a4 C3 A3 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 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 ■ Ol MgO 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 e c "H. £ W O CO o> OS »• 00 CM OS OS OS - s to IN CO 00 00 i-O CD 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. BIBLIOGRAPHY Anderson, C. A. (1936) Volcanic history of Clear Lake area, California: Geol. Soc. America Bull., vol. 47, pp. 629-664. Anderson, C. A., and Russell, R. D. (1939) Tertiary formations of northern Hacra- mento Valley, California : California Jour. Mines and Geol., vol. 35, pp. 219-253. Anderson, F. M. (1945) Knoxville series in the California INIesozoic : Geol. Soc. America Bull., vol. 56, -po. 909-1014. Averill, Charles V. (1929) Lake County : California Div. Mines and Mining Rept. 25. 66 LOWER LAKE QUADRANGLE [BuU. 166 Averill, Charles V. (1947) Mines and mineral resources of Lake County, California : California Jour. Mines and Geol., vol. 43, pp. 15-40. Bailey, E. H. (1946) Quicksilver deposits of the western Mayacmas district, Sonoma 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., vol. 24, ser. 5, pp. 151-217, esp. pp. 214-215. Bradley, W. W. (1918) Quicksilver resources of California: California Min. Bur. Bull. 78. Bradley, W. W. (1914) Lake County: California Min. Bur. Rept. 14. Carpenter, E. J., Storie, R. E., and Cosby, S. W. (1931) Soil survey of the Clear Lake area, California : U. S. Dept. Agr., Bur. Chem. and Soils, ser. 1927, no. 13. 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, Dept. Geol. Sci. Bull., vol. 11, no. 3, pp. 235-432. Dickerson, R. E. (1914) Fauna of the Martinez Eocene of California, Univ. California, Dept. Geol. Sci. Bull., vol. 8, pp. 89-99. Diller, J. S. (1891) U. S. Geol. Survey Bull. 79, 33 pp. Diller, J. S., and Stanton, T. W. (1894) The Shasta-Chico series: Geol. Soc. America Bull., vol. 5, pp. 453-464. 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, California : California Jour. Mines and Geol., vol. 42, pp. 125-153. Forstner, William (1903) The quicksilver resources of California: California Min. Bur. Bull. 27, 273 pp. Gabb, W. M. (1866) Cal. Acad. Sci., Proc, vol. 3, p. 302. Hanks, H. G. (1883) Report on borax deposits of California and Nevada: California Min. Bur. Rept. 3, pt. 2, pp. 15-26. Harker, A. (1909) The natural history of igneous rocks. Huey, Arthur S. (1948) Geology of the Tesla quadrangle, California : California Div. Mines Bull. 140. Hutton, C. O., and Turner, F. J. (1936) Metamorphic zones in northwest Otago : Royal Soc. New Zealand Trans., vol. 65, pt. 4, pp. 405-406. Iddings, J. P. (1890) On a group of volcanic rocks from the Tewan Mountains, New Mexico : U. S. Geol. Survey Bull. 66, 34 pp. Jenkins, O. P. (1948) Copper in California: California Div. Mines Bull. 144, p. 257. Jenkins, O. P. (1950) Manganese in California: California Div. Mines Bull. 152. Johannsen, A. (1932) A descriptive petrology of the igneous rocks, vols. I-IV. Krynine, P. D. (1941) Triassic sediments of Connecticut: (abstract) Geol. Soc. America Bull., vol. 52, p. 1919. Lacroix, A. (1893) Les enclaves des roches volcaniques, pp. 43-48. Larsen, E. S., Irving, John, Gonyer, F. A., and Larson, E. S. 3rd (1936-37-38) Petro- logic results of a study of the minerals from the Tertiary volcanic rocks of the San Juan region, Colorado : Am. Mineralogist, vol. 21, pp. 679-701 ; vol. 22, pp. 889-905; vol. 23, pp. 227-257. Peacock, M. A. (1931) Classification of igneous rock series; Jour. Geol., vol. 39, pp. 36-37. Petti John, F. J. (1949) Sedimentary rocks. Harper and Brothers, New York. Read, H. H. (1923) The petrology of the Arnage district in Aberdeenshire: Geol. Soc. Ijondon Quart. Jour., vol. Ixxix, pp. 446-486. Stanton, T. W. (1895) The faunal relations of the Eocene and Upper Cretaceous on the Pacific Coast : U. S. Geol. Survey 17th Ann. Rept., pp. 1024. 1953] BIBLIOGRAPHY 67 Self ridge, George C. (1936) An X-ray and optical investigation of the serpentine minerals, Am. Mineralogist, vol. 21, pp. 463-502. Taliaferro, N. L. (1941) Geologic history and structure of the central Coast Ranges : California Div. Mines, Bull. 118, pp. 118-163. Taliaferro, N. L. (1943) Franciscan-Knoxville problem: Am. Assoc. Petroleum Geolo- gists Bull., vol. 27, no. 2, pp. 109-219. Thomas, H. H. (1922) On certain xenolithic Tertiary minor intrusions in the island of Mull : Geol. Soc. London Quart. Jour., vol. Ixxviii, pp. 229-259. Turner, F. J. (1948) Mineralogical and structural evolution of the metamorphic rocks, Geol. Soc. America, Mem. 30. Turner, F. J., and Verhoogen, Jean. (1951) Igneous and metamorphic petrology, McGraw-Hill Company, New York, 602 pp. Waring, G. S. (1951) Springs of California : U. S. Geol. Survey Water-Supply Paper 338, pp. 88-98. Weaver, Charles E. (1949) Geology and mineral deposits of an area north of San Francisco Bay, California ; California Div. Mines Bull. 149, 135 pp. Williams, Howel. (1935) Newberry volcano of central Oregon, Geol. Soc. America Bull., vol. 46, pp. 296-298. Yates, R. G., and Hilpert, L. S. (1946) Quicksilver deposits of eastern Mayacmas district. Lake and Napa Counties, California : California Jour. Mines and Geology, vol. 42, no. 3, pp. 231-286. 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 tirinleJ in California state printing office 68207 10-52 2M THIS BOOK IS DUE ON THE LAST DATE THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW BOOKS REQUESTED BY ANOTHER BORROWER ARE SUBJECT TO IMMEDIATE RECALL REICEiVED m RECEIVED JUN 3 199't PHYSICAL SCS. 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