drains
into it. The lake has no outlet, but the water remains moderately fresh,
probably escaping through the porous bottom of Cacbe rocks. The origin
of the lake has already been suggested by Davis (1933, p. 218) : "Thur-
ston Lake . . . occupies a deep hollow that is accidentally enclosed by the
up-building of several volcanic mounts around it . . .". This assumes
that the dacitic flows were too viscous to coalesce, and it is difficult to
suggest an alternative hypothesis.
A similar origin is proposed for a number of flat meadows, surrounded
by dacite or obsidian, which are located to the south and west of Thurston
Lake, into which they drain. These meadows are not all interconnecting,
but the floors stand at about the same elevation. Cache sediments are
exposed in the most westerly of the meadows (Ely Flat), and probably
underlie the others, although they are covered by alluvium.
Evidently, before extrusion of the obsidian, the meadows formed part
of a surface which drained northward. There is further evidence that
drainage was also to the north after the obsidian had partly covered this
old surface. A road-metal quarry on the east border of Ely Flat has been
cut into a deposit of obsidian pebble gravel. The gravel deposit is lo-
cated on the north side of an obsidian ridge, and the layers, made distinct
by a thin intercalated bed of gray clay, dip toward the north. The evi-
dence therefore indicates that the pre-lava topography drained toward
the present basin of Clear Lake, at a calculated slope of 250 feet per mile.
This of course does not establish the fact that Clear Lake was already in
existence in pre-volcanic times, but only that the lavas did not create the
lake basin, although they have restricted it.
From the main volcanic areas south of Konocti, a broad but irregular
and locally discontinuous tongue of basalt extends for some 15 miles to
54 LOWER LAKE QUADRANGLE [BuU. 166
the southwest, narrowing as it goes. Its surface is not flat, but surmounted
by broad mounds, which rise as much as 500 feet above the general level.
Flats between these mounds form the floors of a number of intermittent
lakes, such as the Stienhart Lakes. Compared to the highly dissected
topography of the older rocks which it overlies, the surface of the basalt
area is much subdued. Lava cliffs, 50 to 150 feet high, form the edges of
the basalt cap, and testify to a former greater extent. There is little evi-
dence to show what this former extent might have been; but Childers
Peak, located 1^ miles south of the basalt tongue, is capped by a tiny
remnant of basalt, at nearly the same elevation as the base of the main
flow. This suggests that the flow extended farther to the south. The large
mounds on the lava cap mark the probable sites of extrusion ; the lava
cap was no doubt thickest at this point, hence its preservation. As re-
vealed by the elevations of the base of the lava, the surface over which it
poured had a rolling topography with a maximum relief of some 750
feet.
Borax Lake basin was apparently formed when obsidian flows dammed
the western end of a valley cut in Franciscan rocks. Although the water
level fluctuates considerably, the lake is always shallow, drying up com-
pletely during periods of drouth. In March, 1944, the water surface stood
only 6 feet above that of Clear Lake ; such close correspondence of level
suggests that the valley was occupied b}'- the waters of Clear Lake before
formation of the lava dam.
Borax Lake is so named because of the considerable quantities of borax
crystals which were removed from its muds in the 1860 's. It has been
described in detail by Becker (1888) and also by Anderson (1936). The
source of the borax appears to have been a group of hot solfataric springs
issuing from the obsidian at the southeastern end of the lake. Becker in
1888 noted that the ground was hot and moist, that impure sulfur had
been found in excavations, and that no water was flowing at the time of
his visit. At present the ground around the former springs is bleached
white over an area of several acres, and the odor of sulfur can be de-
tected ; there has been no renewal of flow from the springs. Becker showed
by analyses that no borax is present in the surrounding rocks, and that
the springs must have been the source of the borax crystals. If the springs
are indeed extinct, no replenishment of the borax deposit may be ex-
pected.
Clear Lake. Only the narrow southern part of Clear Lake, less than
one-fourth of its total area, lies within the Lower Lake quadrangle. The
lake broadens considerably in its upper part: it is locally described as
having the shape of a tadpole with two tails, the broad upper part being
the head, and the part in this quadrangle being one of the tails. The lake
has a total area of about 60 square miles, a length of about 18 miles, and
a maximum width of about 7 miles. A contour map of the bottom, based
on more than 100 soundings compiled by the Wallis Marine Service at
Clear Lake Park, shows that the lake floor has the configuration of a
shallow, irregular basin, whose deepest part (52 feet) is directly east of
Mount Konocti.
Geologic work in this quadrangle has shed some light on the origin of
the large topographic depression which is occupied, although not fully,
by the waters of Clear Lake, A casual examination indicates that the
southern edge of the basin is formed entirely of lava flows, but sediments
1953] GEOLOGIC STRUCTURE 55
appear from beneath the lava at Baylis Point, and, as previonsly noted,
there is evidence that the pre-lava surface sloped toward this depression.
That the Cache formation is somehow related to the present depression is
indicated by the fact that lacustrine deposits of the Cache — marls, lime-
stones, and diatomites — appear only in the vicinity of Clear Lake. Fur-
thermore, the dips of the Cache beds are mostly gentle in the immediate
vicinity of Clear Lake, and although the beds on the north shore dip
gently beneath the lake, the dips steepen away from the lake.
Two hypotheses regarding the origin of Clear Lake have been pro-
posed: (1) Clear Lake occupies the lowest part of a shallow downwarp
or fault depression, which is related to the basin in which the Cache
sediments were deposited. This is essentially the origin proposed by
Becker. (2) The waters of Clear Lake occupy an intermontane basin
plain whose outlets have been dammed in some manner. The detailed
sequence of events for this hypothesis has been worked out by Davis
(1933), and Anderson concurred in general with these, after making a
few minor corrections. As for the age of the lake, Anderson showed
that high-level lake sediments at Sulphur Banks and at Buckingham
peninsula are older than volcanic activity at these localities, and it
seems likely that ' ' Clear Lake came into existence some tens of centuries
ago, prior to some, if not all, of the recent volcanic activity. ' '
Davis' hypothesis hinges in part upon the formation of a lava dam at
the southern outlet of the lake. Although the present outlet of the lake
is cut though sediments of the Cache formation rather than through
lava, there are small remnants of lava flows scattered in and about the
southernmost tip of the lake. Whether these might represent remnants
of a former barrier now destroyed, or whether there is perhaps a buried
channel filled with lava is not known. Kecently, however, the elevation
of the channel of Cache Creek just below the Clear Lake Water Company
dam, which has a bottom of resistant Cretaceous sandstone, was deter-
mined as 1300 feet above sea level, whereas the bottom of the lake at
its deepest part stands at 1284 feet. Thus the lake would not be com-
pletely drained if any possible barrier were removed from its southern
end.
It therefore seems that the origin of Clear Lake is more fundamentally
related to the origin of the large topographic depression which it par-
tially fills, than to barriers across its outlets. No direct evidence bearing
on the origin of the depression was found, but its relationships with the
Cache formation are considered highly suggestive. The presence of Cache
sediments around the southeastern border of Clear Lake, extending as
far west as Kelseyville, indicates that the area has been one of instability
in the recent geologic past. Two large depressions have thus occupied the
same general areas, in part overlapping, and the beginning of the younger
is essentially continuous with the end of the older. The Cache depres-
sion, in which thousands of feet of sediment accumulated, is unques-
tionably a downwarped or downfaulted feature, and the Clear Lake
depression is probably of similar origin.
GEOLOGIC STRUCTURE
The Mesozoic and Tertiary sedimentary rocks of the Lower Lake quad-
rangle strike persistently in a northwesterly direction, and the dip is
generally moderate to steep. The sediments are interrupted by numerous
irregular areas of serpentine rock, which are broadly aligned with the
56 LOWER LAKE QUADRANGLE [Bull. 166
rofiional strike. These f>eneralizations apply to the greater part of the
northern Coast Kange, and tlie geologic structural features of this quad-
rangle will be regarded as parts of this larger structural unit.
Geologic work in adjoining areas of the Coast Range has shown that
the basic structural features are large complex folds, several miles in
lengtli and moderately narrow in proportion, having northwestward-
trending axes ; and northwestward-trending faults, some having a length
of many miles and displacements measurable in hundreds of feet. Most
investigators have believed that these faults were steeply dipping,
although Weaver (1949) has postulated low-angle thrust faulting in the
Napa Valley region. Evidence seen in isolated localities, as in mines or
cuts by road or stream, indicates that these large structural features
are very complex in detail, so that the large folds include many folds
and are complexly faulted, and the larger faults are perhaps wide zones
rather than single planes of faulting. Unraveling of tliese complexities
is precluded by poor exposures and lack of suitable map units. None of
the large structural features which had been distinguished by geologic
work in quadrangles adjoining to the south and west could be traced
directly into the Lower Lake quadrangle.
Although there is no apparent difference in degree of deformation
between Cretaceous and Tertiary rocks of this quadrangle, the Fran-
ciscan rocks show a somewhat greater degree of deformation. In
particular, the Franciscan area is crossed by a large number of north west-
Avard-trending shear zones, along which the sediments are sheared on
a microscopic scale, and crumpled into open folds ranging in size from
microscopic to several feet across. Outside the shear zones, the variable
attitudes in the Franciscan may be explained either by complex folding,
or by complex high-angle faulting whereby the different blocks are
tilted in different directions; available evidence in this quadrangle
suggests the faulting.
The structural role of the serpentine bodies is important but difficult
to evaluate and to distinguish from the efl'ects of other agents. Shearing
Avithin and at the contacts of serpentine bodies indicates that they have
been squeezed into their present positions while solid, or nearly solid.
As emplacement by assimilation or even by stoping is not reasonable, the
intruded sediments have doubtless been thrust up and aside, perhaps
before they were fully consolidated. The apparent structural effect of the
serpentine is to locate the movements of major faults, which commonly
follow the border of a mass of serpentine.
Folding
The dominant sti'uctural features of the quadrangle are broad, plung-
ing, northwest-trending folds, several miles in width and extending nearly
across the quadrangle. These folds are neither simple nor symmetrical,
but include minor folds, and are extensively faulted. Their borders
are not sharply defined, because the stratigraphic units which form
them differ, for purposes of mapping, only in their different relative
proportions of sandstone and shale.
The large wedge-shaped area of Cretaceous rocks ending just north of
Middletown has the general form of a doubly-plunging syncline, but its
structure is much complicated by faulting and minor folding, so that
most of the rocks composing it dip to the northeast. At the eastern end,
it terminates abruptly against a large body of serpentine. There is evi-
1953] GEOLOGIC STRUCTURE 57
dence of strong faulting within the eastern end of the syncline, which
may have raised a block near the center, exposing detrital serpentine
near the base of the Cretaceous.
The large area of Cretaceous rocks in the center of the quadrangle
form a broad, well-defined syncline, but this, too, includes numerous
structural complications. The belt of Cretaceous forming the northern
limb is not so wide as that forming the southern limb, and it appears that
movement along a fault trending near the fold axis may have caused
uplift of the northern limb. Such direction of movement of the fault is
contradicted by the presence of a patch of Martinez rocks north of the
fault, and a reversal of fault movement must therefore be proposed.
Such an assumption is not justified by the evidence, but reversal of
movement along faults has been demonstrated in the Coast Range (Huey,
1948).
The isolated patch of Paleocene rocks east of Lower Lake has been
identified as synclinal in structure by Dickerson (1914) and by Stanton
(1895). Both men based their opinion largely upon faunal evidence:
similar fauna appeared at localities 2 miles apart, and younger fauna
appeared in the intervening rocks. The present study indicates that these
rocks are folded into a complex syncline which plunges gently to the
north. The Martinez rocks of the northern limb swing southward beneath
the cover of Cache beds on the west, and may join with those of the
southern limb, forming part of a basin. The center and southern limb of
the syncline are complexly faulted by northwestward-trending faults
and l3y cross-faults. Within the Martinez outcrops, there appears an
elongate area of Tejon conglomeratic sandstone, folded roughly into
synclinal shape, but structurally complex in detail, as indicated by many
steep dips and erratic strikes. The Martinez rocks were probably folded
and faulted before deposition of the Tejon, as well as afterwards. As in
the Mesozoic rocks, the lack of suitable map units precludes detailed
mapping of geologic structure.
The Cache beds are considerably less deformed than the older rocks,
having dips which rarely exceed 30 degrees and commonly approach the
horizontal. In the northeastern part of the quadrangle, the Cache beds
are folded into a broad but well-defined anticline whose axis trends
northwestward, nearly parallel to the North Fork of Cache Creek.
As for the lavas, some of these are interbedded with the Cache forma-
tion, and have been tilted. However, outcrop patterns of most flows indi-
cate that they are essentially undisturbed, if allowance be made for the
relief of the surface over which they flowed. Slumping is prevalent near
the edges of lava flows, and care must be taken not to confuse this with
folding.
Faulting
Faulting in this quadrangle is indicated by zones of crushed and
slickensided shale, by abnormally straight contact lines, by linear out-
crops of silica carbonate rock, and, for some minor faults, by the observed
displacement of strata. Faults between the major rock units were traced
for distances up to several miles, and where well exposed these may show
zones of gouge and fault breccia several tens of feet in width ; other large
faults are probably present within the major rock units, but are not
discernible because of l^e uniformity of the unit and the soil cover. That
most of the faults are steeply dipping is indicated by the fault trends,
which are nearly straight or broadly curved.
58 LOWER LAKE QUADRANGLE [Bllll. 166
The longest fault which could be continuously traced extends for
some 8 miles, from Coyote Valley to Seigler Canyon, and it passes beneath
lava flows at both ends. It forms the contact between the Knoxville and
the Cretaceous rocks, and is marked by zones of sheared and breeciated
rocks, also by silica carbonate rocks near Childers Peak,
The Cache formation seems to be commonly downfaulted at its contacts
with older formations. Such a fault contact is well exposed east of Dead-
man Canyon, in the northeastern corner of the quadrangle, where it
shows a minimum displacement of 150 feet. Furthermore, the Cache-
Franciscan contact in Burns Valley, although concealed by alluvium,
may be traced northwestward into the Bartlett Springs quadrangle,
where it is well exposed and clearly faulted. The south contact of the
main area of Cache sediments trends for over four miles in a nearly
straight line. The actual contact with older beds is covered by slumped
material from the unconsolidated Cache beds; but because the Cache
beds strike into the contact while consistently appearing at loAver topo-
graphic elevations than the older rocks, the contact is believed to be
faulted.
Minor faults in the lavas on the east flank of Mount Konocti and the
adjoining lava fields are marked by sharp breaks in the topography. The
faults show clearly on the aerial photographs, but no pattern or general
trend emerges. Slumping and consequent tilting of lava blocks, some of
very considerable size, is common along the lava cliffs.
GEOLOGIC HISTORY
The geologic record in this quadrangle begins in Upper Jurassic time,
some 125 million years ago, with the deposition of Franciscan sediments.
The Franciscan lithologic association is typical of geosynclines which
are orogenically and volcanically active ; the high ratio of graywaoke to
shale suggests that transitional or perhaps continental conditions pre-
vailed in the geosyncline, although other evidence indicates that the rate
of subsidence was irregular both in space and time. The source of sedi-
ments is thought to have been a volcanic archipelago located to the west
of the present coastline, but much of the later sedimentary material was
probably derived from the reworking of earlier sediments, uplifted
within the geosjmcline. During Knoxville time, subsidence was more
rapid than deposition, as indicated by the predominance of gray clay
shale, and the outlying island arcs projected only slightly above sea level.
Although Franciscan rocks crop out in only a small portion of the quad-
rangle, they undoubtedly underlie the whole, being covered in most places
by Knoxville or younger rocks. Thus the Jurassic sea covered the whole
quadrangle for a long period of time, sufficient to deposit some 15,000
feet of sedimentary rocks. As for the geographic extent of the Jurassic
sea, Taliaferro concluded from a regional study that it covered the region
now occupied by the central and northern Coast Ranges of California,
and reached northward into Oregon. Although there is no recognizable
break between Franciscan and Knoxville sediments, the greater defor-
mation of Franciscan rocks indicates some orogeny before deposition of
the Knoxville. Such orogeny would not necessarily be accompanied by
uplift.
The beginning of Cretaceous time is not marked by any recognizable
break in the rock record, although the somewhat greater degree of
1953] GEOLOGIC HISTORY 59
deformation of Knoxville rocks suggests that mild orogeny, perhaps
accompanied by uplift, preceded Cretaceous deposition. The Cretaceous
lithologic association is characteristic of non-volcanic geosynclines which
may develop adjacent to geosynclines such as the Franciscan-Knoxville.
The high ratio of sandstone to shale suggests that water depths generally
exceeded 120 feet, and the relatively small amounts of chert and mud-
stone fragments suggests that earlier geosynclinal sediments had been
stripped from old Franciscan-Knoxville source areas, exposing the
granitic basement. The Cretaceous sea occupied, according to Taliaferro
(1943), a "long, probably continuous but far from uniform trough
which lay along the west border of the Great Valley." It is questionable
whether this quadrangle was entirely covered by the sea, but large parts
of the quadrangle were covered for long periods of time.
The Paleocene rocks are similar to the Cretaceous, and clear-cut con-
tact relationships were not observed ; but the areal distribution of sedi-
ments shows that uplift and erosion preceded Paleocene deposition.
Martinez deposition of massive feldspathic sandstone followed by shale
was closed by uplift, deformation, and erosion before deposition of the
overljdng Tejon coarse conglomeratic sandstone. These Paleocene rocks,
confined to a small area east of Lower Lake, are evidently but remnants
of more widespread deposits latd-down in a shallow marine geosyncline
which extended northward from the region of San Francisco Bay.
Both Martinez and Tejon rocks are considerably more faulted and
folded than is the overlying Plio-Pleistocene Cache formation. Probably
the Tejon and older rocks underwent deformation at several times
during the Tertiary, but there are no sediments or other evidence to
record the diastrophic history. In late Pliocene time, the Cache formation
began to accumulate in a large structural basin. Streams from the sur-
rounding highlands carried debris into the subsiding basin, forming a
large basin plain whose surface was probably covered with lakes from
time to time. A maximum thickness of about 6,500 feet of clastic sedi-
ments accumulated in the basin. Toward the end of Cache deposition,
a large lake was formed in the eastern part of the basin, in which marl
and diatomite accumulated in association with tuffaceous sediments and
flows of basalt. The volcanism continued intermittently through the
Pleistocene, with the extrusion, from scattered northwest-trending fis-
sures, of a number of separate lava flows, including three distinct major
flows of basic lava. In addition, there were extruded, from fissures or
centers, flows and bulbous protrusions of dacite and andesite (Cobb
Mountain, Mount Hannah, Mount Konocti), and an extensive flow
of obsidian. Following the extrusion of the earliest volcanics, but before
the extrusion of most, the Cache formation was folded and locally
downfaulted against older rocks. The basin in which the Cache forma-
tion accumulated has been uplifted in the western part, but its eastern
part coincides with the present structural basin which Clear Lake
partially fills.
The most recent volcanic activity formed the cinder cone named
Roundtop Mountain, and this episode occurred many thousands of years
ago, judging from the effects of weathering and erosion. Still more recent
volcanism in the area may be evidenced by the accumulation of ' ' recent-
appearing ' ' pyroclastic-material found by Anderson on Mount Konocti.
60 LOWER TiAKE QUADRANGLE [Bull. 166
ECONOMIC GEOLOGY
By James C. Brick and J. Orant (Ioodwin *
The Jurassic (?) Fraiicisoan and Tertiary-Quaternary volcanic rocks
of the Lower Lake quadranj^ile are a potential source of a number of
mineral commodities in Lake County. Quicksilver occurs in Franciscan
sandstone and chert adjacent to serpentine bodies. It is also associated
with silica-carbonate rock, resultinj? from alteration of the serpentine.
The cinnabar is thoujjht to have been deposited by the carbonate solutions
which altered the serpentine late in the Tertiary period. Mineralization
has pjenerally taken place along serpentine contacts in shear zones.
Chrysotile asbestos is common in the sheared ser])entino bodies. Tlie
mineralized zones of anastomosing veinlots trend roughly parallel with
the elongation of the serpentine bodies. Chromite, disseminated and in
pods, also occurs in the serpentine and some high-grade ore has been
mined in the area. Sulfur has been produced from the Tertiary-Quater-
nary volcanic rock where sublimation around solfataric orifices has
occurred. Hot springs containing sulfur and carbon dioxide gases are
still active in the area. The volcanic rocks are also a source of building
materials such as pumice, plaster sand, lightweight aggregate and
ornamental stone.
During both "World Wars the shortage of critical minerals stimulated
prospecting and small-scale development of chromite, asbestos, and
quicksilver deposits.
Asbestos
Copsey and Jones prospect, located by Arthur Copsey of Spruce Grove
and Herbert Jones of Lakeport, is located in the NW^ sec. 32, T. 12 N.,
R. 7 W., in Big Canyon about 1 mile southeast of Howard Springs. This
property was prospected in 1928 by Johns-Manville during which time
they are reported to have taken out 7 or 8 tons of chrysotile asbestos
(Averill, 1947, p. 17). The main working is an open cut about 150 feet
long by 30 feet wide by 20 feet deep. Five smaller pits have been opened
in the mineralized zone of the serpentine. Some asbestos was seen in
place and much of the serpentine on the dump is cut by anastomosing
veinlets of chrysotile with fibers which average about one-quarter inch
in length and are of good quality. Maximum fiber length is three-
quarters of an inch. About 6 sacks of fiber have been handcobbed from
the serpentine and remain on the dump near the largest open cut.
Marvlvne prospect, claimed bv Mr. Ira E. Klein, is located in the NE:J
sec. 3, T." 12 N., R. 6 W., 2,500 feet north of the U. S. Geological Survey
Bench Mark on Brushy Sky High. The prospect is reached by a bull-
dozer trail from the Halle Bond Ranch in Morgan Valley. In the spring
of 1952, soil was removed with a bulldozer and prospect trenches were
cut at 4 points across the mineralized zone, wliieh trends N. 44" E. and
dips 52° W. The zone of chrysotile is about 18 inches wide at the point
of discovery and pinches to 6 inches within 100 feet along the strike.
The serpentine is highly sheared and altered to picrolite in the vicinity
of the prospect. The fibers average only an eighth of an inch and are
slightly brittle; however, the total asbestos content of the vein is hiuh.
An asbestos prospect located in the NE^ of sec. 4, T. 11 N., R. 7 "W. has
been prospected bj^ shallow pits at 4 points along the 200-foot length of
outcrop which strikes about N. 30 W. The mineralized zone ranges from
• Junior Mining Geologist, California Division of Mines.
1953] ECONOMIC GEOLOGY 61
about 2 to 4 feet in width with an asbestos content of from 15 to 25 per-
cent consisting of good quality fibers of chrysotile about a quarter to
half an inch in length.
Borax
Borax was probably first produced in California from Borax I^ake, 8
miles west of north from Lower Lake, and 2 miles south of Sulphur Bank
mine (Hanks, 1883, pp. 15-26). Commercial production of 590 tons of
refined borax was made from 1864-68 by the California Borax Company.
For analysis of Borax Lake, see section on soda.
Chromite
Chromite, disseminated and in pods, occurs throughout much of the
serpentine in this area. Production has been small, but considerable
tonnage of low-grade ore is present.
Copsey chromite prospect, located by Arthur Copsey of Spruce Grove,
is in the NE^ of sec. 4, T. 11 N., R. 7 W., about half a mile north of the
Big Canyon road. A 25-foot open cut was made and a 30-degree inclined
shaft was sunk. Chromite on the dump is of fair grade and low-grade ore
occurs as float along the entire hillside. Three other claims were filed by
Copsey along the same ridge in sec. 33, T. 12 N., R. 7 W., near Childers
Peak.
Gordon Springs prospect (Averill, 1929) is in sec. 2, T. 11 N., R. 8 W.,
a quarter of a mile northwest of Cobb Valley School at the serpentine and
Franciscan sandstone contact. The adit which ran north into the hillside
is now caved and the workings inaccessible. No ore was found on the dump.
Harpe and Sons Ranch (Averill, 1929) chromite prospect is in the
NE^ of sec. 29, T. 11 N., R. 7 W., a quarter of a mile S.E. of Harbin
Springs. The Sawyer Tanning Company mined several pockets of high-
grade ore containing 50 to 52 percent chromic oxide. Low-grade float is
common along this entire ridge.
Popp and Nichelini prospect is in the NE^ of sec. 24, T. 12 N., R. 7 W.,
just north of Seigler Springs in a small body of serpentine. Low-grade
float is common, but nothing of commercial interest was seen during this
investigation.
Other areas where considerable float is reported are sec. 14, T. 11 N.,
R. 8 W., just east of Whispering Pines (Averill, 1947) ; Mastick Ranch
(Averill, 1947), sec. 3, T. 10 N., R. 6 W., just south of McCreary Lake;
and east of Deadmans Canyon in sees. 7 and 13, T. 13 N., R. 6 W.
Clay
Clay of doubtful economic interest was found in the Cache formation
in sec. 8, T. 13 N., R. 6 W., along the North Fork of Cache Creek. The
material is a silty clay, interbedded with sand and pebble beds. The
low-grade clay might have limited uses in the brick and cement industry.
Copper
Three places (Jenkins, 1948) in sec. 19, T. 11 N., R. 7 W., in a body of
gabbro-diabase show traces of copper mineralization. Small prospect pits
have been sunk along fracture zones showing azurite- and malachite-
stained rock. The pits disclosed some primary ore which is disseminated
chalcopyrite cut by veinlets of chalcocite in zones of altered gabbro. One
of the prospects was in a 5-foot vein of aragonite. Most of the ore seen is
highly oxidized and it seems probable that disseminated primary sulfides
62 LOWER LAKE QUADRANGLE [Bull. 166
exist at depth on a much larger scale than that suggested in the oxidized
outcrops,
Diatomaceous Earth
An impure deposit of freshwater diatomaceous earth is in sec. 27,
T. 13 N., R. 8 W., along Thurston Creek on property owned by Henry
Murphy, Rt. 1, Box 260, Kelseyville. This material was deposited in a
small Tertiar}^ freshwater lake. Data from wells drilled on the property
indicate a thickness of about 100 feet. Near-surface material is quite
high in clay and silt and the entire deposit is capped with from 2 to 5 feet
of pyroclastic debris.
Gem Materials
Clear Lake Gem Mining Company of Woodland, California, conducted
considerable exploration and development work on a * * gem stone ' ' pros-
pect in the SE^ of the SE^ of sec. 20, T. 12 N., R. 7 W., in 1929 (Averill,
1929). Two large open cuts (100 feet long by 10 feet wide by 5 feet deep,
and 200 feet long by 20 feet wide by 5 feet deep) and numerous smaller
pits were cut in the Perini Hill andesite in search of gem material. The
stones, a very clear, high-temperature variety of quartz, occur as irreg-
ular masses sprinkled through the andesite. These were originally thought
to be hyalite, a variety of opal. Purple cordierite which originally was
thought to be amethyst, is also present.
A pale blue opal which occurs as irregular masses in hydrothermally
altered andesite is common at the Sulfur Bank mine.
Jasper is common as stream cobbles along Putah and Cache Creeks.
Manganese
Several manganese prospects have been recorded in the literature
(Averill, 1929; Jenkins, 1950). However, recent investigation has failed
to disclose the presence of any manganese of commercial grade. Consid-
erable low-grade siliceous ore occurs in the Franciscan chert in Pine Flat ;
on the Herman Ranch 4 miles west of Middletown ; around the Thome
mine near Anderson Springs ; and near the intersection of Herman and
Dry Creeks. Manganese staining is very common throughout the chert of
this entire area.
Mineral Springs
Lake County probably has a greater number and variety of mineral
springs than any other area in the United States. Bradley (1914) has
described these springs in detail and further description and additions
were published in later journals (Averill, 1929 ; Jenkins, 1947).
Quicksilver
Six quicksilver mines in the Lower Lake quadrangle have produced
considerable amounts of mercury. The cinnabar occurs both disseminated
and in high-grade stringers in silica-carbonate rock and in Franciscan
chert and sandstone adjacent to serpentine bodies. None of these mines
were operating in May 1952.
Anderson (Schtvartz) mine (Averill, 1929) is in sec. 25, T. 11 N.,
R. 8 W., about 1000 feet northeast of the Big Chief mine. The ore occurs
along a shear zone of brecciated and iron-stained chert and green sand-
stone which continues southwestward through the Big Chief. The ore
minerals are cinnabar and native mercury, which occur in high-grade
stringers and pockets. A D-type retort was used and production was
small.
1953] ECONOMIC GEOLOGY 63
Baker mine (Averill, 1929) owned by Mr. Lawrence Fuqua of Lower
Lake, has been sold to the Three Friends Mining Company. This mine
was worked as early as 1870, and in 1917 a two-corapartment shaft was
sunk. All of the old workings are now caved and abandoned. Some
cinnabar can be seen near the surface associated with serpentine and
decomposed material heavily stained with oxides of iron. The owner
states that 150 flasks were produced prior to World War I during which
time another 45 or 50 flasks were produced. The most recent work was
in 1931 when a shaft inclined at 45 degrees was sunk for a distance of
230 feet south-southeast from the present surface opening. Drifts were
opened from the end of the shaft ; the longest, extending 89 feet westward
is reported to have cut good ore.
Big Chief mine (Averill, 1929), in sees. 25 and 35, T. 11 N., R. 8 W.
on the Anderson Springs property, produced approximately 500 flasks of
mercury with a rotary furnace and oil burner. The condensing system
consisted of sewer tile and redwood tanks cooled by a spray of water.
The property was developed by a 350-foot tunnel which cut 20 feet of
ore in chert and another 20 feet of ore in sandstone. A second tunnel
driven at a point around the hill from the first, and at a right angle to
it, was 450 feet long. This tunnel penetrated 12 feet of ore in sandstone
and 12 feet of ore in chert on a level 22 feet below the first tunnel. Later
development included a small glory hole operation.
Big Injun mine (Averill, 1929) , in sec. 35, T. 11 N., R. 8 W., was owned
in 1929 by Ellis Armstrong of Calistoga. This property is described by
Bradley (1918) as follows:
"The mine is apparently on a contact of serpentine and sandstone,
but the formations are considerably broken up at this point. There are
two 'veins' or ore zones, the principal development having been done
on the eastern one. The strike is NW, and the dip is SW. The width
varies up to 30 feet, with ore shoots showing 1 to 4 feet wide. The ore is
characterized by the presence of considerable native mercury with
cinnabar ; and the gangue minerals are quartz and dolomite. There are
three main crosscut adits, the lowest being 550 feet, reaching a depth
of 150 feet below the outcrop. At 350 feet in on this crosscut there is a
hot sulfur spring." The equipment at the Big Injun was later moved
to the Big Chief.
Sulphur Bank mine (Bverhart, 1946) in sees. 5 and 6, T. 13 N., R. 7
W., was originally opened in 1865 as a sulfur mine, and in 3 years had
produced nearly 2 million pounds of sulfur. A drop in the price of sulfur
and an increase in contamination by cinnabar made production of that
product unprofitable. From 1873 through 1944, Sulphur Bank was oper-
ated intermittently as a quicksilver mine during which time more than
126,000 flasks was produced. The cinnabar is deposited as an incrustation
on boulders and blocks of altered andesite. At depth along the faults
which act as orifices for hot sulfur gases, cinnabar has been deposited
in Quaternary lake beds and Tertiary volcanic rocks. Hot waters and
vapors charged with carbon dioxide, hydrogen sulfide, methane gas,
and nitrogen still issue from vents at Sulphur Bank.
During periods of peak production, about 200 tons of ore was handled
per day and about 65 men were employed. Open pit methods have been
64 LOWER LAKE QUADRANGLE [Bull. 166
employed almost exclusively since heat and gases discourage under-
ground development. Since 1944 some diamoiul drilling has been done
and ore blocked out, but uncertainty about continuing high prices for
mercury has discouraged dcAvatering of the pits and reconditioning of
the plant. The property is still held by the Bradley Mining Company.
TJiorne mine (Averill, 1929), in sec. 36, T. 11 N., R. 8 W., about 1 mile
south of the Big Chief, is part of a group which includes the Big Chief
and Anderson mines owned by H. H. Barrows, 1648 16th Street, Oakland.
Weipcr mine, in sees. 16 and 17, T. 12 N., R. 6 W., is on property
belonging to Charles and William Anderson and includes 2 claims. Two
short tuniiels were opened about 1930; one struck no ore, but the other
crossed a zone about 12 feet wide which contained considerable cinnabar.
In 1944, the Bradley Mining Company made surface explorations to a
depth of several feet, but removed no ore.
Soda
Borax Lake, in sees. 7, 8, 17, and 18, T. 13 N., R. 7 W., is a playa-type
lake from 3 feet to 5 feet deep and covers an area of 200 to 300 acres. In
summer the lake drys up until only 2 or 3 inches of brine remain resting
on trona beds. The following anah'sis was made in January 1947 when
the lake was from 3 to 4 feet deep and covered about 300 acres. (Jenkins,
1947.)
Sodium carbonate 18.535
Sodium borate 0.484
Sodium fbloride 1.760
Potassium cbloride 1.780
Sodium sulfate 0.010
Magnesium acid carbonate 0.099
Misc. and organic 0.541
Total salinity 23.209 gm/liter
Rock, Sand, and Gravel
Aggrelite Compa^iy, owned by John C. McFayden and William Spi-
vack of 1734 Webster Street, Oakland, operates a concrete block plant
at the Sulphur Bank mine in sec. 6, T. 13 N., R. 7 W. Mine dump material,
which is kaolinized and opalized andesite formed by acidic hj-drothermal
activity near the solfataric orifices, is crushed and screened to minus
one-half inch and mixed with one-third part of furnace calcines from
the dump below the quicksilver plant. Iron oxides in the calcined rock
adds shades of pink, red, and brown to the finished bricks which are made
as requested in 27 different sizes and shapes. Plant capacity is 2000
bricks per 8-hour shift. The plant is operated by Mr. Keith Ward, the
plant manager, and 6 other employees. The finished product is used
locally and trucked to the San Francisco Bay area. Mr. Lewis Rapport
of Oakland is the sales manager and distributor.
Bonanza Springs Quarry is in sec. 30, T. 12 N., R. 7 W., about a quarter
of a mile south of Bonanza Springs in rhyolite tuff of Quaternary age.
The soft white rock is extremely porous if not sealed. It has been used
locally, especially in construction of the buildings at Bonanza Springs.
Camp and Yates gravel pit is on property leased from Herman Heinkel
in sec. 34, T. 11 N., R. 7 W., on the east bank of Dry Creek three-quarters
of a mile northwest of Middletown. From 8 to 16 feet of the gravel bars
1953] BIBLIOGRAPHY 65
are removed by a drafjline fitted with a 30-foot boom and a one-half
3^ard bucket. Gravel Avhieh is composed essentially of quartz, jasper, ande-
site, and basalt, is trucked a short haul to the plant and dumped on a
6-inch grizzly. A 50-foot conveyor belt carries the gravel to a 1-inch
trommel. The oversize is crushed by a Telsmith jaw-crusher and returned
to the circuit by a bucket-elevator. The crushed product and undersize
are carried by conveyor-belt to bins for unscreened material or to a
second trommel screen for further classification. The plant produces
plaster sand, coarse gravel, pea gravel, f-inch rock, and l^^-inch rock.
Trucks are loaded from bins or from a conveyor belt. The plant is
operated by Dariel Camp and 4 employees.
Coleman quarry, leased and operated by Mr. Coleman of Clear Lake
Park, is in sees. 16 and 17, T. 13 N., R. 7 W. about a quarter of a mile
northeast of Clear Lake Park. The material, an incoherent vesiculated
glass, is quarried by open-cut and screened to remove obsidian fragments
and coherent masses. The glass (obsidian) sand ranges from white
through shades of gray and brown. Random pods of obsidian have been
encountered in different zones. The obsidian is sold as ornamental stone.
The "glass" sand is used locally as a plaster sand and for road-bed
construction. This material also appears suitable for the manufacture
of light-weight blocks.
Coleman Ornamental Stone quarry is just north of the "glass" sand
quarry in a superjacent flow of highly vesiculated pyroxene dacite. The
light-weight rock is stained deep red brown by oxides of iron. Contorted
flow bands add to the beauty of this rock, which is very popular locally
for rock gardens and walls.
Seigler Springs quarry is in sec. 24, T. 12 N., R. 8 W., about a quarter
of a mile northeast of Seigler Springs Hotel. This is a light brown pyro-
clastic rock composed of rhyolite tuif and mixed rock fragments. The
strength and only slight porosity of this rock permits its use as a building
stone. Older buildings at Seigler Springs are constructed from this rock.
Sulfur
The Sulphur Bank mine (Everhart, 1946), originally owned by the
California Borax Company, was a sulfur mine. Between the years 1865
and 1868, 2 million pounds of sulfur valued at $53,500 was produced.
The sulfur is a product of sublimation in fissures around the solfataric
vents. The same solutions and gases which deposited the cinnabar depos-
ited the sulfur as a superficial cap under surface and near-surface condi-
tions. Falling prices and increased contamination by cinuabar brought
an end to mining operations until the mine was reopened in 1873 for
its quicksilver.
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County, California : California Jour. Mines and Geol., vol. 42, pp. 199-230.
Becker, G. F. (1888) Geology of the quicksilver deposits of the Pacific slope: U. S.
Geol. Survey Mon. 13.
Bowen, N. L. (1928) The evolution of the igneous rocks, Princeton Univ. Press.
Bowen, N. L., and Schairer, J. F. (1935) The system MgO-FeO-SiOz : Am. Jour. Sci.,
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Conrey, Bert Louis (1947) Geology of a southern portion of the Morgan Valley
quadrangle ; unpublished M. A. thesis, Univ. California.
Davis, W. M. (1933) Lakes of California: California Jour. Mines and Geo!., vol.
29, pp. 197-200.
Davis, E. F. (1918) The radiolarian cherts of the Franciscan group : Univ. California,
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Dickerson, R. E. (1914) Fauna of the Martinez Eocene of California, Univ. California,
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Eardley, Armand J. (1951) Structural geology of North America, Harper and
Brothers, New York, 624 pp.
Everhart, D. L. (1946) Quicksilver deposits at the Sulphur Bank mine, Lake County,
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INDEX
Aggrelite Company, 64
Anderson, 9, 30, 32, 33, 34, 38, 39, 40, 51, 52, 54, 59
mines, 64
Springs, 13, 19, 62, 63
Big Chief mine, 62, 63
Canyon road, 25, 61
Injun mine, 63
Bear Canyon, 13, IS, 19
Baylis Point, 55
Bald Mountain, IS
Baker mine, 63
Blackeye Canyon, 33
Boggs Mountain, 36, 41, 44, 47, 50, 53
Bonanza Springs, 33
quarry, 64
Borax Lake, 35, 38, 54, 61, 64
Bradley Mining Company, 64
Brushy Sky High, 19, 60
Buchia, 14, 22
(Aticella) piochii (Gabh),ll
crassicolis, 23
var. graciles, 23
piochii, 22
Gabb, 23
stantoni, 22
Buckingham, 55
Burns Valley, 17, 30, 31, 50, 58
Cache beds, 7, 31, 35, 53, 57
Creek, 9, 55, 62
formation, 26, 30, 32, 33, 35, 51, 58, 61
Lake, 30
sediments, 30, 32, 55
terrain, 50
California Academy of Sciences, 31
Borax Company, 61, 65
Coast Ranges, 7
Calistoga, 63
quadrangle, 35
Camp and Yates gravel pit, 64
Chico fossil, 29
Childers Peak, 54, 58, 61
Cinder Cone, 43
Clear Lake, 7, 8, 10, 30, 31, 32, 33, 35, 38, 39, 41, 43, 46, 50, 53, 54, 55
Gem Mining Co., 62
Highlands, 8
hydrographic basin, 9
Park, S, 54, 65
series, 35
volcanics, 35
Water Company, 9, 28, 55
Coast Ranges, 22, 26, 56, 58
Cobb Mountain, 9, 18, 35, 37, 45, 47, 50, 53, 59
Valley School, 61
Coleman quarry, 65
Collayomi Grant, 19
Valley, 8
Copsey Creek, 27, 29
Coyote Valley, 9, 20, 30, 32, 58
(69)
70 INDEX [Bull. 166
Cretaceous, 10, 21, 27
age, 7
rocks, 14, 16, 26, 36, 56, 57, 59
sandstone, 24, 27, 33, 55
sea, 59
system, 23
time, 58
Dariel Camp, 65
Deadman Canyon, 33, 58, 61
Dry Creek, 31, 62, 64
Elder Creek, 22
Ely Flat, 53
Eocene rocks, 51
Excelsior Valley, 13
Franciscan, 7, 14
area, 17, 18, 20, 56
chert, 62
greenstone, 17
group, 11, 21
-Knoxville, 11, 13, 14, 18, 26, 59
rocks, 37, 50, 54, 56, 60
sediments, 58
time, 10
Fraser Point, 39
Gordon Springs, 61
Great Valley, 26, 59
Grindstone, 22
Halle Bond Ranch, 60
Harbin Springs, 14, 61
Harpe & Sons Ranch, 61
Herman Creek, 62
Ranch, 62
Herndon Creek, 29
High Cascade volcanoes, 45
Hills Half Acre, 30, 32, 52
Howard Springs, 60
Johns-Manville, 60
Jurassic, 10, 11, 22, 23, 58, 60
rocks, 16
sea, 58
Kelsey ville, 33, 55, 62
Knoxville, 27, 58
clastic beds, 26
group, 11, 13, 21, 22, 23, 31
rocks, 36
shales, 7, 15, 16
time, 10
Konocti, 53
Lake County, 7, 43, 60
"Lake County diamond," 43
Lakeport road, 31
Lassen Peak, 43, 45
Lincoln Rock, 20
Loch Lomond, 33, 35
Long Valley, 50, 51
Lower Cretaceous, 22, 23
Lake, 8, 36, 45, 50, 57, 59, 60, 61
quadrangle, 9
region, 25
1953] INDEX 71
Manning Flat, 35
Martinez, 26, 28
formation, 23, 27
rocks, 28, 29, 57, 59
sandstone, 29
Marylyne prospect, 60
Mastick Ranch, 61
Mayacmas quicksilver district, 9
Range, 9, 37, 53
Mesozoic rocks, 34, 51, 55, 57
sediments, 51
Middletown, 8, 14, 15, 17, 19, 21, 25, 26, 50, 52, 56, 62, 64
formation, 35
Morgan Valley, 60
quadrangle, 23
Mount Hannah, 9, 38, 39, 47, 52, 53, 59
Konocti, 33, 39, 47, 52, 53, 54, 58, 59
Shasta, 45
Murphy, Henry, 62
McCreary Lake, 61
McFayden, John C, 64
Napa Valley, 56
Newville, 22
North Fork, 31, 57, 61
Oxytenthis tehamaensis, 22
Paleocene, 10
age, 23
Martinez, 27
rocks, 57, 59
Palmer Creek, 19
Perini Hill, 36, 41, 44, 46, 47, 62
Phipps Canyon, 33
Pine Flat, 62
Plagioclase phenocrysts, 7
Pleistocene, 10, 35
time, 51
age, 34, 35
Plio-Pleistocene, Cache formation, 7, 59
Popp & Nichelini prospect, 61
Portlandian, 22
post-Nevadan, 22
pre-Cretaceous, 22
Putah Creek, 9, 19, 52, 62
Quackenbush Mountain, 30, 41
Quaternary age, 64
Recent, 35
Riverview lode, 26
Rocky Creek, 7
Roundtop Mountain, 35, 40, 59
Russian River, 9
Sacramento Valley, 9
San Francisco, 7
Bay, 59
area, 64
County, 7
Juan lavas, 43
region, 43
Sawyer Tanning Company, 61
72 INDEX 1 Bull. 166
Seigler Canyon, 30, 58
Mountain, 9, 30, 47, 52
Sprinjcs, 33, 61
Springs Hotel, 65
quarry, 65
Shasta group, 23
Soda Creek, 13, 14, 50
Sonoma County, 7
volcanics, 32, 35
South Westland, New Zealand, 18
Spivack, William, 64
Spruce Grove, 60, 61
Stanton, cited, 22, 29, 57
State Highway 53 ; 14
Steinhart Lake, 54
Sulphur Bank mine, 7, 61, 62, 03, 64, 65
Banks, 40, 55
Creek, 9
Tejon, 30
conglomerate, 57
formation, 7
fossil, 29
rocks, 28, 59
sandstone, 59
Telsmith jaw-crusher, 65
Tertiary, 59
-Quaternary, 60
rocks, 29, 55, 56
Tesla, 16
Thorne mine, 62, 64
Three Friends Mining Company, 63
Thurston Creek, 62
Lake, 53
Tithonian stage, 22
United States Geological Survey, 9, 00
Weather Bureau, 10
University of California, 8, 23
Wallis Marine Service, 54
Weiper mine, 64
Whispering Pine, 61
Woodland, 62
i
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