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[IBRARYQ^ rV s ^ \ VDJO"^ # ^^'' '^^WOJI]VJ-JO> >s^ ^'i^iUlSVSUr AOSANCElfj> va3AiNn]\\v 4 -< ^OFCAllFOff^ V^ S: .^OFCAIIFO/?^ M:^ AOSANCElfj> ^^ilJONVSOl-"^ %a3AINn-3WV' aWEUNIVERS'/a vvlOSANCElfj> ^^ILIBRARYO/: ^tllBRARYQr %a3AINn-3V\V ^OFCAIIK)% ^OFCAllFO/?^ ^^ ^(f/OJIlVJ-JO"^ \ME UNIVER^/A v^lOSANCElfj> o JO^ ^^ ^ 0%- .^OFCAIIFO% ^ ^WEUNIVERV/^ o v^lOSANCElfj> o ^Aa]AINn3WV ^OFCAIIFO/?^ ^^ .^^ ^(?Ativaan-^^'^ ;r% ^>clOSANCEl% ^IIIBRARYQ^^ ^^t-LIBRARYQ^, %a3AINn-3\\V^ ^.!/0JI]V3JO^ ^(l/OJIWO-JO^ ^WE UNIVERS-ZA o ;r% v^lOSANCElfj> ^OFCAIIFO/?^ ^OFCAIIFO/?^ ^^ ^WE•UNIVER% ^lOSANCElfj> o "^/^aiAiNn ]wv ^OF;CAUFO/?^ ^ ^OAavaaiH^"^ mih o ?3 v^lOSANGELfj^ -^^IIIBRARY^/ -5^lllBRARY6?/ \WEUNIVER% \1U^7U*V4 >2 DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, Director FOLIO 193-FlELD EDITION "Q^^r V5"^r GEOLOGIC ATLAS OF THE UNITED STATES San Francisco Folio BY ANDREW C. LAWSON WASHINGTON U. S. GEOLOGICAL SURVEY 1915 stack Annex 6 50:3^ CONTENTS. Page. Geography 9 Situation and general divisions of the area 9 Area east of San Francisco Bay 10 Relief 10 Drainage 11 Soil 13 Vegetation 14 Climate 15 San Francisco Peninsula 16 Relief and drainage 16 Shore lines 18 Soil 18 Vegetation 19 Climate 20 Marin Peninsula 20 Relief 30 San Andreas rift valley 22 Point Reyes Peninsula 22 Soil 23 Vegetation 23 Climate 24 Bay of San Francisco 25 Geology 28 Stratigraphy and areal geology 38 Geologic formations of the middle Coast Ranges 28 Pre-Franciscan rocks 31 Character and distribution .. 31 Gavilan limestone 31 Quartz diorite ("Montara granite") 31 Jurassic (?) rocks 34 Franciscan group and associated igneous rocks 34 Character 34 Stratigraphy 34 Cahil, Marin, and Bonita sandstones 35 Character 35 Calera limestone member of Cahil sandstone 38 Sausalito and Ingleside cherts . 40 General character 40 Petrographic features 41 Fossils 43 ij Distribution and tiiickness 44 (3) (ieology — Continued. Page. Stratigraphy and areal geology — Continued. JuraBsic (?) rocks — Continued. Franciscan group and associated igneous rocks — Continued. Conditions of sedimentation 44 Contemporaneous volcanic rocks 46 Intrusive rocks associated with the Franciscan group. 47 Peridotite and serpentine 47 tSilica-carbonate rock 50 Spheroidal basalt and diabase 50 Metaniorphic rocks 52 Age of the Franciscan group 54 Cretaceous system 57 Distribution 57 Lower Cretaceous (Shasta) series 58 Knoxville formation 58 Upper Cretaceous series 60 Chico foriuation 60 Oakland conglomerate member 60 Upper part of Chico formation 62 Fossils 63 Tertiary system 64 Eocene series 64 Subdivisions 64 Martinez formation 65 Character and distribution 65 Fossils 66 Rocks of San Pedro Point 67 Tejon formation 68 Distribution and character 68 Fossils 69 Miocene series 70 Monterey group 70 Petrographic character 70 Relations to older formations 73 Subdivisions 75 Sobrante sandstone 78 Fossils of lower faunal zone 78 Claiemont shale 79 Oursan sandstone 80 Tice shale . ^ 80 Hambre sandstone ,._. 81 Rodeo shale 81 Fossils of juiddle faunal zone 81 liriones sandstone 82 ( 'haracter and distribution 82 Hercules shale member 82 Fossils of upper faunal zone ^^3 Partly differentiated Monterey strata in the Con- cord quadrangle 83 Undifferentiated Monterey sti-ata in the Tamalpais -^ q uad ran gle 84 Hasalt . __. 85 Geology — Continued. Page. Stratigraphy and areal geology — Continued. Tertiary system — Continued. Miocene series — Continued. San Pablo formation 86 (jreneral features 86 Distribution 86 Thickness 87 Age 87 Fossils 88 Pliocene series 89 Leona rhyolite __. 89 Distribution and chax'acter 89 Chemical composition .. 91 Field relations 92 Age 92 Northbrae rhyolite 93 Distribution __. 93 Correlation 93 Petrographic features 93 Pinole tuff 94 General features 94 Peti'ogi-aphic character 95 Distribution 96 Orinda formation 97 General features 97 S*tratigraphic relations 98 Fossils 99 Age 100 Merced formation 101 General character and distribution .__ 101 Stratigraphic relations 101 Fossils L 103 Berkeley group ^ 104 General description 104 Moraga formation ^ 105 Siesta formation 106 Bald Peak basalt . 106 Relations of the group to adjacent formations 106 Tertiary and Quaternary deposits 107 Santa Clara formation 107 Quaternary system 108 Pleistocene series 108 Campus formation 108 Alameda formation , 109 San Antonio formation . Ill Merritt sand 112 Terrace gravel . 113 Recent series ; 113 Temeseal formation . 113 Other recent deposits 114 Terrace deposits 114 Travertine 114 6 Geology — Continued. Page. Stratigraphy and areal geology— Continued. Quaternary system — Continued. Recent series — Continued. Other recent deposits — Continued. Dunes 114 Salt-marsh deposits 114 Structure 115 General features 115 Montara block 116 Faults 116 Folds : 118 San Francisco-Marin block -._ 131 Faults 121 Folds 123 Berkeley Hills block 125 Faults 125 Folds 133 (ieologic history 138 Pre-Franciscan time 138 Franciscan epoch 139 Knoxville epoch 140 Chico epoch 141 Martinez epoch 142 Tejon epoch 143 Monterey epoch _. 143 San Pablo epoch A 145 Merced epoch 146 Berkeley epoch 147 Deformation at close of Tertiary period 147 Campus epoch ■. 148 Alameda epoch 148 Post- Alameda diastrophism 148 San Antonio epoch 149 Merritt and Temescal epoch 149 Recent uplift and depression 150 Changes in drainage 152 Economic geology 154 Available resources 154 Water . 154 Brick clays _ 159 Orinda formation 159 Siesta formation . 160 Alluvial and marsh clay 160 Shale for making bricks and cement IGI Limestone 161 Crushed rock 162 Gravel and sand. . 163 Abrasive 164 Greensand _ 164 Oil 164 Salt... __. 165 Pyritc 165 V Economic geology — Continued. Page. Quicksilver 166 Mangane'e 16T Lead 169 Gold 169 Copper I'i^" Asbestos, chroniite, talc, and niagnesite ITO Minerals IW Earthquakes and construction 171 Literature l^-") ILLUSTRATIONS. Maps, etc. (in pocket): Topographic maps. Colnnmiar section (on back of Tamalpais topographic map). Areal-geology maps. Structure-section sheets. Plates (at end of text) : I. Steeply tilted rocks of Ban Pedro Point, San Mateo quadrangle. II. Western front of the Berkeley Hills, viewed from the north across the canyon of Strawberry Creek. III. Calera limestone member of Cahil sandstone, Franciscan group, Calera Point, San Mateo quadrangle. IV. San Francisco Bay, from the Berkeley Hills. V. Ellipsoidal structure in intrusive basalt, Hunter Point, San Francisco. VI. Ellipsoidal basalt intrusive into thin-bedded radioiarian chert of Franciscan group, Hunter Point, San Francisco. VII. Thin-bedded chert and shale of the Claremont formation, Monterey group, Clai-emont Canyon, Berkeley Hills. VIII. Minutely folded thin-bedded radiolarian chert of Franciscan group exposed in quarry in Golden Gate Park, San Francisco. IX. San Andreas rift valley; view southeastward toward Crystal Springs Lake, San Mateo quadrangle. X. Trace of San Andreas fault, made by the movement which caused the earthquake of 1906. Figures: • Page. 1. Index map of central California 10 2. Map of vicinity of San Francisco Bay, showing topographic features and the geomorphic divisions resulting from the adjustment by tilting of great fault blocks 10 3. Outline map of the Tamalpais, San Francisco, Concord, San Mateo, and Haywards quadrangles, showing the limits of the great fault l)locks, the larger faults, and the axes of folds 11") 4. Outline map of the western slope of the Berkeley Hills in the southwestern jjart of the Concord quadrangle and adjacent parts of the San Francisco and Haywards quadrangles, showing the deflection of streams by the longitudinal rift valley of the Haywards fault zone 127 DESCRIPTION OF THE SAN FRANCISCO DISTRICT. By Andrew C. Lawson." GEOGRAPHY. SITUATION AND GENERAL DIVISIONS OF THE AREA. The five sheets of the San Francisco folio — the Tama 1 pais, Ban Francisco, Concord, San Mateo, and Haywards sheets — map a territory lying between latitude 37° 30' and 38° and longitude 122° and 122° 45'. Large parts of four of these sheets cover the waters of the Bay of San Francisco or of the adjacent Pacific Ocean. (See fig. 1.) Within the area mapped are the cities of San Francisco, Oakland, Berkeley, Alameda, San Rafael, and San Mateo, and many smaller tOAvns and vil- lages. These cities, which have a population aggregating about 750,000, together form the largest and most important center of commercial and industrial activity on the west coast of the United States. The natural advantages afforded by a great harbor, where the railways from the east meet the ships from all ports of the world, have determined the site of a flourishing- cosmopolitan, connnercial city on the shores of San Francisco Bay. The bay is encircled by hilly and mountainous country diversified by fertile valley lands and divides the territory mapped into two rather contrasted parts, the western part being again divided by the Golden Gate. It will be convenient to sketch the geographic features under four headings — (1) the area east of San Francisco Bay; (2) the San Francisco Peninsula; (3) the Marin Peninsula; (4) San Francisco Bay. Figure 2 shows the topography of this general region. " Seo note on page i^. (9) 10 Montcj-e y{^ 121° 120 Figure 1. — Index map of central California. The location of the five quadrauKles described in the San Francisco folio is shown by the darker ruling (No. 193). Published folios describing other quadrangles are indicated by lighter ruling and the proper serial numbers and are included in the numerical list on tlie back cover of this folio. AREA EAST OF SAN FRANCISCO BAY. Relief. — The area east of San Francisco Bay embraces a belt of billy country lying between the bay and the western flank of Mount Diablo. The ridges trend generally northwest and southeast. Portions of two wide valleys lie within this area. One of these is the valley of San Francisco Bay, on whose shores stand the cities of Berkeley, Oakland, and Alameda; the other is Ygnacio Valley, which occupies the northeast corner of the Concord quadrangle and extends with a very flat slope northward beyond the limits of the quadrangle to the shores of Suisun Bay. The southern extension of this valley, up the dniinago line of Wnlnut Creek, is the well-defined flat-bottomed V Figure 2. —Ma The eastern boundi of the Berkeley The submerged land was siibme e >f )f e a e )f Figure 2. —Map of vicinity of San Francisco Bay, showing topographic features and the geomorphic divisions resulting from the adjustment by tilting of great fault blocks. The eastern boundary of the Berkeley Hills block is not sharply deflced; it may be drawn along a general zone of overthrust folding and faulting just e«st of the Berkeley HUls. on which the Mount Diablo thrust block has ridden westward. This thrust movement antedated the tilting of blocks to the west The submerged bar outside the Golden Gat« originated as a delta deposit of the stream that flowed in the old valley of San Francisco Bay before tJiir The five quadrangles described in this folio are shown by heavier lines than the b s of other i|uadranglei C 11 San Kanion Valley, which sharply separates tlie belt of hills above mentioned from Mount Dial)lo, the greater mass of which lies farther east, in the adjacent quadrangle. The domi- nant range of hills is that which forms the southwestern limit of the belt and which immediately overlooks the Bay of San Francisco. The culminating point on this range is Bald Peak, east of Berkeley, which stands at an altitude of 1930 feet above sea level. Otiier peaks whose altitudes afford an idea of the general height of the range are Grizzly Peak, 1759 feet; Round Top, 1750 feet; and Redwood Peak, 1608 feet. This range is commonly referred to as the Berkeley Hills, although the area to which that term is applicable appears to be rather vaguely defined. It is also often referred to as the Contra Costa Hills, but this term apparently applies more properly to the broad group of hills between the Bay of San Francisco and Mount Diablo. A rather well defined line of valleys, including San Pablo and Moraga valleys, separates this dominant range from the more eastern portion of the hilly belt. The hills thus lying between the dominant ridge and Ygnacio and Ramon valleys show a less pronounced linear trend, are much more mature in their geographic expression, and in general are much lower. The culminating points in this part of the belt are Rocky Ridge (2000 feet) on the south, and the Briones Hills (1432 feet) on the north. In the Haywards quadrangle there is a notable sag in the longitudinal profile of the range, and at Haywards there is a gap at the level of the alluvial plain, which here stands about 100 feet above sea level. This gap is the outlet of a remark- ably open low valley known as Castro Valley. Drainage. — The drainage of the area includes several features worthy of special mention : 1. The dominant ridge of the hill belt does not form the divide that separates the waters flowing directly to the Bay of San Francisco from those flowing to Suisun Bay by way of Walnut Creek, in San Ramon and Ygnacio valleys. The divide runs through tlie center of the belt of hills, so that a very considerable portion of the lower ground northeast of the dominant ridge is drained either through or around the end of 12 the ridge. The soiitliern half of the range drains through gaps in the ridge, which form outlets to the bay for San Lean- dro and San I^orenzo creeks. Tlie northern half is drained by San Pablo Creek, which flows around the end of the main ridge, and by Pinole Creek, which flows to San Pablo Bay. 2. A second feature of the drainage of especial interest is that, though the streams on the northeast side of the dominant ridge are manifestly subsequent, those draining the southwest- ern slope of the same ridge directly to the bay are as mani- festly consequent, presenting a condition which suggests that the slope to San Francisco Bay is of more recent origin than the maturely dissected hill country farther northeast — a suggestion which is more fully discussed in this text, under the heading "Structure." 3. A third feature is the prevailing alluviation of the valley bottoms and the steep-sided stream trenches cut in the alluvium, which do not as a rule reach the bedrock, clearly indicating that the former conditions in this area favored more vigorous downward corrasion by the streams, at a time when the can- yons and valleys were deeper, and that with the passing of these conditions others ensued which reduced the transporting power of the streams and caused them to drop their load of detritus in the bottoms of the canyons and valleys, making these flat and broad, as well as helping to give the country its geomorphically mature aspect and adding to its agricultural value. Since these flat-bottomed valleys were thus formed there seems to have been a slight but distinct tendency toward a recurrence of tlie older conditions, indicated by the trench- ing of the valley floors, but this trenching may be due, in part at least, to the disturbance of natural conditions caused by culture. Attention may be called also to the rather noteworthy con- vergence of drainage in Castro Valley near Haywards. Most of this drainage is carried l)v San Lorenzo Creek throuR'h a pronounced ))i'eak in the Berkeley Hills. The only other notable feature of the drainage is the fact tliat the stream flowing in San Bamon Valley, the largest, most mature, and broadest valley in the Concord quadrangle, 13 has cut tliroii.i;li tlie alluvium on the vallev floor at several places, particuhn-ly from Alamo to the village of Walnut Creek, where it runs on bedrock. Soil. — The soils of the country may be divided into two classes. The soil on the hillsides and ridge tops above the level of the valley floors is sedentary — that is to say, it has been formed in place by the chemical and mechanical disinte- gration of the underlying rocks. This soil has l)een modified chiefly by tlie abstraction of certain constituents which nour- ished the generations of plants that have grown upon the sur- face and by the addition of organic matter formed by the decay of the same vegetation. On these hill slopes earthworms ai-e uncommon, probably because the soil is very dry and parched during the summer, so they have not aided in turning over and mixing the soil and in thus making it more useful for agricul- ture. This work, however, lias been performed, probably with equal efficiency, by several kinds of burrowing mammals, such as the gopher (Thomomys) and the ground squirrel (Sper- mophilus). These animals formerly infested the region in great numbers and have persisted there until very recently, in spite of the eff'orts of the farmers to destroy them, but during the last few years, by more systematic efforts, the health authorities have almost completely exterminated them, because they are regarded as a menace to the public health as propagators of the bubonic plague through the fleas wdiich infest them. These sedentary soils, having been formed from the immedi- ately underlying rocks, vary in character from place to place, and here and there the slopes are so steep that little or no soil can accuniulate. The soils derived from the Cretaceous and Eocene formations are perhaps those best adapted to agricul- ture, but they lie chiefly on high ground that is cut by rather steep canyons, so that they are not so generally cultivated as the soils of the lower ground. The sandstones of the Monte- rey group, which are very quartzose, yield nearly everywhere a light sandy soil, whereas the shale and chert formations of the same group yield scant and poor soils. Considerable areas that are underlain by the shale and chert carry no soil what- 14 ever, the bare mechanically disintegrated rock forming the sur- face of the ground. The fresh-water deposits of the Orinda formation have generally yielded deep and excellent soils, which, however, are in man}^ places heavy and clayey. The soil in the bottoms of the vallej'^s is not sedentary but has been derived from various sources in the course of the degradation of the surrounding hills and is excellent. It varies from a sandy to a clayey loam and in certain localities is even gravelly, but in practically all places it is well adapted to suc- cessful tillage. Vegetation. — The region is almost devoid of forest. Hill- tops and slopes are bare of trees (see PI. II) and the prevail- ing mantle of vegetation in uncultivated tracts is composed of the wild oat {Aveiia fatua) and other wild grasses {Danthonia californica, Festuca myuros, Lolium temulentum, and species of Hardeum); but on the south sides of steep canyons the slopes may be covered with a more or less dense growth of brush. The only native timber to which the term forest might be applied is the grove of redwoods (Sequoia semper- virens) on the west side of Redwood Canyon, extending from the creek to the summit of Redwood Peak. The only other conifers in the district are the digger pine {Pinus sabiniana), which is a feature of the lower slopes of Mount Diablo on the east side of San Ramon Valley, and the knobcone pine (Pinus alter iiata), which grows locally on the summit of the first ridge east of Redwood Canyon. The most abundant tree is the live oak (Quercus agrifolia), common in canyons and on north and east slopes and also notable for filling shallow gulches or south and west slopes of otherwise treeless hills. The valley oak (Quercus lobata) is characteristic of the open valleys, growing onl}'^ sparingl}'^ or not at all on the hills. It is most common in San Ramon and Ygnacio valleys. The blue oak (Quercus douglasii) is scattered over dry hills in the eastern part of the area. The buckeye (JEsculus californica) grows along the bases of low hills. The laurel (Umbellularia californica) is the commonest tree in can- yon bottoms. It also clusters about rocky knolls on ridges and slopes. Along the stream courses are found the red alder 15 [Alnus oregona) and the white alder (Alnus rhomb if olia), the latter, however, only east of the crest of the Berkeley Hills. There are three willows, the most abundant and widely distrib- uted being the white willow or arroyo willow (Salix lasiolepis), which thrives in dry gulches in the hills as well as along living streams. The yellow willow (Salix lasiayidra) and the red wil- low [Salix Iwvigata) are mostly confined to living streams. The madrone [ArhiLtun menziesii) is rare and is found chiefly with the redwoods but extends northward to Strawberry Canyon. The big-leaf maple {Acer macrophyllum), the box elder {Negundo caUformcwu), and the sycamore {Platanus racemosa) are rare. The California walnut [Juglans californica) grows along streams at Walnut Creek and Lafayette. Taking the territory as a whole, the brush-covered areas are more striking than the wooded areas. The brush of the Berkeley Hills is composed chiefly of nine-bark [Neillia capitata), cofl*ee berry [Rhamnus californica), hill scrub [Baccharis pilularis), poison oak (Bhus diversiloba), and mountain lilac {Ceanothus thyrsiflorus and C. sorediatus). Typical chaparral is not common, although colonies of manzanita {Arctostaphylos manzanita) and similar shrubs are found, particularly east of the crest of the Berkeley Hills. Chamiso grows more or less extensively on Las Tranipas Ridge, on the hill east of Bed wood Peak, and at the base of Mount Diablo." Climate. — The crest of the Berkeley Hills is a dividing line for the climate of this region. The climate of the area east of the crest is somewhat like that of the interior valleys; that of the west slope of the Berkeley Hills is like that of the coast. In the area east of the crest the summers are hotter and the winters are colder than in the area farther west. The sea breezes and fogs that temper the summer heat on the west slope have a greatly diminished influence on the east side of the Berkeley Hills. The annual rainfall at Berkeley is about 27.48 inches, and it falls almost wholly in winter, the summer being rainless. Snow rarely falls, even on the highest ground, and no snowfall lasts more than a day. "Furiijucli of the information contained in thiss and otiier paragraphs dealing with the vegetation the writer is indebted to Prof. W. L. Jepson. 16 SAN FRANCISCO PENINSULA. The territory west of the Bay of San Francisco is naturally divided into a northern and a southern part by the Golden Gate. South of the Golden Gate lies the San Francisco Penin- sula, with the city of San Francisco at its northern extremity; north of it lies the Marin Peninsula, w^hose most notable fea- ture is Mount Tamalpais. Relief and drainage. — The San Francisco Peninsula is divided into two parts by Merced Valley. Each of these parts has the general profile of a much dissected orographic block having a gentle slope to the northeast -and a crest line on its southwest margin. The culminating crest of the northern block is San Bruno Mountain (elevation 1315 feet), and the corresponding- crest of the southern block is Montara Mountain (elevation 1952 feet). The structural line separating the surface areas of the blocks is the trace of the San Bruno fault, which lies at the base of the steep southwest front of San Bruno Mountain. In character of relief these two blocks present an interesting- contrast. The surface of the northern block is irregularly hilly and, except in San Bruno Mountain, shows but little linearity in the disposition of its crests and valleys. Its geo- inorphy is fairly mature, although a characteristic feature of that maturity is a certain ruggedness of profile due to the presence of formations composed of radiolarian chert. The exceptional resistance which these offer to erosion causes the areas they occupy to present a marked contrast with adjoining areas composed of sandstone. San Bruno Mountain, however, is a simple linear ridge with an immature frontal slope over- looking Merced Valley. The southern block is marked by notable linearity in its crests and valleys, and its geomorphy is in general much less mature than that of the northern block. The most remarkably linear feature is the valley in which lie San Andreas and Crys- tal Springs lakes (see PI. IX), a short segment of the San Andreas rift valley, a feature due to repeated faulting in recent geologic time. The trace of the fault of 1906 follows this rift for about 300 miles. The valley is due in part directly to earth movement but in large measure also to the mashing 17 of the rock in the fault zone and its consequent easy erosion. Northeast of the San Andreas rift lies the equally linear Buril)uri Ridge, which slopes down to Merced Valley and to the Bay of San Francisco. San Mateo (^reek cuts across this ridge transversely at nearly its widest part, draining the valley of the San Andreas rift, both tlie part which lies northwest and that which lies southeast of the point where the creek leaves the rift to flow through a sharply incised gorge toward the Bay of San Francisco. The position of this trunk drainageway of the rift valley indicates that it is a superimposed stream, the course of which was established when the slope was mantled by softer formations, now removed. The slope of the Montara orographic block in the area southwest of the rift valley is dissected by a series of sub- parallel steep-sided canyons and intervening ridges, which are in large part remnants of the original tilted surface. Viewed in a large way, however, this tilted slope, taken as a wdiole, from the crest of Montara Mountain to the Bay of San Fran- cisco, presents a broadly terraced aspect. It comprises two ter- race levels or steps, one the flat-topped ridge that is elsewhere called the Buriburi Plateau, the general elevation of which is about 700 feet, and the other the Sawyer Plateau, comprising a number of flat-topped ridges that stand at elevations of 1100 to 1200 feet. The crest of the tilted block known as Montara Mountain is composed of quartz diorite and is somewhat serrate in its longi- tudinal profile, its serration being due to its incision by the head- water erosion of the high-grade streams on its southwest front. On the precipitous shoulders and ridges of the southwest face of the mountain there are obscure traces of terraces at elevations of 400 to 500 feet. At the base of the mountain front, but separated from it by an alluviated valley, is a low ridge, known as Miramontes, terminating in Pillar Point. This ridge is com- posed of folded late Pliocene strata, which rest upon the quartz diorite. A little north of this, at Montara Point, is a fine example of a superimposed stream cutting through a foothill ridge, with lower ground between it and the base of Montara Mountain. San Francisco— 2 18 The sand dunes of the city of San Francisco are notable features of the north end of the peninsula. The sand is blown in from the beach south of the Golden Gate by the westerly winds and drifts eastward over the western part of the city as a wide expanse of ripple-marked dunes. The encroachment of the sand is now, however, in large measure checked by the extension of city improvements toward the beach and by the planting of suitable vegetation in the sand to restrain its movement. Shore lines. — The shore lines of the two sides of the peninsula present a marked contrast. On the bay side the shore contour is im maturely serrate, with tidal marshes and other evidences of silting in the bays between the points, cliff cutting is relatively feeble, and there are no clean sandy or pebbly beaches. On the ocean side the attack of the waves has developed extended lines of sea cliffs with sandy beaches at their bases, and the embayments between the cliffs are filled out. This process has proceeded so far as to give the shore a simple and mature con- tour, only a few short points — as, for example, San Pedro Point — projecting seaward. The shore features, particularly those on the bay side of the peninsula, indicate the recent sub- sidence of an erosionally dissected land mass, and a like subsi- dence on the seaward side is indicated by Merced Lake, which before it was modified for use as a reservoir was a flooded stream valley shut off by drifting sands. Soil. — The soil of the peninsula is largely sedentary on the gentler slopes of the surfeee that are underlain by the sandstones of the Franciscan group and the comparatively soft strata of the Merced (Pliocene) and Quaternary formations. But little soil accumulates upon areas occupied by the radiolarian cherts of the Franciscan group, by the serpentine associated with the Franciscan, or by the quartz diorite ("Montara granite"). At the base of the hills and in some of the valleys there are belts of rich soil formed by the accumulation of alluvium washed from higher levels. In the vicinity of San Francisco the loose sandy soil is intensively cultivated, with the liberal use of fertil- izers, for growing vegetables for the local market. A part of the hill laud is adapted to cattle grazing and dairying, but the large reserves maintained by the Spring Valley Water Co. to 19 keep the city water supply free from pollution have restricted the areas available for these industries. Veyelation. — Climatic conditions are adverse to forest growth on the San Francisco Peninsula. Most of the high ground is bare of trees. In the gullies and canyons, however, there is a more or less luxuriant growth of shrubs and low trees, compris- ing California laurel (UmbeUidaria californica), coffee berry (Rhamnus californica), creek dogwood (Corniis piibescens), arroyo willow [Salix lasiolepis var. bigelovii), wax myrtle (Mi/rica californica), coast live oak [Quercus agrifolia), and blue blossom (Ceanothus thyrsijiorus). On the open hills may be found the following common and widely distributed species of shrubs: Old man [Artemisia cali- fornica), bush monkey flower (Diplacus ghitinosus), woolly painted cup {Castilleia foliolosa), JSricameria ericoides, eoifee berry [Rhamnus californica), Christmas berry {Hetero7ncles arbutifolia), poison oak {Rhus diversiloba), and bush lupine (Lupiniis arboreus). The following are the more common plants that grow on the sand dunes and tend to restrain their movement: (Enothera cheiranthifolia, coyote scrub [Baccharis doiiglasii), sea fig {3Iesembryanihemum mqui later ale) (introduced), dune tansy (Tanacetum camphoratum), Franseria chamissonis, Roubieva midtifida, Franseria bipinnatifida, Crotou californicus, Polygo- 71U771 paronychia, sand grass (Poa douglasii), and beach grass (Ammophila and Arenaria, introduced). On the sea cliffs may be found a flora peculiar to these steep and exposed slopes, comprising the sea daisy [Erigeron glaucus), lizard tail (Friophylhim stwchadifolium), yarrow (Achillwa millefolium), seaside painted cup (Castilleia latifolia), and Eri- gonum latifolium. A few plants are peculiar to the beaches, such as the sea plantain (Plantago maritima), sand strawberry (Fragaria chilcnsis), and beach pea [Lathyrus littoralis). The vegetation of the salt marshes comprises Salicomia ambigua, Triglochin maritima, Frankenia grandifolia, Atriplcx patula, Atriplex hastata, Tissa macrotheca, and Cotula coronopi- folia. Near the tidal channels and in the parts of the marshes that liave been longest reclaimed to vegetation there may be 20 found a variety of sedges and tules, such as Scirpus lacustris var. occidentalis, Scirpus olneyi, Scirpus robushts, Cyperus bron- gniartii, Juncus jjatens, Juncus xip/iioides, and Typha latifoUa. Climate. — At San Francisco the only marked seasonal change is due to difference in precipitation. The mean annual temper- ature is 56° F. The coldest month is January, which has a mean temperature of 50°, and the warmest is the period from the middle of September to the middle of October, during which the mean temperature is 60°. On summer afternoons a layer of fog 1700 feet deep spreads over -the city. The mean annual rainfall is 22.83 inches, which falls in winter, the sum- mer being rainless. The prevailing winds are westerly, and their average velocity is 9.7 miles an hour. The mean relative humidity is 88 per cent in the morning and 73 per cent in the evening. At Pilarcitos Lake, about 15 miles south of the center of San Francisco, the annual rainfall is about double that at the city; but the rainfall decreases southeastward, along the valley of the bay, from 22.83 inches at San Francisco to 15 inches at San Jose. Places on the bay side of the peninsula are colder in winter and warmer in summer than the city of San Francisco. In winter the valley bottoms on the bay side are subject to frequent morning frosts, but on the slopes of the surrounding hills frosts are less frequent and less severe. In summer the valleys may be bathed in bright sunshine while San Francisco and the Golden Gate are mantled in dense fosr. The climate of the peninsula thus presents marked local variations, and these variations are doubtless related to the relief. MARIN PENINSULA. Relief. — The Marin Peninsula presents the features of a dis- sected mountain mass which has been depressed sufficiently to permit the waters of the ocean to enter the stream valleys and so convert them into bays or inlets. Its highest point is Mount Tamalpais, which has an elevation of 2604 feet. From this peak a ridge with a fairly even crest extends westward and then northwestward beyond the Tamalpais quadrangle. From the peak and crest the surface falls away in steep slopes on all sides, in an alternation of canyons and steep-crested ridges. 21 The canyons are all of steep grade and have steep sides, but where they reach the sea level they widen very notably and their bottoms become flaring, flat valley floors, which are occupied for the most part by salt marshes. This description applies particularly to the bay side of the peninsula, where Tiburon Peninsula and San Quentin Point separate three embayments that reach far inhmd to the base of the mountain and that are very evidently drowned valleys. On its w^est side the main mass of the Marin Peninsula has a very even steep slope, which in part descends to the sea and in part to the bottom of the straight, narrow valley that separates the Point Peyes Peninsula from the mainland. This declivity ranges in width from 1^ to 2 miles and except at its southern end is not deeply trenched by the streams that cross it. This western shore of the Marin Peninsula, which is without notable embay- ments and promontories, presents a marked contrast to the extremely indentate and irregular eastern shore on the bay side, and the geomorphic asymmetry of the peninsula suggests that it is a tilted orographic block, elevated on its western and depressed on its eastern margin. This interpretation of the origin of its geomorphic features agrees with that given for the north end of the San Francisco Peninsula, for, as the west or southwest boundary of the Marin block is similar to the south- west boundary of the San Francisco block, as exemplified in the front of San Bruno Mountain, and as the two lie in the same general straight line, the mainland of the Marin Penin- sula and the north end of the San Francisco Peninsula are evidently parts of the same tilted crustal block and have had a common geomorphic history. This block is transected by the Golden Gate, which separates it superficially into two parts. The Golden Gate is without doubt the gorge of a trunk stream which maintained itself across the block dur- ing the slow progress of its tilting and which subsequently, after the depression of the region, became a marine strait through which the sea flooded the valley on the lower side of the block. Besides the Golden Gate, another valley completely transects the tilted block. This is Ellk Valley, which extends across the 0'7 ]\I;irin Peninsula from the head of Richardson Bay to Tennessee Cove. It is a narrow valley of low gradient, which has steep mountain slopes on both sides and which lies at right angles to the axis of the block. The bottom of this valley, though nar- row, is alluviated throughout by the wash from the adjacent slopes. The highest point of the valley bottom is about mid- way between the bay and the ocean and lies between the 175-foot and 200-foot contours. This point is evidently the headwater portion of a stream that crossed a part of the block prior to the tilting. Rodeo Lagoon, just north of the Golden Gate, occupies the only drowned valley on the west side of the mainland of the Marin Peninsula.. San Andreas rift valley. — The Point Reyes Peninsula is geo- graphically distinct from the mainland of the Marin Peninsula and is separated from it by a long, narrow, straight valley, the north end of which is occupied by Tomales Bay and the south end by Boliuas Lagoon, a lake cut off from the ocean by a sandspit. This forms a notable segment of the San Andreas rift valley, and the trace of the fault of April 18, 190G, runs completely through it. Its straight, linear course was undoubt- edly determined by the existence of the zone of recurrent fault- ing which, partly by displacement and partly by excessive ero- sion induced by rock mashing, finds topographic expression in the rift valley. Point Reyes Peninsula. — The dominant feature of the relief of the Point Reyes Peninsula is a comparatively straight ridge on its eastern margin, parallel to and close to the rift valley. From this ridge the ground slopes, w^itli many minor irregu- larities, westward to the seashore. The streams that drain this slope have in their lower stretches been drowned by subsi- dence, and both Tomales Bay and Bolinas Lagoon were also formed by subsidence. At the south end of the Point Reyes Peninsula is one of the most clearly evident wave-cut terraces on the coast of Cali- fornia. It has the form of a very even topped, gently sloping plateau, the rear of which, where it abuts against ancient and now much degraded sea cliffs, stands about 250 feet above the sea. This wave-cut terrace has a maximum width of over a 23 mile and u half and terminates on its seaward side at the brink of the modern sea cliffs. It is now dissected by numerous small streams, which run seaward across it from the high ground on the north. We thus have side by side — in the drowned streams and the elevated sea cliifs — abundant evidence of the opposite move- ments of elevation and subsidence of this part of the coast, subsidence having been probably the later movement. It is remarkable that although there is so fine an elevated wave-cut terrace on the Point Reyes Peninsula there is no well-defined trace of elevated strands on the western shore of the mainland of the Marin Peninsula. Soil. — The soil of the Marin Peninsula is chiefly sedentary, so that its character in most places depends on that of the underlying formations. The principal formation, the sand- stone of the Franciscan group, yields a sandy loam soil, which, however, is scant on the prevailingly steep slopes. There is but little soil upon the radiolarian chert formations of the Franciscan group and the associated serpentine. The splie- roidal basalt yields a red soil, which is deep enough to be till- able on gentle slopes. In the valley bottoms above the limit of the salt marsh the soil is a fertile alluvium composed of the wash of the valley slopes. Cattle raising and dairying are the chief agricultural industries on the peninsula. Vegetation. — The woody vegetation of the Marin Peninsula is either chaparral or tree growth of various sorts. Chaparral is tlie dominant type. It cov^ers the main slopes of Mount Tamalpais up to its summit and considerable areas farther north, along Bolinas Ridge and toward San Rafael, becoming less abundant toward the north. It includes mainly the fol- lowing species: Arctostaphylos tomentosa, A. manzanita, A. nummular ia, Cennothus thyrsifiorus, C. foliosus, C. rlgidus, C. prostratus var. divergens, C. cuneatus, C. soredlatus, Garrya elliptica, Rhamnus californica, Quercus duniosa, Q. wislizenii ynr. frutescens, and Adenostoma fasciculatum, or chamiso. The tree growth is confined mostly to canyons or occurs as a scattered stand on northern slopes. In tlie canyons it con- sists mainly of Sequoia sempervirens, Pseudotsuya taxifolia, and 24 Umbellularia californica. On the lower northern slopes of Mount Tamalpais is an open stand comprising- Quercus cali- fornica (the most abundant species), Q. garryana, and Arbutus menziesii. Along the streams may be found Fraxinus oregana, Alnus rubra, and Acer macroiphyUum. Associated with the scattered oaks there is considerable RIiub diversiloba, and the redwoods of the canyons are accompanied by the plants that commonly form the undergrowth of the main redwood belt, comprising Vaccinium ovatum, Gaultheria shallon, Oxalis oregana, Scoli- opus bigelovii, Trillium ovatum, and Clintonia andrewsiana. On the western slope of Mount Tamalpais are a few patches of the rare Cupressus sargentii, and in the valleys near San Rafoel is a form of valley oak that shuns the coast. The vegetation on the west side of the San Andreas rift valley is radically different from that on the east side. From Bo Unas Lagoon northward the eastern slope of the main ridge of the Point Reyes Peninsula is covered with a forest which, though not continuous, is fairly dense in the areas where it is best developed. This forest is composed almost exclusively of Pinus muricata, which is accompanied by a little Pasania densijiora and Quercus agrifolia and by considerable Umbel- lularia californica on very steep slopes. The densest part of the forest is, however, pure Pinus muricata. The shrubs of the Point Keyes Peninsula are northern types, which have here their southernmost or nearly their southernmost repre- sentation. These shrubs include Rubus spectabilis var. men- ziesii, Ledum glandulosum, and Rhododendron callfornicum. The vegetation of the marsh lands is practically the same as that which characterizes the marshes of the San Francisco Peninsula, already listed. Climate. — The climate of the Marin Peninsula, unlike that of the city of San Francisco, is characterized by foirly well marked seasonal changes in temperature. The mean annual temperature is dfy^ F., nearly the same as that of San Fran- cisco, but the mean January temperature is 40'' and the mean July temperature is 70\ The mean annual rainfall at Kent- field (elevation Go feet) is ol.34 inches, which is over double 25 that of San Francisco; but at Point Reyes Light it is only 30.80 inches. Tlie prevailing winds are northwest, and their average velocity on Mount Tainalpais is 17.8 miles an hour. The maximum velocities occur at Point Reyes Light. The summer afternoon fogs generally do not extend to the top of Mount Tamalpais, and the upper surface of the fog seen in the bright sunlight from the summit is a most remarkable and beautiful sight. The coast north of the Golden Gate, particu- larly in summer, is covered by a sea fog 100 to 1700 feet thick, which lies along the coast wdth its bottom frequently a hundred feet or less above sea level. BAY OF SAN FRANCISCO. The Bay of San Francisco is a submerged valley and is a most notable example of a great harbor formed by the influx of the sea into the low parts of a subsiding coast. (See PI. IV.) If the region were uplifted so that the water were drained out of the bay, the depression would not differ in its essen- tial features from the Santa Clara, Santa Rosa, or Napa val- leys. If, on the other hand, the coast were still farther depressed, so that these valleys also were flooded, they would have features entirely analogous to those of the Bay of San Francisco. The isolated hills and some of the foothill ridges would become islands similar to Angel Island and Goat Island, the tributary valleys would become embayments or inlets simi- lar to Richardson Bay and San Rafael Bay, and the inter- vening ridges would become peninsulas or promontories like Tiburon Peninsula, Hunter Point, and San Pablo Point. Before the valley of the Bay of San Francisco was submerged there flowed through it a river that drained the great interior valley of California. This river probably ran between the Tiburon Peninsula and Angel Island, where, in Raccoon Strait, there is a deep channel witli a sounding of 39 fathoms. Thence it flowed through the gorge of the Golden Gate, where the present maximum depth of water is 69 fathoms, between Fort Point and Lime Point. The position of this ancient river in Raccoon Strait suggests that it w^as a superimposed stream whose course had been determined when the region was 26 covered with soft Pliocene formations, which have since been carried away by erosion. The waters of the bay that lie away from the main channel are comparatively shallow. From Raccoon Strait to San Pedro Point the main channel has a maximnm depth of 16 fathoms in the narrowest place, but the depth of the water in general in this part of the bay ranges from 4 to 8 fathoms. The channel through San Pablo Bay at most places beyond San Pedro Point does not exceed 4 fathoms in depth, but it deepens notably at Carquinez Strait, where it is constricted. The water in San Pablo Bay is shallow, its depth averaging perhaps 7 feet at low tide. In the area between San Francisco and Goat Island the maximum depth is 20 fathoms, and south of this area the deeper water channel has in general a depth diminish- ing from 10 fathoms to 6 fathoms, with rather shallow water on both sides. The fact that the deepest channel lies in the most constricted parts indicates that these depths are maintained by tidal scour and that the present deep-water channel can not be assumed to represent throughout its course the trench of the ancient river. Before the submergence of the valley now occupied by the bay the streams draining the surrounding hills flowed clown over more or less gravelly or sandy bottoms to the trunk drain- ageway and spread gravel and sand over the valley bottom. As subsidence proceeded these gravels and sands became buried by finer silts. This process continued by stages, so that beneath the floor of the valley and beneath the bay there is an alternation of sands or gravels with finer silts or clays, and some of these deposits contain marine fossils, which have been discovered bv borins;. This recurrent burial of fluviatile gravels and sands by fine silt or clay involved many changes in the courses of the stream channels as they ran out from the canyons over the floor of the valley, but each of these gravelly channels, wliatever its course at any stage of the infilling of the valley, remained connected as a strip of sand and gravel with the portion of the stream that lay above the zone of aggra- dation. Thus were established the conditions of an artesian 11 basin, the features of which will be further described under the headino-" Economic oeolo2;v." The Bay of San Francisco is evidently a submerged land valley, and the geomorphic features of its periphery and the valley itself were manifestly shaped by the ordinary agencies of erosion, but as tlie valley lies on the relatively depressed sides of the San Francisco-Marin and Montara crustal blocks it was probably in large part outlined by the movement that tilted these blocks, and to that extent it is of diastrophic origin. A second movement of depression appears to have affected the region as a whole and allowed the sea to enter the gorge at the Golden Gate, which had been cut by stream erosion across the crest of the rising side of the northern block. That the earth movements in this region were not simple is shown by the fact that on the southeast side of San Pablo Bay and about the west end of Carquinez Strait there are wave-cut terraces and elevated deposits of marine shells of species that are still living ; whereas in the area south of San Pablo Bay there are no such terraces or elevated late Quaternary marine deposits. The evidences of uplift on San Pablo Bay appear on a third and quite distinct crustal block, represented by the Berkeley Hills, and the only place where this block touches the Bay of San Francisco is at San Pablo Bay. The Hay- wards fauk, which skirts the western flank of the Berkeley Hills, is in the zone of dislocation between this block and the block on whose depressed side lies the greater part of the bay. Outside of the Golden Gate, extending out to the Farallon Islands, there is a broad submerged embankment, which lies beneath an area of very shallow water. (See fig. 2, p. 10.) This embankment probably in part represents the delta of the ancient river that once flowed through the Golden Gate before the depression, but it has been also in 2:)art built up by deposits of fine silt, which in the flood season are carried through the Bay of San Francisco and dropped outside the Gate. 28 GEOLOGY." STRATIGRAPHY AND AREAL GEOLOGY. GEOLOGIC FORMATIONS OF THE MIDDLE COAST RANGES. The middle Coast E,anges of California, within which lie the quadrangles described in this folio, are composed of many dif- ferent kinds of rock, both igneous and sedimentary. The geo- logic history of the region is varied, including records of depo- sition, erosion, diastrophism, and volcanic eruptions, and the geologic structure is correspondingly complex and interesting. The formations comprising the sedimentary rocks are graph- ically represented in the columnar section on the back of the Tamalpais topographic map (in pocket). The oldest known rocks are certain quartzites, limestones, and crystalline schists, which are best exposed in the Santa Cruz, Santa Lucia, and Gabilan ranges. The age of these rocks is not yet known, but some of them are probably early Mesozoic and some are possibly Paleozoic. These older rocks are intruded by the granitic and dioritic rocks of the ranges just mentioned and their extensions north- ward through Montara Mountain, the Farallon Islands, and the Point Peyes Peninsula as far as Bodega Head. Upon the eroded surface of the complex of plutonic and metamorphic rocks rests the Franciscan group, composed chiefly of sandstones, radiolarian chert, foraminiferal limestone, and lavas, associated with which are intrusive masses of spheroidal "The geologic mapping of the area covered by this foUo has afforded an opportunity for training in field geology many students in the University of California, who have contributed observations that are recorded in the text. The list of these contributors is long, however, and the same ground has been worked over by different students, so that it is impracticable to make individual acknowledgments for the aid rendered. The nominal author of the folio is familiar with all parts of the field and assumes responsibility for the correctness of the observations made and conclusions reached in the work done under his direction, but he gratefully acknowledges his ol)liga- tions to all who have aided in this work. In the study of the fossiliferous formations the author has had the active cooperation of Prof. J. C. Merriam, whose contributions, with those of his students, have been indispensable in unraveling the intricate geology of the Concord quadrangle All the fossils named in the lists here published have been identified by Prof. Merriam. 29 basalt and serpentinized peridotite. These rocks are widely dis- tri])iited in the middle Coast Ranges, oceuiTing notably in the Mount Hamilton and Mount Diablo ranges, about the Bay of San Francisco, and in areas north of the bay. Upon the Franciscan group the Shasta series (Lower Creta- ceous) rests in unconformable relation, and upon this group lies the Chico formation (Upper Cretaceous). These Cretaceous formations were once coextensive with the territory now occu- pied by the present Coast Ranges, and although removed by erosion over large areas where the Franciscan and older rocks now appear at the surface, they still constitute one of the largest elements in the stratigraphy of the region. They are composed chiefly of shales and sandstones and in the ranges north of the bay have a measured thickness of between 5 and 6 miles. The Eocene rocks, which succeed the Chico, are much less widely distributed. They comprise two assemblages of sandstones and shales known as the Martinez and the Tejon formations, which near the Bay of San Francisco aggregate between 4000 and 5000 feet in thickness and in the area farther south are prob- ably much thicker. Evidences of unconformity between the Eocene and Cretaceous rocks have been observed in some places, but the discordance is not very pronounced. The fossil faunas of the two series are, however, very different. Some strata referable to the Oligocene series have been observed and recorded, but the next great group of rocks is of Miocene age and is known as the Monterey group. The for- mations of this group have a wider distribution south of the Bay of San Francisco than those of Eocene age, and in some places they rest directly upon Cretaceous or older rocks, neither the Martinez nor the Tejon intervening. The most character- istic feature of the group is its content of bituminous shale, with which nearly all the oil of California is directly or indi- rectly associated. These shales alternate with sandstones, and the basal formation of the group is at many places conglom- eratic. The group attains a thickness of several thousand feet, and in areas where it rests upon the Eocene the superposition is unconformable. The rocks of this group were doubtless originally deposited over the greater part of the area of the \0 Coast Ranges from the Bay of San Francisco southward, but since their deformation and uplift they have been extensively eroded. Their remnants, however, form a considerable element of the stratigraphy of the region. The next overlying formation, the San Pablo, is unconform- able with the Monterey group and is much less widely distrib- uted. In its southern areas the discordance is strongly marked, but in some of the northern areas it is scarcely discernible. The rocks, which are chiefly marine sandstones that are locally intermixed with tuffs, are found on both flanks of the Coast Ranges. Their thickness in the best-known sections ranges from 1500 to 2000 feet. Above the San Pablo unconformably, but in few places resting directly upon it, lies the Merced formation, a thick accumulation of marine sandstones, clays, and conglomerates, which were laid down in Pliocene time in deep local troughs that sank as fost as the sediments were deposited. These basins of Pliocene marine deposits were apparently confined to the coastal side of the Coast Range region. On the inland side of that region similar geosynclinal troughs were devel- oped to corresponding depths, in which accumulated fluviatile and lacustral sediments, constituting the Orinda" formation. The Orinda and Merced formations are each more than a mile thick. Interstratified with the beds of both formations are layers of volcanic ash. Upon the Orinda and Merced lie various lavas and volcanic tuffs alternating with lacustral clays, limestones, and sand- stones. Of these lacustral formations, the Siesta" (Pliocene) and the Campus" (Pleistocene) are the most extensive. The later Quaternary formations comprise marine shell beds, sands, and clays overlain by a thick deposit of alluvium that is ricli in the bones of extinct Mammalia. "In this folio the names Orindan, Siestan, and Cauipan have been changed to Orinda, Siesta, and Campus. Tiie names orio:inally eiuj^loyed have been in use for more than 13 years antl tlie autlior prefers them. 31 PKE-FRANCISCAN ROCKS. CHARACTER AND DISTRIBUTION. Tbe oldest rocks of that part of tlie Coast Ranges wliicli is here especially considered comprise the quartz diorite ("Mon- tara granite") and some fragments of the formations into which that rock was intruded as a batholithic mass. In the southern Coast Ranges these pregranitic formations constitute a large volume of strata, of unknown age, made up chiefly of crystalline limestones, quartzites, and schists. They are exposed on the flanks of the Montara batholith in Santa Cruz County (see Santa Cruz folio, No. 163) and on the Point Reyes Peninsula in Marin County, according to F. M. Anderson." GAVILAN LIMESTONE. Only one of the pregranitic formations, the Gavilan lime- stone,'' which takes its name from the Gabilan" Range, between San Benito and Salinas valleys, is represented in the area here described. It occurs only in isolated masses included in the quartz diorite of Montara Mountain, in the southern part of the San Mateo quadrangle. This limestone is a coarsely crys- talline marble in which the calcite crystals show striations due to multiple twinning. Besides the carbonate of lime of which it is chiefly composed it generally contains a considerable pro- portion of magnesia, some silica, and some carbon in the form of lustrous flakes of graphite. The silica may occur in the form of silicates, such as wollastonite, which is abundant in the same rock on the Point Reyes Peninsula. QUARTZ DIORITE ("MONTARA GRANITE"). The so-called "Montara granite" is a coarse-grained gray rock made up of quartz in abundance, plagioclase, orthoclase, and biotite or hornblende. One common facies of the mass con- tains both biotite and hornblende. In an earlier paper the "California Univ. Dept. Geology Bull., vol. 2, No. 5. * Becker, G. F., U. S. Geol. Survey Mon. 13, p. 181, 1888. "This name, Spanish for sparrow hawk, is spelled Gavilan in modern orthography and in the geologic literature of California. The United States Geographic Board, however, has decided in favor of the older form Gabilan (pronounced Gah-vee-lahn) for the mountain range. 32 writer stated that the rock is a hornblende-biotite granite, but subsequent examinations have shown that a facies of the mass which contains no liorn blende but includes biotite is perhaps more widespread than that in which hornblende occurs either in association with biotite or alone. It was found also that by an increase in the proportion of plagioclase the rock at many places passes into quartz diorite, so that this designation is petrographically more correct for the mass as a whole, and the term granite is justified only by popular usage. Titanite and apatite are common accessories and titanite is locally abundant. The rock is more or less deeply weathered so that it is disintegrated in almost all its exposures except those on the shore and in the deeper canyons, where corrasion is rapid. The mass is at some places characterized by blotchlike inclu- sions of a dark, more basic rock and contains small dikes of aplite and pegmatite, two rocks that are in places intimately associated and locally include small crystals of garnet and magnetite. The rock shows occasionally a foliated or gneissic structure, which is clearly due to deformation, and evidences of deformation appear in the microscopic structure of the rock even where no foliation is apparent. The quartz diorite makes up the bulk of Montara Mountain, a bold ridge nearly 2000 feet high, which extends from north- west to southeast across the southwest corner of the San Mateo quadrangle. The ridge affords two complete and easily acces- sible transverse sections, one on the coast bet^veen San Pedro Point and Halfmoon Bay, the other on the road from Crystal Springs to Spanish Town, along Pilarcitos Canyon, just beyond the southern border of the quadrangle; and the rock is well exposed over the greater part of the surftice of the ridge, some of the slopes being mantled with a coarse arkose sand resulting from its disintegration. Along the northeastern slope of Montara Mountain the quartz diorite is flanked by sandstones, grits, shales, and con- glomerates of probably Eocene age. These lie directly on the worn surface of the quartz diorite and dip away from it to the northeast. At the northwest extremity of the mountain the sandstones and basal conolomerate mantle over the axis of the ridge and rest at low angles upon the quartz diorite, as may be easily seen on the coast road near the Devils Slide. At Pilarcitos Lake the plane of contact of the sandstones and quartz diorite has a lower angle than the slope of the moun- tain, so that an isolated outcrop of the underlying rock appears as an inlier in the sandstone at the south end of the lake. At places farther north these Eocene rocks appear to be faulted down against the rocks of the Franciscan group and in the earlier reports they were assigned to that group, although their difference from the normal type of Franciscan rocks was pointed out and the desirability of segregating them was suggested. In the area southeast of Pilarcitos Lake the quartz diorite is bounded by a fault that brings the Franciscan against it. The southern limits of the quartz diorite lie beyond the San Mateo quadrangle and are mapped in the Santa Cruz folio. On the strip of land between the mountain slopes and Half- moon Bay and in an area lying farther north, between that bay and Seal Cove, the quartz diorite is covered by alluvium composed of its own debris, which has been spread out by alluvial fans formed by streams that run down the mountain slopes. Near Seal Cove the quartz diorite, as may be seen in the sea cliffs, forms the basement on which rests the littoral Merced formation. Here a Pliocene terrace cut in the quartz diorite and encumbered by beach material stands only a few feet above the present sea level. From Seal Cove to the Devils Slide the quartz diorite forms the shore of the Pacific. The quartz diorite area of Montara Mountain thus outlined is only a part of a much more extensive mass. It is in reality but an inlier of a granitic terrane which certainly extends from Santa Cruz to Bodega Head and which emerges from beueath the overlying mantle of Mesozoic and Tertiary sediments in the Santa Cruz Mountains, Montara Mountain, the Farallones Islands, the Point Reyes Peninsula, and Bodega Head. This great batholith has an extent from northwest to southeast of not less than 120 miles and may even be regarded as the continuation of the similar rocks of the Gabilan Range. The mountain mass of which this was the core was truncated by erosion, the overlying stratified rocks of the crust into which San Francisco— 3 34 it was intruded were removed except some remnants on its flanks, and a large part of the quartz diorite itself was worn away prior to the submergence which permitted the deposition of the sediments of the Franciscan group. JURASSIC (?) ROCKS. FRAXCISCAX GROUP AND ASSOCIATED IGNEOUS ROCKS. CHARACTER. The Franciscan group was named from San Francisco, where it occurs in extensive exposures, from which it was first described. It comprises (1) a vokimiuous accumulation of sedimentary for- mations, some of them clearly marine, others doubtfully so; (2) some intercalated lavas of contemporary age; and (3) cer- tain crystalline schists produced by the metamorphism of both the sedimentary and the igneous rocks. The formations of the Franciscan group are pierced at many points by igneous intrusives, which are so intimately associated with the sedimentary rocks, both as to age and as to distribu- tion, that they constitute one of the most characteristic features of the group and they will therefore be briefly described in connection with the Franciscan rocks. This treatment is also desirable because these intrusives produced the metamorphism that formed the crystalline schists and so gave to the Fran- ciscan group one of its most interesting features. The sedimentary rocks of the group comprise (1) sandstones, conglomerates, and shales; (2) limestone; and (3) radiolarian cherts. The igneous rocks are (1) basalt or diabase, in many places having a strongly pronounced spheroidal or ellipsoidal structure, (2) peridotites, which have in general become thor- oughly serpen tinized. The dominant rock in the cr\^stalline schists is glaucophane schist, which is so abundant in them that the schists as a whole are commonly referred to as " the glaucophane schists," although other varieties of crystalline schist are associated with them. STRATIGIIAPHY. Owing to the general absence of fossils from the sandstones of the Franciscan group and to the similarity of these rocks at S5 several horizons, as well as to the general obscurity of their planes of stratification, only the broader features of the stratig- raphy of the group can be determined; but although these sandstones are not susceptible of subdivision and correlation by means of their petrographic character or their faunal eon- tent, two radiolarian chert formations that occur in them constitute well-defined and easily recognizable stratigraphic horizons that traverse the group. As neither of these cherts forms the base or the summit of the group they serve to divide the Franciscan sandstones into three distinct formations, which, with the cherts themselves, constitute the five formations of the group. Very few sections of the Franciscan rocks, how- ever, include all five of these formations. Some sections com- prise only the lower part of the group, others only the upper part, but as the sequence is constant these partial sections may be combined to construct a complete stratigraphic column. The formations which by this mode of division constitute the Franciscan group are as follows, the series beginning at tlie top: Bonita sandstone. Ingleside chert (radiolarian chert). Marin sandstone. Sausalito chert (radiolarian chert). Cahil sandstone (including Calera limestone member and some volcanic rocks). CAHIL, MARIN, AND BONITA SANDSTONES. Character. — The Cahil sandstone is named from Cahil Ridge, in the San Mateo quadrangle; the Marin from Marin Penin- sula, in Marin County; and the Bonita from Bonita Point, on the north side of the Golden Gate. These three formations are petrographically very much alike and may therefore be described together. The prevailing rock in all three formations is a massive, obscurely bedded sandstone of dark greenish-gray color and medium texture. Where it is fresh, or unweathered, it is so strongly cemented that when it is broken the fracture traverses its constituent grains. The cementation has involved a considerable amount of recrystallization of the finer interstitial 36 material of the rock, but the larger sand grains appear to have been little affected by this secondary crystallization of the matrix in which they are embedded. The sand of which it is composed was not well washed and sorted at the time of its deposition and comprises several other minerals besides quartz, which is, of course, its principal constituent. When exam- ined under the microscope it is seen to contain more abundant fragments of plagioclase, orthoclase, biotite, hornblende, and zircon than are usually found in rocks of this class. Besides these minerals, it contains pieces of chert, volcanic rock, and schist. This heterogeneity of composition and the prevailing angularity of the grains suggest that the rock may be an arkose rather than a beach-washed sand. It includes flakes of black carbonaceous matter, which in some places are so abundant as to o'ive the rock a rude cleavage. This carbonaceous material indicates that the sands w^ere deposited in places to which were carried also the remains of vegetation. In certain localities, too, the sandstone includes thin seams of coal, and the carbon- aceous fragments are exceptionally abundant near these seams. The coal is of no commercial value. The sandstones of these formations include lenses of pebbly conglomerate and beds of dark shale, but most of these are difficult to trace for more than short distances and they form beds that are thin in comparison with the general mass of the sandstones. Where erosion is not exceptionally active the sandstones of these formations, by their secular decay and disintegration, yield an abundant soil. Wherever it may be examined in cliff sec- tions or in cuttings the sandstone beneath the soil presents a more or less shattered appearance, being traversed in all directions by intei*secting parting planes. Some of these part- ings are joints or planes of differential movement, but most of tliem show no indications w'hatever of movement or of shat- tering action. Nearly all of these partings have been formed by weathering, and their development seems to be due to the slow secular disintegration of the sandstone. They are more numerous near the surface, and they have divided the sand- stone immediately beneath the soil into small angular pieces, 37 wliich, after they are loosened by surface agencies, become incor- porated into the soil itself. The sharply marked alternation of wet and dry seasons and the general absence of trees in this region are peculiarly favorable to this disintegration. In steep canyons and on tlie crests of ridges these massive sandstones are usually well exposed, but on the intervening- slopes they appear only in isolated knobs that protrude above the generally smooth surface, so that the distribution of the formations must be to a large extent determined by examining the rock fragments in the soil. The weathered rock is com- monly of a tawny-yellow color, due to surface oxidation, and this discoloration ordinarily extends below the zone of mechan- ical disintegration mentioned above. Although these sandstones are prevailingly massive, signifi- cant glimpses of their bedding obtained at many places show that they are normally stratified. The bedding in these places is usually made apparent by intercalated beds of shale rather than by any notable differences from horizon to horizon in the character of the sandstones themselves. The sandstones at certain horizons might even be described as thin bedded, but where this thin bedding occurs the mantle of soil is heavy and the exposures of the rock are few and small. No positive indi- cation of dynamic metamorphism has been observed in these for- mations, but where they lie near certain intrusive rocks they have been aifected by a peculiar kind of contact metamorphism, wdiich has produced the rocks that are described farther along in this text under the heading " Metamorphic schists." In the Caliil formation there is a conspicuous foraminiferal limestone, an oceanic deposit, laid down far from the shore, which separates the sandstones below" and above it into distinct divisions. The attempt to indicate this separation in the map- ping was, however, not entirely successful, and on the geologic maps the limestone is therefore treated as a member of the Cahil sandstone, under the name Calera limestone member. On the summit of Fifield Ilidge, in the San Mateo quadrangle, a thin stratum of obscurely fossiliferous impure limestone appears as a lens in the Cahil sandstone at a horizon several hundred feet above the Calera limestone. It has been observed 88 at only a few points and is apparently not persistent. This bed probably nowhere exceeds 10 feet in thickness. It does not resemble the Calera limestone -and conld not easily be con- founded with it. Owino- to the obscurity of the stratification the thickness of the Cahil, Marin, and Bonita sandstones can not be determined Avith precision. The Cahil, which is approximately 2560 feet thick, is made up of about 60 feet of limestone (Calera lime- stone member) with 500 feet of sandstone below and 2000 feet of sandstone above. The Marin sandstone in its best expo- sure on the Marin Peninsula is about 1000 feet tliick and the Bonita is about 1400 feet thick, though this estimate is less certain, as tlie exposures are obscure. These three formations occur principally on the San Fraur- cisco and Marin peninsulas, but it is only where they are immediately associated with the Sausalito and Ingleside cherts that they can be distinguished from one another and so be assigned to their proper stratigraphic positions in the geologic sequence. All three formations are remarkably deficient in fossils, yielding only a few isolated forms or fragments of forms, which are of no value in determining the age of the rocks. Calera limestone member of Cahil sandstone. — The Calera limestone is so named from Calera Valley, in the San JNIateo quadrangle, where it is well exposed on the sea cliffs at the lower end of the valley. It is a gray compact rock of aphan- itic texture, resembling lithographic limestone. Its weathered surface is much lighter in color than its freshly broken faces. Smooth surfaces or thin sections of the rock show a great many clear, hyaline spots, the largest measuring half a millimeter across. These spots are the remains of foraminiferal shells. Under the microscope the limestone appears to be uniformly dense and structureless and is composed of a cryptocrystalline aggregate of calcite. The individuals of the aggregate are so small that one can not be discriminated from another. The character of the limestone and the distribution of the forami- niferal shells through it indicate that it is essentially a chem- ical precipitate in which the Foraminifera were sporadically 39 entombed. Minute veinlets of calcite traverse the rock in all directions. The limestone is more or less distinctly stratified and con- tains lenses of chert, which generally lie parallel to the planes of stratification. (See Pi. III.) These lenses are rather irreg- ular in form and the longer axes of some of them are oblique to the bedding. The chert consists of nearly pure silica and may be either dark or light colored. The lenses range in thickness from about an inch to about a foot and have ordi- narily rather abrupt or obtusely rounded terminations. Some of them appear to have very much the same relation to the limestone that the flint nodules and lenses in the chalk of the south of England have to the chalk. The limestone itself contains very little silica, even in the immediate vicinity of the chert lenses, and although it is tra- versed by numerous small veins these are composed of calcite. The chert lenses are easily separable from the limestone, which is a very pure carbonate of lime with but little admixture of magnesium carbonate, as may be seen from the following analyses, made by Wm. L. Lawsou, of samples of the limestone from Permanente Canyon, San Mateo County, where it is quarried for use in the manufacture of beet sugar. Analyses of limestone from Permanente Canyon, San Mateo County, Cal. 1 2 CaO _ ._ 54.44 .20 42.93 .56 .10 1.98 .05 Trace. Trace. 54.84 MgO .14 CO, AI2O3+ FOoOa .___ 43.23 .47 Soluble SiOa Insoluble ......... HoO . 1.52 .15 SO3 _. Trace. P,06 Trace. 100. 25 100. 35 40 The Calera limestone lies in a belt of discontinuous outcrops that extend from northwest to southeast across the southern part of the San Mateo quadrangle. The thickness of the lime- stone in these outcrops varies greatl}^ but averages about 60 feet. SAUSALITO AND INGLESIDE CHERTS. General character. — These formations consist of radiolarian chert and are so much alike that one description will serve for both. The Sausalito chert, the lower of the two formations, is named from the town of Sausalito, in the Marin Peninsula, near which it is extensively exposed; the Ingleside chert tiikes its name from Ingleside, in the San Francisco Peninsula. The rocks of w^hich these two formations are composed are the most remarkable of the Franciscan group. They are neither so thick nor so persistent as the sandstones, but their great hardness and their resistance to weathering make them the best-exposed formations of .the group, and they constitute the most rugged features of the relief. The interest which these rocks excite by their bold outcrops is intensified by the most cursory inspec- tion of their structural and petrographic features. In color they are prevailingly dull brownish red, especially in their thicker and more evenly bedded portions, but they include some rock that is yellow and green. White, colorless, blue, purplish, gray, brown, and black cherts are also seen, but these colors are usually local and are not characteristic of extensive masses of the rock. Where the cherts have been subjected to heat, as at their contact with irruptive rocks, they are a brilliant vermilion-red. One of the most remarkable features of the cherts is their bedding, wdiich is well displayed in numerous excellent sec- tions, not only ^ natural outcrops but even better in many rock cuttings that have been made in the hills of San Francisco for extending and grading streets and for procuring road metal. All these exposures show a strikingly constant form of bedding the essential feature of which is the alternation of thin sheets of chert with partings of shale. (See PI. VIII.) The shale and the chert are usually of about the same color. The thick- ness of the sheets of chert in the typical sections generally ranges from about 1 to 3 or 4 inches, averaging perhaps 2 or 3 41 inches. Some beds are much thicker, but the sections of the chert nevertheless in general show thin and even bedding. The shaly partings between these sheets usually range from about one-eighth to one-half inch in thickness, but many of them are mere films. As the formations are in some places exposed in sections that are several hundred feet thick they present the remarkable phenomenon of an alternation of thou- sands of layei-s of chert with as many layers of shale. In the common red phase of the formations the regularity of this thin-sheeted stratification is amazing. In other phases, in which red iron oxide is not so abundant, the regularity is nnich less marked and the sheets assume lenticular forms. In these less ferruginous phases the chert beds reach their maximum thickness and the shaly partings their minimum. In these phases the chert beds may not be separated by shale and may be several feet thick, so that the formations may locally present a massive aspect having litttle resemblance to the prevailing thin-bedded facies. There are also gradations from the massive to the thin-bedded variety. Petrographic features. — The radiolarian cherts are not petro- graphically uniform. In many places they are true jaspers and have been so designated in some of the earlier descriptions of them. In other places the silica of which they are composed is chiefly amorphous and the rocks resemble flint or hornstone. In still other places the proportion of iron oxide and other pig- ment they contain is so large that they are not cherty but are so soft that they can be easily scratched with a knife. In a few phases, notably those that contain no coloring matter, they become a quartz rock that is not flinty or jaspery and that does not differ essentially from vein quartz. This phase, however, is exceptional and is probably due to local causes. If a selected series of thin sections of these cherts is viewed under the microscope they present a gradation from those that are composed almost wholly of amorphous or isotropic silica to those that are a holocrystalline aggregate of quartz granules. The most isotropic sections, however, exhibit numerous minute scattered points that polarize light and that can not be sharply separated, even by the highest powers, from the isotropic base. These points are not inclusions; they are centers of incipient 42 crystallization in the amorphous rock, corresponding to the prod- ucts of devitrification in glass. In other sections these centers of crystallization are much more thickly crowded and well-defined areas composed of interlocking granules of quartz appear, inter- locking also with the isotropic base. The actual boundaries of these areas can be made out only with difficulty and uncer- tainty, owing to the fact that the quartz grains are under molec- ular strain, which produces undulatory extinction as the stage is revolved between crossed nicols. In still other sections these areas coalesce and the proportion of amorphous base to the whole rock is very small. Finally, some sections show a holo- crystalline aggregate of interlocking quartz grains. Most of the srains are under molecular strain, as is shown bv undula- tory extinction, and somewhat resemble chalcedony. The dis- crimination between the amorphous and the crystalline silica is easy in those varieties of the rock that contain little iron-ore pig- ment but becomes more difficult as the abundance of the obscur- ing pigment increases. The gradation thus observed in a series of thin sections prepared from specimens taken at random seems clearly to be a gradation in time and not merely a grada- tion in space. It indicates different stages of a process of crys- tallization in a solid amorphous mass. If this be granted, there seems to be no good ground for doubting that in general the holoerystalline cherts, or jaspers, were originally amorphous silica, and that they have reached their present form by a pro- cess of crystallization quite analogous to that of devitrification in volcanic rocks. In addition to these general petrographic features the radio- larian cherts present a few features that are of subordinate or local interest. The oxide of manganese seems by paragenesis to be connected with the cherts. This oxide is not usually found in masses but appears only as films and stains in the crevices of the chert and along its bedding planes. In some places the stain is so abundant that the body of the rock is locally blackened. In the chert at Red Rock, in the Bay of San Francisco, a tliick deposit of oxide in the form of psilome- lane lies in the bedding planes, which ai-e here nearly vertical. (Jther similar deposits occur in these cherts in the Coast Ranges. 43 So far as the writer is aware the manganese ore is confined to these cherts. Another interesting feature of these rocks is their passage locally into an iron ore. Still another character- istic feature, which, however, varies in the degree of its devel- opment, is the system of minute fissures that traverse the hard beds transverse to the bedding planes. Many of these fissures are fault planes along which have occurred tiny dislocations that are apparent in the steps which mark the otherwise even surfaces that form the upper and lower limits of the chert beds. Some of these fissures are lines of very evident veining, and doubtless all of them Avould prove to be veins if examined in thin section. The vein matter is quartz, which is usually white or hyaline, whereas the chert is colored. None of these fissures, faults, or veins pass tlirough the shaly partings, the plasticity of which has prevented their development, so that each thin sheet of chert has its own system of fissures. Fossils. — On a smooth surface of almost any specimen of these cherts a lens will reveal minute round or oval dark, hyaline, or wliitish dots. These dots, which are scattered through the rock, prove on microscopic examination to be the remains of Radiolaria, the characteristic fossils of these forma- tions. The Radiolaria are minute animals that thrive in sea water and secrete siliceous skeletons of very complex structure. These skeletons evidently accumulated in great numbers on the floor of the sea while the radiolarian cherts were being deposited and thus contributed to their formation. As a rule they are sporadically embedded in the siliceous matrix above described, but in some places they are so closely crowded as to constitute the greater part of the chert. Where the Radiolaria are scantily distributed through the chert it is uncertain whether or not the matrix also is derived from these organ- isms, and the alternative hypothesis that it was formed by the purely chemical precipitation of silica, supplied possibly by submarine springs, is wortliy of consideration. If the silica is wholly of organic origin it must have been dissolved and repreci pita ted in its present form as an ooze on the sea bottom. Under the microscope the radiolarian remains appear in ordinary light as circular or oval spaces or as clear rings free 44 from pigment. Between crossed nicols these clear spaces are seen to be occupied by chalcedony. The clear areas are more sharply defined in the amorphous vaiieties of chert and they are somewhat indefinite in outline, yet distinct as areas, in the holocrystalline varieties. In thin section they are most readily observed in the red cherts, by reason of the contrast which they make with the pigmented matrix. In the better preserved remains of these organisms the spines, lattice work, and other structural features may be observed and the genera to which they belong thus determined. Specimens of radiolarian cherts from the Franciscan group and their fossils were described many years ago by Dr. George J. Hinde." Distribution and thickness. — The Sausalito and Ingleside cherts are most extensively displayed in the hills in the north- ern part of the San Francisco Peninsula and in the southern part of the Marin Peninsula, but smaller areas of these cherts and of other cherts of undefined horizons are common through- out the Franciscan terrane. The maximum thickness of the Sausalito chert is about 900 feet and that of the Ingleside chert about 530 feet. The absence or paucity of land-derived sediment in both formations indicates that they were deposited far from the shore, in deep water. CONDITIONS OF SEDIMENTATION. The basal part of the Cahil sandstone was deposited upon the sinking bottom of a transgressing sea. When the sub- sidence had proceeded so far that the land-derived sediments failed to reach the deeper water, detrital accumulation gave way to the formation of foraminiferal ooze on the bottom of a clear- water sea, the shore having migrated far to the east. The product of this period of deposition is the Calera limestone member. An upw^ard movement of the sea bottom caused the shore line to move westward again, the water became too shal- low for foraminiferal life, and the layer of ooze on the sea bottom was buried under the detrital sediments which form "Hinde, Gr. J., Note on the radiolarian chert from Angel Island and from Buriburi Ridge: California Univ. Dept. Geology Bull., vol. 1, No. 7, pp. 285-240, 1894. (Appendix to paper by F. L. Ransonie on the geology of Angel Island.) 45 the upper part of the Cahil sandstone. As these sediments are very thick the sea bottom must have continued to subside dur- ing the period of their deposition. As subsidence proceeded the detrital material washed from the receding shore again failed to reach the region, and organic agencies once more resumed sway. This time, however, calcareous organisms were replaced by those which secrete silica from the sea water, so that the sea bottom was covered with radiolarian ooze, which eventually consolidated as the Sausalito chert. The rhyth- mical oscillation of conditions which produced the remarkable alternation of layers of chert and shale in this formation has not yet been explained but was probably due to alternating conditions in the sea water which affected or interrupted the swarming of radiolarian life. The accumulation of this radio- larian ooze was stopped by a recurrence of the shallowing of the sea and the return of the shore to a line sufficiently near to insure the deposition of sands upon the siliceous deposits. These sands now form the Marin sandstone. After this sand- stone had accumulated to a thickness of about 1000 feet the subsidence of the sea bottom, which had been in progress dur- ing the period of its deposition, caused the shore again to retreat so far that the conditions became once more favorable to marine life, and another deposit of radiolarian ooze was laid down. This ooze formed the Ingleside chert. Again uplift set in, shallow water prevailed, the shore was close at hand, and the sands of the Bonita sandstone were deposited. As these sediments attain a thickness of 1400 feet we must assume that the uplift that started their deposition was followed by gradual depression, which continued during their accumulation. Thus the mere consideration of the character of the forma- tions listed in the fivefold subdivision of the Franciscan group, taken with their sequence in the geologic column, leads to the recognition of a remarkable series of vertical oscillations of the sea bottom and of a consequent series of horizontal migrations of the sea shore during the time occupied by the deposition of the sediments of the group. It may be well to note, however, that in the above outline of the conditions of sedimentation that existed during the deposi- 46 tion of the Franciscan formations it is assumed that the Cahil, Marin, and Bonita sandstones are wholly marine. The char- acter of these formations suggests, as has been indicated in the description of them, that they may be in part at least non- marine or continental deposits. If this should prove to be the fact the record of oscillation and the consequent migration of the continental margin to and fro must have been even more complicated than if they were marine. At many places and at several geologic horizons in the sand- stones of the Franciscan group there are small isolated areas of radiolarian chert that can not be referred to either the Sausalito or the Ingleside formation. Such areas are particularly note- worthy on Buriburi Ridge, in the San Mateo quadrangle. Most of these isolated patches of radiolarian chert are residuals of somewhat larger masses, but many of them appear to have been originally discrete lenses, which indicates that during the deposition of the sandstones of the Franciscan group the condi- tions were from time to time locally favorable to the accumula- tion of radiolarian ooze. CONTEMPORANEOUS VOLCANIC ROCKS. The volcanic lavas which are interstratified with the sedi- mentary deposits of the Franciscan group are basaltic rocks, which are generally vesicular or amygdaloidal and which in most of their outcrops are greatly decomposed. Where thus decomposed they are as a rule not essentially different from other basaltic rocks that are clearly intrusive in the Franciscan. The masses of intrusive rock, however, cut boldly and iri'egu- larly across the stratification of the sedimentary beds, whereas tlie volcanic rocks occur in sheets of moderate thickness that conform to the planes of stratification and are in some places associated with pyroclastic rocks. These distinctly interbedded lavas are much more common in the lower sandstone forma- tions than in higher portions of the group. The most note- worthy sheets of volcanic rock are on the fianks of Cahil and Sawyer ridges, in the San Mateo quadrangle. They are mapped with the intrusive basalt and diabase. IIS'TRUSIVE ROCKS ASSOCIATED WITH THE FRAXCISCAN GROUP. The greater part of the igneous rocks associated with the Franciscan group are not of contemporaneous origin with it but have been intruded into it and are therefore not properly a part of the group. Tliere are several distinct types of these intrusives, two of which are dominant. These are (1) genetic- ally allied peridotites/pyroxenites, and gabbros, the first named preponderating and being generally very thoroughly serpen- tinized; and (2) spheroidal and variolitic basalts and diabases. Peridolite and serpentine. — The intrusive rocks of the first class may be referred to generally as peridotite and serpentine, for although tliey are associated witli pyroxenites and gabbros these are very subordinate in amount and are regarded as dif- ferentiation products of the same magma. These rocks form dikes and laccolithic sills as well as irregular masses, most of which appear to be the eroded remnants of intrusive lenses. Some writers on Coast Kange geology have erroneously sup- posed that these bodies of serpentine are products of the meta- morphism of the sandstone, an idea that still finds occasional expression, although it has been entirely disproved. The ser- pentine in its several variations has the usual character of that rock. Much of it is massive and unsheared, but in places it shows abundant evidence of pronounced internal movement, being composed of slickensided, sheared, and schistified serpen- tine in which lie rounded masses of unsheared rock. The massive varieties contain crystals of enstatite that show lustrous cleavage faces, and this fact and the results of microscopic study indicate that the serpentine w'as derived from a peridotite hav- ing the mineralogic character of saxonite or Iherzolite. These massive varieties of the ser^^entine are also usually traversed by numerous small veins of chrysotile, but none of these veins are large enough to constitute marketable asbestos. Small seams of magnesite also appear here and there in tlie serpentine. Bodies of glaucophane and other crystalline schists lie near some of the masses of serpentine but are too small for repre- sentation on the geologic maps. Perhaps the largest areas of serpentinized peridotite lie in the belt that extends across the city of San Francisco from 48 Hunter Point to Fort Point. This belt apparently includes two distinct sheets of serpentine, which are intrusive in the Franciscan group and are separated by a moderate thickness of Franciscan rocks, the twofold character of the intrusion being very evident in the excellent exposures at Hunter Point, the Potrero, and Fort Point. These sheets lie nearly flat but are in places so warped that they are more oY less discordant with the stratification planes of the Franciscan rocks. The intruded rocks at Fort Point are seamed with veins containing magnesite, datolite, pectolite, and other minerals but apparently have not been altered by contact metamorphism except for a very few inches from the contact surface. Near the serpentine, however, both at Fort Point and at Hunter Point, small loose frag- ments of glaucophane schist have been found. In this belt of serpentine, particularly at Hunter Point, there are stringers and nests of chromite, irregular masses of chlorite several feet across, and small veins of talc. The serpentine is nearly every- where thoroughly sheared by movements that have been set up within its mass, probably in connection with the gradual and irregular increase of volume attendant upon serpen tinization. This sheared mass, which is traversed in all directions by slick- ensided surfaces, contains numerous subangular and rounded bowlders as well as much larger masses of unsheared serpentine, most of which are traversed superficially hj a roughly rectang- ular network of minute veins of chrysotile. The sheared ser- pentine of the Potrero and of Hunter Point incloses many rather short, thick lenses of fresh fine-grained diabase, wdiich are evidently dislocated fragments of dikes that were intruded into the peridotite before its serpentinization and which by this process became sheared apart into discrete lenses. The intru- sive character of these diabase lenses is shown by the fact that tlieir edges that lie against the serpentine liave been chilled and are fine grained and grade into a coarser-textured rock toward the centers of the lenses. Another belt of intrusive peridotite lies along Buriburi and Pulgas ridges, on tlie northeast side of Crystal Springs Lake, in the San JNfateo quadrangle, where it is also thoroughly ser- pentinized. It occurs to a minor extent in the form of dikes 49 but chiefly in the form of lenticular flat-lying laccolithic sheets, froin which the overlying formations have been to a large extent removed by erosion. On the southwest side of Fifield Kidge there is a zone of outcrops of sheets and dikes of ser- pentine which is smaller than that on Buriburi Ridge but lies parallel to it. On the Marin Peninsula there are two well-defined belts of serpentinized peridotite. One of these extends northwestward from a point near the west peak of Mount Tamalpais be3-ond the northern limit of the Tamalpais quadrangle. The serpen- tine here occurs in the form of a rather thin warped sheet, which has been partly exposed by the removal of the overlying formations, into which it is intrusive, and has been so dissected that it is represented by several separate areas. The second belt traverses the Tiburon Peninsula longitudinally and is also in the form of intrusive sheets and dikes, with glaucophane schists at the contact zones, one of the most notable of which is that on Angel Island, described by F. L. Ransome." Still another belt of serpentine lies on the east side of the Bay of San Francisco, near the edge of the Franciscan group, wdiere it passes beneath the overlying Knoxville formation. This belt extends through the San Francisco, Concord, and Haywards quadrangles. Glaucophane schists are intimately associated with these masses of serpentine, particularly at the north end of the l)elr, and the relations of the two rocks indi- cate that the schists are due to metamorphism at the contact with the peridotites. Gabbros and pyroxenites occur as local facies of the serpentinized peridotites. Although the perido- tites from which the serpentines are derived are clearly mi vu- sive in the Franciscan group they appear to be conlin o that group; they do not traverse the overlying Cretacious formation. They were, however, w^ell exposed by denudation before the overlying Knoxville formation was deposited, for they formed part of the floor on wdiich that formation was laid down. In this way they occur in some places at the contact of the Franciscan with the overlying shales, and as both shales "Ransome, F. L., The geology of Angel Island: California Univ. Dept. Geology Bull., vol. 1, No. 7, pp. 193-233, 1904. San Francisco — 4 50 and serpentine have been deformed the relations of the rocks are in places extremely involved, suggesting contacts which really do not exist. Silica-carbonate rock. — Considerable parts of the serpentine have been altered to an aggregate of carbonate and silica, which weathers in very rugged masses that are strongly stained Avith limonite. This rock has been referred to in an earlier paper as silica-carbonate sinter, but more recent studies" have shown that the rock is in many places closely associated with, serpentine and in some places grades into it, so that there is little doubt that the so-called silica-carbonate sinter is a product of the alteration of the serpentine, the bases having been carbonatized and the silica set free to be redeposited in the form of chal- cedony, opal, and quartz. These forms of silica occur as a plexus of small veins traversing an aggregate of carbonates of iron, magnesia, and lime. Under the weather the carbonates are dissolved out at the surface, leaving the silica prominent, as well as rusty with iron oxide. The more notable masses of this rock are found north of Berkeley, in the valley of Temescal Creek, and at San Bruno Poiut, but many other smaller bodies may be seen elsewhere. Splieroidal basalt and diabase. — The second class of intru- sives invading the Franciscan rocks are the variolitic and spheroidal basalts and diabases. These are confined to the Franciscan group; they are not found in later formations. They antedate the peridotites, which at many places appear to cut across them. These masses of basalt and diabase occur at numerous points in the Franciscan group in all the quadrangles and thouo;h most of them are small, several are rather larce. They are of irregular shape, they include no clean-cut dikes or intrusive sills, and their exposed contacts with the rocks they intrude are generally irregular and jagged. (See PI. VI.) Fragments of the incasing rocks, especially of the radiolarian cherts, are abundant at the contacts, where the chert is usually baked to a bright vermilion-red and its structure is in some places also greatly changed. Some inclusions show evidence of "Knopf, Adolph, An alteration of Coast Range serpentine: California Univ. Dept. Geology Bull., vol. 4, No. 18, 1906. 51 partial resorption. The spheroidal structure of these intrusive rocks is clearly revealed only on sea cliifs, as at Hunter Point and Point Bonita (see PL VII), but may also be detected in numerous road cuttings and on natural exposures on hillsides. On sea-cliff exposures the rock presents the appearance of an irregular pile of filled sacks, each sack having its rotundity deformed by contact with its neighbor. The average dimen- sions of these sacklike or ellipsoidal masses are about 3 feet in the longest and about 1 foot in the shortest diameter. The rock between the ellipsoids is usually more decomposed than that elsewhere and so weathers out easily under the action of the waves, leaving the more resistant ellipsoids prominent. Some of the ellipsoids are vesicular, others are variolitic, and still others are both. The cause of this peculiar structure and the mode of its development are not yet understood. At some places, as on the cliffs at Hunter Point, these spheroidal basalts present also a remarkable brecciated appearance, as if made up of innumerable small, angular fragments cemented together. At both Point Bonita** and Hunter Point the mar- gins of such spheroidal masses show a pronounced variolitic struc- ture. At Hunter Point the rock is in places filled with closely packed spherical varioles ranging in size from one-sixteenth to one-half inch. These varioles weather out of the rock easily, being much harder than the matrix in which they are embedded. The intrusive rocks of Angel Island described by Ransome'' under the name fourchite are probably a fjicies of the rocks here grouped under the general designation "Spheroidal basalt and diabase." These spheroidal basalts occur wherever the rocks of the Franciscan group are exposed in this area, but most abundantly on the JMarin Peninsula, the largest masses being in the canyon of Lagunitas Creek northwest of Mount Tamalpais. "Ransome, F. L., The eruptive rocks of Point Bonita: California Univ. Dept. Geology Bull., vol. 1, No. 3, 1893. ''Ransome, F. L., The geology of Angel Island; California Univ. Dept. Geology Bull., vol. 1, No. 7, 1S94. 52 METAMORPHIC ROCKS. One of the most interesting features of the Franciscan group consists of crystalline schists formed by the local metamor- phism of its sedimentary and igneous rocks. These schists vary considerably in petrographic character from place to place and are of widespread though sporadic occurrence. Most of the areas they occupy are small, many of them comprising only an acre or two, but some of them include hundreds of acres. These schists are more or less intimately associated with the serpentinized peridotites or at some localities with the spheroidal basalts and diabases. They are usually exposed in discrete areas on the periphery of the intrusive masses but occasionally appear to be inclusions. In several places where these metamorphic rocks are dissociated from intrusive bodies the dissociation is probably due to erosion. The serpentinized peridotites were in many places originally intruded as lacco- lithic lenses, which have been removed by denudation, and the more resistant crystalline schist wdiich lay on their peripheries or beneath them form knobs that project from the general surface. Exceptionally the metamorphic rock forms a contin- uous belt along the margin of an intrusive mass. The process of alteration seems to have been erratic or selective in its oper- ation, so that the metamorphic rocks occur in patches, various in form and size, which are disposed in a general way about the margins of the intrusive masses. In this respect these metamorphic rocks are quite different from those wdncli form a contact zone due to pure thermal metamorphism on the periphery of a granitic intrusion, a zone that is commonly persistent and uniform. The metamorphism in the Francis- can group apparently occurred only at certain localities on the peripheries of the intruding bodies, either where the emana- tions from the intrusive mass found freer escape, or where the incasing rocks, by reason of their composition, were more susceptible to alteration, or wdiere there was a combination of these two favorable conditions. The composition of a part of the metamorphic rocks suggests that this part was derived from 53 basic igneous rocks preexistent in the Franciscan, but other parts are evidently altered sedimentary rocks.'^ Considered petrographically these metamorphic rocks may be grouped in several classes, among which are (1) amphibo- litic schists, (2) mica schists, (3) quartz schists, and (4) albite schists. The amphibolitic varieties include — 1. Black or dark-green schists, composed chiefly of horn- blende, which is usually accompanied by some albite or quartz. 2. Hornblende-garnet schists. 3. Actinolite schists, with or without chlorite and talc. One facies of these actinolite schists occurs in the form of nodules in serpentine, the prisms of actinolite being long and thick. 4. Glaucophane schists, composed almost wholly of glauco- pliane. 5. Glaucophane-quartz schists. 6. Glaucophane-albite schists. 7. Glaucophane-epidote schists. 8. Glaucophane-lawsonite schists. 9. Crocidolite-quartz schists. 10. Massive aggregates of green or blue amphibole, ompha- cite, and the garnet known as eclogite. The micaceous varieties comprise (1) biotite-quartz schists, (2) muscovite-glaucophane schists, and (3) schists in wdiich muscovite, glaucophane, garnet, and quartz are about equally developed. The quartz schists comprise varieties in which the dominant constituent is quartz, with wdiicli are associated subordinate amounts of glaucophane or mica or crocidolite. In the albite schists the dominant constituent is feldspar, which is accompa- nied by subordinate amounts of glaucophane or crossite. All these schists of course contain numerous accessory minerals, and some of them include large amounts of titanite. "Tliese inetauiorphic rocks are described more fully in several papers, particularly in The geology of Angel Island, by F. L Ransome (California Univ. Dept. Geology Bull., vol. 1, No. 7, 1894), and in The paragenesis of the minerals in the glaucophane-bearing rocks of California, by J. P. Smith (Am. Piiilos. Soc. Proc. vol. 45, 1907). Other papers, dealing chiefly with the mineralogy of the rocks, are mentioned in the bibliographic list at the end of this folio. 54 The mere mineralogic description and nomenclature of these interesting rocks, however, form only the first step toward solving the intricate problem of their genesis. The fact that they are derived from various kinds of rocks, both sedimentary and igneous, and yet have common features, such as their con- tent of blue amphibole, suggests that some regional condition influenced their development. The fact that some of the most characteristic minerals of the body of the schists — for example, lawsouite — also occur in veins that traverse the same schists indicates that the agencies of metamorphism must have been somewhat like those that fill veins, and that the metamorphism is therefore not purely thermal. The further fact that a char- acteristic mineral of these schists, blue amphibole, is developed in different kinds of rocks without deformation indicates that the agencies that formed the schists were not the forces of dynamic metamorphism. If purely thermal contact metamor- phism and dynamic metamorphism be thus both set aside as possible agencies in producing the schists the only remaining explanation of their origin is that they are due to metamor- phism by chemical reactions, which, however, have not greatly modified the original chemical composition of the rocks affected. The nature of these reactions is at present obscure, but they are doubtless various and complicated, and their localization is no doubt intimately connected with emanations from intrusions of basic magma. These inetamorphic rocks occur most extensively in the San Francisco quadrangle, at places north of Berkeley, and on the Tiburon Peninsula. AGE OF THE FRANCISCAN GROUP. The age of the Franciscan group has not been positively determined either in the quadrangles here mapped or elsewhere in the Coast Ranges. Neither the Foraminifera of the Calera limestone nor the Ivadiolaria of the Sausalito and Tngleside cherts appear to be sufiiciently distinctive to determine the age of the rocks in which they are found. Certain plant remains that were discovered by Fairbanks in the Franciscan rocks at vSlate Springs, Monterey County, are regarded by some paleo- 55 botanists as forms that represent an age transitional from Jurassic to Cretaceous time — Cretaceous rather than earlier. F. H. Knowlton, however, expresses the opinion that these fossil plants may be either Cretaceous or Jurassic. Two marine Mollusca, one a single specimen of an Inoceramus, found many years ago on Alcatraz Island, in San Francisco Bay, the other a fragment of either an Inoceramus or an Aucella, found 2f miles south of San Mateo, afford the only other paleontologic evidence available. These forms indicate a Jurassic or Cretaceous age but are otherwise indecisive. Although the paleontologic evidence as to the age of the Franciscan group is thus extremely meager and inconclusive the stratigraphic evidence is curiously self-contradictory and leads to a conclusion that few geologists may be willing to accept. The rocks of the Franciscan group are overlain uncon- formably by the Knoxville formation, wdiich is generally regarded by paleontologists as Lower Cretaceous. Exposures of the Knoxville resting upon the eroded surface of the deformed and locally metamorphosed rocks of the Franciscan group occur on the southwest flank of the Berkeley Hills, particularly north of Berkeley, and similar exposures occur in the San Luis quadrangle and have been described by Fair- banks. The relations exhibited in these exposures would indicate that the Franciscan is pre-Cretaceous in age. Unfor- tunately for the stability of this conclusion, however, other available evidence leads to another decision. In several folios of the United States Geological Survey the granite of the Sierra Nevada is mapped and described as of post-Jurassic age. This granitic rock extends continuously from the southern Sierra Nevada, in the Mount Whitney region, around the southern end of the Great Valley of California, into the Coast Ranges, where it forms a part of the complex upon which the Fran- ciscan rocks rest unconformably, as has been indicated in the San Luis and Santa Cruz folios. In the area here mapped the Franciscan rocks are nowhere superposed upon the granitic rock, which is confined to Montara Mountain and has the facies of a quartz diorite. In an earlier paper the writer described the Franciscan as resting u|)on this quartz diorite ("Montara 56 granite"), but later field work has shown this to be an error, the supposed Franciscan being in reality a block of Eocene strata faulted against the Franciscan, Where the Franciscan lies in contact w^ith the quartz diorite in the San Mateo quad- rangle the juxtaposition is due to faulting. Nearly all writers on Coast Range geology are, how^ever, agreed that the Fran- ciscan is later than the granitic rocks. The considerations that support this view may be summarily stated as follows : 1. The sandstones of the Franciscan group are composed very largely of granitic debris, as was first pointed out by Becker, who regarded these rocks as Lower Cretaceous. 2. At no place has the granite been observed to be intrusive in the Franciscan. 3. The intrusive relation of the granite to the older rocks is clearly shown at many localities. 4. Included fragments of these older rocks are very common in the granite, but no fragments of Franciscan strata have been found in the granite, although the radiolarian cherts are well adapted to preservation as inclusions, as is shown b}^ the fact that they are common in the rocks that intrude the Franciscan strata. 5. The pregranitic rocks are generally metamorphosed and consist chiefly of coarsely crystalline marbles (which are locally graphitic), quartzites, and various crystalline schists, none of wdiich resemble the Franciscan strata. 6. The local metamorphism of the Franciscan is not due to granitic intrusion but is intimately associated with basic irruptives. 7. The Franciscan rocks have not been subjected to the same dynamic metamorphism that lias generally affected the rocks into which the granite is intruded. 8. In general, there are in the Coast Ranges two different assemblages of very diverse sedimentary rocks, into one of which the granite is clearly intrusive, whereas the other con- tains no such intrusions; therefore it is highly probable that the latter assemblage is postgranitic. Considering, then, the fact that the Franciscan is post- granitic, and accepting the view that the granitic rocks of the 57 Coast Ranges are continuous with and of the same age as the granite of the Sierra Nevada, we must conclude that the Fran- ciscan group is post-Jurassic. This conchision is clearly in conflict with that drawn from the fact that the Franciscan lies unconformably below the Knoxville (Lower Cretaceous). At present there appears to be no way of harmonizing the conflict without (1) either extending the geologic time at the interval between the recognized Cretaceous and the Jurassic, an exten- sion that should not be made without the justification of more thorough investigation, or (2) assuming a period of batholithic development in the Coast Ranges that was distinct from and older than that in the Sierra Nevada, an assumption that should also not be made without further and fuller investigation. It is hoped that this statement of the difficulty of determin- ing the age of the Franciscan group may stimulate California geologists to make further field studies directed to the solution of the problem. CRETACEOUS SYSTEM. DISTRIBUTION. The Cretaceous formations constitute an important feature of the geology of tlie region mapped but are confined to the east side of the Bay of San Francisco. Their chief outcrops lie in a belt that extends through the San Francisco, Concord, and Haywards quadrangles, widening toward the southeast. An area of the upper part of the system occurs also in the northern part of the Concord quadrangle. The Cretaceous rocks lie unconformably upon those of the Franciscan group along the southwest front of the Berkeley Hills and are the dominant rocks of that range. They strike northwest and dip generally to the northeast at rather high angles, although in places the dip is reversed by irregular plication of the strata. On the northeast side of the range the upper formation of the Cretaceous rocks is unconform- ably overlain by Tertiary strata that occupy a great trough in the central part of the Concord quadrangle. The outcrop of the Cretaceous rocks along the Berkeley Hills is thus the southwest limb of a great synclinorium, tlie northeast limb of 58 which appears in the northern part of the Concord quadrangle, south of the town of Martinez, and also, more extensively, in the Mount Diablo quadrangle, which adjoins the Concord quadrangle on the east. Of these two areas of Cretaceous rocks, that of the Berkeley Hills is stratigraphically the more important, for it affords complete sections of the system from the Franciscan to the Tertiary, and it will therefore be de- scribed in greater detail. The system as represented in these quadrangles comprises two distinct formations, which, named in ascending sequence, are designated the Knoxville formation and the Chico formation. LOUVER CRETACEOL^S (SHASTA) SERIES. KNOXVILLE FORMATION". The Knoxville formation, named for Knoxville, in Napa County, is exposed in a practically continuous belt along the southwest slope of the Berkeley Hills, being confined to the lower part of the slope in the northwestern part of the range but reaching the summit in the 'southeastern part and de- scending to the foot of the slope beyond Haywards. The belt widens notably from northwest to southeast, across the Concord quadrangle, its width at Berkeley being onl}'" a few hundred feet, whereas at the southern border of the quadrangle it is over tliree-quartei*s of a mile. Farther southeast, in the Haywards quadrangle, the belt of Knoxville rocks may be still wider, but the rocks are in part mantled over by later rhyolite or by the alluvium of the valley floor, and their extent beneath these deposits can not be positively stated. The widen- ing of the belt of outcrop toward the southeast signifies a thick- ening of the formation, although it is in part due to plication of the strata, to which the increased volume of overlying soft rocks is conducive. The maximum thickness of the forma- tion in the Concord quadrangle is not less than 1000 feet and may ])e as much as 1500 feet. Tlie formation thins out rapidly northwest of Berkeley and at its northwestern extremity is represented by outlying patches that rest upon the worn surface of Franciscan rocks. Tliis thinning, however, is probably due in large part to erosion in Tertiary time, for these remnants 59 are uucoiiformably overlain by Pliocene and Quaternary for- mations. The Knoxville, being the basal formation of a series that rests unconformably upon tlie Franciscan rocks, might reason- ably be expected to consist of tlie coarse detritus that is charac- teristic of transgressive beds — that is, it should be in large measure conglomeratic. This, however, it is not, and the pre- vailing absence of conglomerate in a formation so situated is interesting and significant. The formation is composed almost wholly of dark, more or less carbonaceous, argillaceous, or finely arenaceous shale but includes occasional small lenses of com- pact gray limestone. In some sections it consists of alternate beds of shale and thin beds of rather fine sandstone; in others, particularly in the San Francisco quadrangle, it contains beds of fine pebbly conglomerate, the pebbles being on the average not much larger than peas. Freshly exposed sections show that this shale is evenly and thinly laminated, but owing to the readiness with which it slacks under the atmosphere and breaks down into soil fresh exposures are uncommon, and the dip of the strata can not be easily observed even in road cuttings only a few years old. The stratification in the more recent expo- sures, however, indicates that the formation as a whole dips to the northeast, beneath the next higher formation. Locally the dip is reversed, as, for example, north of Laundry Farm, in the southwestern part of the Concord qujidrangle, wdiere the shale wraps around an area of underlying serpentine in anticlinal fashion, and also near the southern border of the same quad- rangle, where a synclinal fold occurs. The shaly character of the Knoxville formation is not merely a local anomaly but is a general feature, which prevails over a large part of the Coast Ranges of California, This fact would seem to preclude the idea of a slow transgression of the Knox- ville sea over an uneven surface in the region of tlie present Coast Ranges; it suggests rather the sudden submergence of a land surface of low relief, or, to put it in another way, a sudden transition from a condition of very advanced aerial erosion to a condition of marine deposition in extensive shallow lagoons and swamps. 60 At several places along the belt of outcrop of the Knoxville formation the characteristic Knoxville fossil Aucella piochii has been found, and an ammonite obtained in it at the mouth of Strawberry Canyon, at Berkeley, was identified by T. W. Stanton as probably a species of Hoplites. At the same locality a small Pecten Avas also found, perhaps a new species. Frag- ments of Belemnites are common in the formation, particularly north of Berkeley, wdiere they occur in a calcareous sandstone and in the fine pebbly conglomerate already mentioned. Near the northwestern limit of the formation as mapped, north of Berkeley, it includes a few^ patches of impure limestone and some rusty-Aveathering pebbly grits. This limestone has yielded a number of fossils, which have been studied by F. M. Ander- son, who identifies them as forms characteristic of the upper Knoxville. These fossils are Ifodiola major, Liicina colusa- ensis, Pecten complexicosta, Cardinia?, Myoconcha?, Turbo, Atresias liratus, and Phylloceras onoense. The lower Knox- ville is here wanting, the limestone resting directly upon the Franciscan rocks. Besides these fossils obscure remains of plants are fairly abundant in the shales. In the southern part of the San Mateo quadrangle two small areas of conglomerate are represented on the geologic map as probably of Knoxville age. This reference is based on the fact that in the territory just south of the quadrangle a simi- larly isolated patch of conglomerate contained remains of Aucella. A like area of conglomerate of probable Knoxville age is mapped at the head of Bolinas Lagoon in the Tamalpais quadrangle. UPPER CRETACEOUS SERIES, CHICO FORMATION. Oakland conglomerate member. — In the Berkeley Hills the basal portion of the Chico formation consists of a conglomerate to which the name Oakland conglomerate member is applied, from its typical exposure at the city of Oakland. This con- glomerate outcrops along a belt paralleling that of the Knox- ville formation, upon which it lies conformably. Like the Knoxville it increases in volume from the northwest toward 61 the southeast, but the increase is not uniform. At the mouth of Strawberry Canyon the conglomerate has not been discov- ered. In the valley southeast of Claremont Creek it is exposed in rather small outcrops, which are not mapped, and at Temes- cal Lake it is exposed to a thickness of perhaps 100 feet. From this point it may be traced southeastward ahnost continuously to Redwood Peak, where it bulges out to a thickness of about 1000 feet. Beyond Redwood Peak it diminishes in volume for about a mile to 200 or 300 feet and then expands again, reach- ing a width of about 1000 feet northeast of Laundry Farm. From this point southeastward its thickness ranges from 500 to 700 feet, and still farther southeast, in the Haywards quad- rangle, it again expands. The average thickness of the con- glomerate in the Concord quadrangle may be about 500 feet. In the Haywards quadrangle its average thickness may be nearer 1000 feet. The conglomerate at many places shows distinct stratification and the strata dip uniformly to the northeast, beneath the over- lying sandstones and shales of the Chico formation, at angles ranging from 45° to 65°. The conglomerate exhibits none of the subordinate plication observed in the underlying shale of the Knoxville formation, a fact due to the more massive char- acter of its strata and their competence to propagate the stresses to which the region was subjected. Here and there beds of coarse sandstone are intercalated with the conglomerate, but the mem- ber as a whole is distinctly conglomeratic. Its constituent pebbles are very much waterworu and rounded and range in si/e from the dimensions of a marble to those of a man's head. This conglomerate rests conformably u])on the soft shales of the Knoxville formation and its stratigraphic position is significant of an important event in the geologic history of the region, for it indicates an abrupt change in the conditions not only of deposition but of erosion. When it is recalled that the Knoxville is the basal formation of the Cretaceous system over a very wide extent of the Coast Ranges the inference is that the region surrounding the basin of deposition must have been one of low relief, with low-grade streams carrying fine silt rather than coarse sand. The sudden appearance of the 62 conglomerate above the shales, not only in this quadrangle but at several other localities in the Coast Ranges, indicates the encroachment of the deltas of high-grade streams upon the basin. It would seem clear from this that the period of accu- mulation of the Knoxville deposits was brought to a close by the orogenic deformation of the continental margin of the basin in which tliey were deposited, though such deformation did not cause the floor of the basin to emerge from the water. Upper part of Cliico formation. — In the Berkeley Hills the principal part of the Chico formation — the part tliat conform- ably overlies the Oakland conglomerate — comprises a thick accumulation of sandstones and shales, the sandstones prepon- derating. The upper beds of the Oakland conglomerate grade rather abruptly into the overlying sandstones of the Chico for- mation, by transitional beds of pebbly sandstone. The outcrop of the sandstones and shales occupies a broad belt along the summit of the Berkeley Hills, parallel to the belts occupied by the Knoxville formation and the Oakland conglomerate. This belt widens gradually but steadily toward the southeast, its increase in width being due to an increasing volume of strata in this direction. Although the Chico sandstones and shales form the prominent ridges of the Berkeley Hills these rocks disintegrate readily under the weather and yield a heavy mantle of sedentary soil, so that, as a rule, their stratification can be seen only in steep-walled canyons, w^here observations show that though the strike of the strata remains fairly con- stant, the dip is at some places reversed, indicating sharp fold- ing. The strata and the formation as a whole dip in general to the northeast at angles ranging from 30° to 80°. The thick- ness of the strata is difficult to measure on account of the obscurity of the exposures and the uncertainty as to repetition by folding, nearly all parts of the formation being very nuich alike. It is estimated, however, that the part of the formation above the Oakland conglomerate mend:)er has a thickness of about 2000 feet at the northwest end of the belt and of about 5000 feet at tlie southeast end, within the Concord quadrangle. The belt of outcrop of the sandstones and shales of the Chico 6,- formation widens steadily from a feather edge at a point north of Berkeley to over 4 miles in the vicinity of Haywards. The only fossils discovered in the Chico of this belt are, frag- ments of a large Inoceramus and a small Pecten, found in Strawberry Canyon, and a number of echinoderms and other more obscure fossils, found at the sandstone quarry in Shepard Canyon. On the northern border of the Concord quadrangle, south of Martinez, an area of sandstones and shales of the Chico formation lies in the heart of an anticlinal fold, which is flanked on both sides by synclines of Tertiary strata. The general trend of the axis of this fold is northwest and southeast and the pitch of that part of it which lies within the Concord quadrangle is to the southeast. The belt has a maximum wndth of 2|- miles and the dips are prevailingly steep, so that the belt seems to include a great thickness of strata, but this apparent thickness probably much exceeds the real thickness, the volume of strata being multiplied by folding. The anti- cline has not been eroded deeply enough to reveal the base of the Chico and the underlying rocks, but the partial section exposed has a thickness of not less than 3000 feet. This body of strata is composed almost wholly of massively bedded sand- stone but includes subordinate beds of shale and one lens of conglomerate. The sandstone is of medium fine texture, is well cemented, and has a bluish-gray color. Under weather- ing, however, it becomes rather incoherent and crumbling, takes on a tawny color, and affords an abundant soil, so that fresh faces of the rock are rarely seen. Fossils. — Numerous characteristic Chico fossils have been found in these beds. The locality within the area here described tliat has yielded them most abundantly is in the upper part of the section exposed on the east side of the Arroyo del Hambre, about three-quarters of a mile south of the north- ern border of the Concord quadrangle. The more important fossils found here, as determined by Prof. J. C. Merriam, are as follows : 64 Fossils of the Chico formation. Corbula cultiforinis Gabb. Meekia sella Gabb. Meekia navis Gabb. Meretrix arata or fragilis Gabb. Mytilus quadratus Gabb. Mytilus pauperculus Gabb. Nueula tx'uncata Gabb. Pecten mai-tinezensis Gabb. Pectunculus veatchi Gabb. Tellina hoffnianniana Gabb. Tellina? aequalis Gabb. Venus varians Gabb. Cinulia obliqua Gabb. Cylindrites? brevis Gabb. Dentalium cooperi Gabb. Gyrodes expansa Gabb. Perissolax brevirostris Gabb n. var. Pugnellus hamulus Gabb. Solarium inornatum Gabb. Helicoceras vermicularis Gabb. Sharks' teeth. Teleost fish scale. TERTIARY SYSTEM. EOCEKE SERIES. SUBDIVISIONS. As Prof. J. C. Merriajii has clearly shown, the Eocene rocks of the California Coast Kanges may be divided into two paleontologically well-defined formations — the Martinez and the Tejon. The line of stratigraphic separation betw^een these two formations is not w^ell marked, and, as the sandstones of both yield an abundant soil, which is generally cultivated, tlie mapping of the dividing line is not easy. Certain differences in the character of the rocks, however, taken in connection with the fairly frequent occurrence of fossils in the beds, facilitates an approximately correct separation, which has been indicated on the map. These formations are found only in the northern and eastern parts of the Concord quadrangle and the southern part of the San Mateo quadrangle. Along the northeast flank of the Berkeley Hills strata of Miocene age rest directl}^ upon the Chico. Tbis absence of Eocene rocks in this part of the field is significant in the geologic history of the region. It signifies either that no Eocene rocks were deposited over tbis part of the region, it being a land area bordering the Eocene basin, or that, if tliey were deposited, they had been lifted above sea level by orogenic movements and completely removed before the subsidence which permitted the deposition of the next series of rocks. Tlie relative probabihty of these two interpretations is con- sidered under tbe heading "Geologic history." 65 Although the Eocene series is not found on the northeast flank of the Berkeley Hills, Avhere it should have been depos- ited if the sequence were conformable, it is abundantly repre- sented on the flanks of the anticline south of ]\Iartinez, named the Franklin anticline, from Franklin Creek, which runs almost across it. Here both the Martinez and Tejon for- mations are important elements of the stratigraphy. The Tejon appears also in the core of a large anticline northeast of Sobrante Ridge, which has not been dissected deeply enough to reveal the underlying formations. It also occurs on Shell Ridge and Lime Ridge, two northwestern spurs of Mount Diablo, although the paleontologic indications at tliese places are not so satisfactory, as Avell as northw^est of Pacheco, where the underlying Martinez outcrops at the north border of the quadrangle. MARTINEZ FORMATION. Character and distribution. — The Martinez formation, named from the town of Martinez, in Contra Costa County, is, in its typical form, composed of heavily bedded sandstones in which much glauconite occurs, giving the rocks a greenish-gray color. The formation includes also some reddish sandstones and intercalated shales. Roth sandstones and shales break down readily under the weather. A thick lens of rather coarse, well-cemented congloine]-ate also occurs in the south- easternmost extension of the terraue, south of Grayson Creek. These strata dip away from the Chico in apparently conform- able sequence on both flanks of the Franklin anticline at angles ranging from 30° to 80°, the steeper dips being on the northeast flank. The anticline is, however, complicated by a subordinate fold and perhaps by an axial fault, wdiich brings a tongue of the Martinez rocks into the center of the Chico area and causes the beds to dip apparently under the Chico. The sections tliat best reveal the strata show that the INIartinez for- mation is probably not less tlian 2000 feet thick. Northeast of the Franklin anticline is a broad syncline of Martinez strata, in the trough of wliich lie folded strata of the Tejon forma- tion, and above these, in the axis of the trough, is a belt of San Francisco — 5 66 Monterey rocks. The Martinez rocks in the northeast limb of this syncline are exposed in low hills about a mile north of Pacheco. Southwest of the Franklin anticline the outcrop of the Martinez strata on this limb of the fold is cut off a mile or more south of the Santa Fe tunnel by a thrust fault which causes the Chico to override the Monterey. Fossils. — Fossils occur at many places in the Martinez formation, most abundantly on the southwest limb of the Franklin anticline. These fossils have been studied by Stan- ton, Merriam, Weaver, and Dickerson, in whose papers, as well as in earlier papers by Gabb, the paleontology of the formation is fully discussed. The work of these paleontologists shows that the Martinez formation contains a fauna which is quite distinct from that of the underlying Chico and from that of the overlying Tejon. So far as is now known only four of the species of this fauna range downward into the Chico and only twenty-five upward into the Tejon. Ninety-seven species are confined to the Martinez. It has been even possible to separate the fauna into two parts, corresponding to an upper and lower division of the Martinez formation. This distinc- tively Martinez fauna comprises the following species, the list of which has been revised by Dr. Dickerson." Characteristic fossils of the loiaer part of the Martinez formation. Flabellum remondianum Gabb. Troclioeyathus zitteli Merriain. Sehizaster leoontei Merriam. Cardimn cooperi Gabb. Cuculhea mathewsoni Gabb. Leda gabbi Conrad. Lima niultiradiata Gabb. Meretrix sp. Modiolus inerriami Weaver. Nucula of. truncata Gabb. Pholadoinya nasuta Gabb. Tapes a(f. quadrata Gabb. Tellina martinezensis Weaver. Actseon lawsoni Weaver. Cylicbna costata Gabb. Dentaliuin cooperi Gabb. Discohelix californicus Weaver. Fusus fequilateralis Weaver. Neptunea mucrouata Gabb. Perissolax tricarinatus Weaver. Siphonalia lineata Stanton. Urosyca caudata Gabb. Urosyca robusta W^eaver. Xenophora zitteli Weaver. Teredo sp. Turbinella crassatesta Gabb. "Dickerson, Roy E., Fauna of the Martinez Eocene of California: Cali- fornia Univ. Dept. Geo). Bull., vol. 8, No. G, Berkeley, 1914. 67 Characteruiic fossils of the upper part of the Martinez forynat ion. Schizaster lecontei Merriain. Cancer ep. Cardium cooperi Gabb. Cucullsea inathewsoni Gabb. Leda gabbi Conrad. Modiolus merriami Weaver. Modiolus ornatus Gabb. Dentalium cooperi Gabb. Dentalium stramineu)n Gabb. Heteroternia gabbi Stanton. Heteroternia trochoidea Gabb. Heteroternia sp. indet. Natica 6p. Perissolax tricarinatus Weaver. Perissolax blakei Gabb. Nucula truncata Gabb. Pholadoniya nasuta Gabb. Solen stantoni Weaver. Tellina martin ezensis Weaver. Tellina hornii Gabb. Tellina undulifera Gabb. Ampullina cf. striata Gabb. Brachysphingus liratus Gabb. Bullinula subglobosa AVeaver. Siphonalia lineata Stanton. Architectonica tubereulata Weaver. Strepsidura pachecoensis Stanton. Tiitoniuui pulehrum Weaver. Turritella infragranulata Gabb. Turritella pachecoensis Stanton. Turritella conica Weaver. Turris sp. indet. Urosyca caudata Gabb. Rocks of San Pedro Point. — In the San Mateo quadrangle a belt of early Tertiary sedimentary beds lies on the north flank of Montara Mountain and extends from Pilarcitos Lake to the coast at San Pedro Point, where the beds are well exposed in cliff sections. (See PL I.) These beds were at one time assigned to the Franciscan group, although they were recognized as a pecu- liar facies of that group which might possibly be segregated from it. The recent discovery in these rocks of a stratum con- taining poorly preserved remains of early Tertiary fossils has led to their separation from the Franciscan group and to their tentative assis'nment to the Martinez formation. The beds con- sist of conglomerates, coarse arkose sandstones or grits, finer laminated micaceous sandstones, dark shales, and thin strata of limestone. The conglomerates rest directly on the quartz diorite ("Montara granite"), and are followed, in ascending sequence, fey sandstones and shales, the whole having an aggre- gate thickness of several hundred feet. The beds in the coast section are folded in at least two irregular and rather twisted synclines and an intervening anticline, and the folds are tra- versed by a number of faults. These beds were probably depos- ited across the contact of the Franciscan and the quartz diorite and after being folded were faulted against the Franciscan on their north side. 68 The fossils found in these beds indicate that they are of early Eocene age, and they are referred to the Martinez foi'ma- tion by R. E. Dickerson, who has kindly supplied the follow- ing list of forms identified by him : Fossils found in the beds at San Pedro Point. Flabellum sp. Paracyathus (?) sp. Cidaris sp. Terebratulina ef. tejonensis Stan- ton. Cardium cf. cooperi Ciabb. Cucullgea cf. inatheAvsoni Gabb. Dosinia ef. lawsoni n. sp. Glycimeris sp. Glycimeris cf. veatchii var. major Stanton. Macroeallista (?) packi n. sp. Meretrix stantoni n. sp. Modiolns cf. bakeri n. sp. Ofetrea buwaldana n. sp. Phacoides diaboli n. sp. Phacoides quadrata n. sp. Semele (?) sp. Tapes (?) quadrata Gabb. Teredo sp. Venus (?) sp. Venerieardia sp. Amauropsis (?) sp. A I aria sp. Chlorostoma (?) sp. Cylichna costata Gabb. Dentalium sp. striated Dentalium cooperi Gabb. Discohelix sp. Fissurella sp. Galerus excentricus Gabb. Hipponyx sp. Natica sp. a. Natica (?) sp. b. spiral lined. Patella sp. Ringinellacf. pinguis Gabb. Spiroglyphus (?) sp. Tritonlum iiiartinezensis n. sp. Tritonium sp. a. Turritella sp. Turritella cf. pachecoensis Stanton. Urosyca cf. caudata Gabb. Crustacean fragments. TEJON FORMATION. DistribiUion and character. — The beds of the Tejon forma- tion are best exposed for study in the area northeast of the Franklin anticline, where they lie in a steeply dipping syncline and where their thickness is not less than 2000 feet. The rocks are generally free from glauconite and are composed of more cleanl}^ washed sand than that which formed the sandstones of the Martinez formation. They are either of a very Jight color or are stained red with oxide of iron. They are also much more strongly cemented and more resistant to degradation than the Martinez rocks. The belt of Tejon represented by the two limbs of this syncline is continued on the southeast side of Ygnacio Valley in the two spurs of Mount Diablo that embrace the lower part of Pine Canyon. On the southern of these spurs the Tejon rocks rest upon and dip away from younger (Monterey) rocks on the limb of an overturned syncline. On 69 the northern most spur the strata dip at varying angles to the northeast. The rocks are sandstones that are oenerallv as hard and resistant as those of tlie Pacheco syncline, except some of the heavy-bedded sandstones southeast of Concord, which are highly calcareous and softer. The only other local- ity at which the Tejon appears is in the heart of the Sobrante anticline, where it is flanked on both sides by strata of Monte- rey age. The formation is named from Fort Tejon, in Kern County, where it is typically developed. Fossils. — In the Tejon formation of the Concord quadrangle about 43 species of fossils have been found. Dr. Roy E. Dickerson has kindly supplied the following list as a result of his recent studies of this fossil fauna : Fossils of the Nummuloid (?) sp. Orbitoides sp. Trochocyathus striatus Gabb. Turbinolia n. sp. Cardiuiii cooperi Gabb. Cardiuin bi'eweri Gabb. Glycimeris sagittata Gabb. Glyeimeris cor Gabb. Leda gabbi Conrad. Modiolus ornatus (Gabb). Modiolus inerriami (Weaver). Meretrix hornii Gabb. Meretrix uvasana Conrad. Meretrix ovalis Gabb. Nucula (Acila) n. sp. Spisula n. sp. Tellina hornii Gabb. Tellina cf. remondii Gabb. Tellina longa Gabb. Tellina martinezensis Weaver. Thracia karquinezensis Weaver. Solen parallelus Gabb. Tejon formation. Solen stantoni Weaver. Venericardia planicosta var. hornii Gabb. Dentaliuui stramineum Gabb. Amanropsis alveata (Conrad). Architectonica sp. Ancilla(01iverato)californica Cooper. Bela cf. clathrata Gabb. Cylichna costata Gabb. (Jonus remondii Gabb. Ficopsis remondii Gabb. Megistostoma striata Gabb. Morio tuberculatus Gabb. Perissolax blakei (Conrad). Perissolax n. sp. Rinella ciinalifera Gabb. Spiroglyphiis (?) tejonensis Arnold. Turris monolifera Cooper. Turritella uvasana Conrad. Turritella conica Weaver. Tritonium eocenicum Weaver. Tritonium impressum Weaver. Only two of these species extend down into the Cliico and eight into the Martinez, and four of these eight are rare or non- characteristic. The fauna as a whole is therefore quite dis- tinctive for the Tejon, and, as Merriam has shown, the sharp difference between this fauna and that of the Martinez forma- tion is significant of an important event in the geologic history of the region in Eocene time and thus warrants the separation of the two formations on the geologic map. 70 MIOCEXE SERIES. MONTEREY GROUP. PETROGRAPHIG CHARACTER. The Monterey group, named for the town of Monterey, consists of a thick mass of sediments of Miocene age, which forms a prominent feature of the stratigraphy and structure of the Coast Ranges, particularly the part south of the Bay of San Francisco. On the California coast north of the bay the strata of this group have been found only in the vicinity of Point Reyes and Point Arena. In the area here considered the group is represented in the Concord, Haywards, San Fran- cisco, and Mount Tamalpais quadrangles, and although it does not occur in the San Mateo quadrangle it forms a well-exposed terrane just south of it. The best sections are those in the Concord quadrangle, where the group is composed of two kinds of rock. One of these is ordinary sandstone, generally of light color and not strongly cemented except where it con- tains many fossils, which give it a firm bond of carbonate of lime. The other kind of rock of the Monterey group embraces a number of varieties, all of which have been grouped together in the older writings on California geology as "bituminous slates." In this text "slates" will be replaced by "shales," a word that expresses more correctly the character of their lamination, the term "bituminous shale" being a convenient and fairly expressive designation for these peculiar and inter- esting rocks. The bituminous shale is in part of organic origin. Some of its beds resemble diatomaceous earth; they are white, soft, chalky, and more or less pulverulent. The microscope shows that they contain abundant organic remains, and chemical examination shows that they are composed chiefly of silica. Other varieties of these white, chalky beds are somewhat harder and are harsher to the touch; they consist in part of fine volcanic ash and may even grade into varieties in which this constituent seems to predominate. Though still remaining soft and chalky, this white bituminous shale grades into creamy or purplish or brownish varieties. The color appears to be due wholly to organic matter, for it can be burnt out, the 71 rock becoming white or assuming a slightly reddish tint, due to the presence of a small quantity of ferric oxide. When heated on platinum foil both the wiiite and the colored varieties emit a very distinct bituminous odor. These rocks generally con- tain a large proportion of soluble silica, which may be removed by digestion with caustic potash. The proportion of soluble silica in many of the chalky facies, presumably those that are richer in organic remains, averages probably 50 per cent. The residue after digestion is an isotropic cloudy substance con- taining a variable admixture of angular chips of doubly refract- ing minerals, chiefly quartz and some angular fragments of glass. This isotropic residue is probably only in small part detrital material and is doubtless chiefly a palagonitic decom- position product of very fine volcanic glass dust. This chalky facies of the bituminous shale may occur either in thinly and evenly laminated beds or in rather massive thick beds that are traversed by numerous irregular shrinkage joints. Locally the chalky facies may pass into an opaline rock, which is dense and compact but may be easily scratched with a steel point. These nondetrital rocks, which were undoubtedly deposited far from the continental margin, grade into clayey and finely arenaceous varieties, which indicate that at certain stages of their accumulation, or over certain areas of Monterey sea bottom, there were distinct influxes of detrital material. A quite different facies of the bituminous shale is prevail- ingly cherty or flinty and is usually characterized by very even lamination. This laminated rock may consist wholly of chert or, as is more common, it may be composed of an alter- nation of thin partings of soft shale with beds of chert that generally range in thickness from 1 to 3 inches. This facies is well displayed in the band of bituminous shale which forms in part the crest of the Berkeley Hills (see PI. V) and which presents a very striking resemblance to the typical beds of radiolarian chert of the Franciscan group in the Coast Ranges. Many of the chert beds that lie between the shale partings are laminated, though they show not the slightest tendency to cleave alono- the lamimie. In these beds and those in which 72 the shale partings are absent the lamination is made apparent by differences in the color of the layers of chert. The colors commonly observed are dull yellow, gray, purplish, and black. These cherts, like the chalky facies, contain some silica that is soluble in caustic potash, but very much less — not more than 3 or 4 per cent. Traces of microscopic organisms may be dis- cerned in some thin sections of the cherts, but these traces do not resemble the diatomaceous remains found in the chalky shale, and their character is indeterminate. Nearly all these cherts, of whatever color, weather light yellowish or whitish and break down under the atmosphere into small, sharply angular or subcubical fragments. The bituminous matter of the rocks is found chiefly in the soft shale partings but may also be detected in the cherts themselves. These two chief kinds of bituminous shale, the chalky and the cherty, usually occur at fairly distinct and separate hori- zons in the Monterey group, but in some places one grades into the other. A bituminous shale at one horizon may be persistently cherty over a wide area, whereas a shale at another horizon in the same deposit may be as persistently chalky. In the Concord quadrangle, where the bituminous shale is sepa- rated into distinct formations by intervening formations of sandstone, one of the shale formations may consist typically of chert beds interlaminated with thin partings of shale, while another may be uniformly chalky. Any account of the petrography of the bituminous shales, even such a general account as is here attempted, would be incomplete without a reference to certain rock formations that are rather constantly associated with them, although in very subordinate volume. One of these formations is a light-colored or whitish quartzose rock, which resembles in texture and appearance an indurated sandstone; the other is a gray com- pact limestone, which generally weathers ocherous yellow and in places contains considerable cherty silica. Both these rocks occur rather commonly at several horizons in the bituminous sliale and by their hardness, texture, mode of weathering, and other physical properties may be readily discriminated from both the clierty and tlie chalky types of the shale. They gen- 73 erally occur in distinct beds not more than 2 or 3 feet thick. The quartzose beds as seen in hand specimens appear to be aggregates of vitreous particles througli Avhich consideralile earthy white material and some dark grains are interstitially distributed. The microscope shows that the body of the rock is composed of fragments of quartz, orthoclase, and acidic plagio- clase and siliceous rocks, the quartz greatly preponderating. All these are remarkably angular and are embedded in an amorphous cloudy matrix which resembles the insoluble residue of the chalky shales. The sharply angular fragments of quartz and feldspar are very uniform in size, ranging in diameter from about 0.12 millimeter to 5 millimeters. The dark particles are not uniform in composition, but many of them are volcanic glass. This remarkable rock, though here called a sandstone, has rather the character of a quartzose tuff than of a detrital rock, and its intercalation in the cherts and shales, which generally include little deti-ital material, supports this sugges- tion. The question of its origin is, however, left open for the present. No fossils have been found in these rocks. The limestone is generally magnesian and is usually devoid of fossils. Specimens of rock taken from certain of its beds, however, contain a small percentage of phosphoric acid, and moUuscan remains, as well as bones of large cetaceans, have been found in it at some places, as at Miner's ranch, on the east side of San Pablo Valley, in the Concord quadrangle, where the shells and the bones occur in the same matrix, RELATIONS TO OLDER FORMATIONS. The Monterey is exposed in superposition upon older rocks at five localities. At three of these it rests upon the Tejon, at one upon rocks that are probably of Martinez age, and at one upon the Chico. The simplest relations are those exposed in the Pacheco syncline, where a fairly symmetric synclinal trough of Monterey strata with low southeastern pitch is fianked on either side by Tejon strata. The basal portion of the Monterey is characterized by thick lenses of conglomerate, and the sandstones associated with these lenses contain fossils which, in the opinion of Prof. J. C. Merriam, are characteristic 74 of the lower Monterey. There is no apparent structural dis- cordance, but the basal conglomerates are significant of an unconformable relation. On the northeast side of Shell Ridge the Monterey is similarly in contact with the Tejon, but with reversed dips, owing to the overturning of the syncline in which the strata are folded. In the Shell Ridge section there is no structural discordance, but farther along the contact, in Pine Canyon, in the Mount Diablo quadrangle, there are abundant conglomerates at the base of the Monterey, which again indicate an unconformity. In the Bear Creek anticline, in the northwestern part of the Concord quadrangle, the lower Monterey strata rest upon the Tejon, but here the structure of the Tejon is so obscure that it is not possible to discover w^hether or not there is structural discordance, and no con- glomerate has been observed at the base of the Monterey. The consideration of these three sections thus affords evidence of no very profound degradation of the Tejon in the pre- Monterey interval of uplift. In the section exposed in the Berkeley Hills, however, about 5 miles southwest of the Bear Creek anticline, the Monterey rocks rest directly upon the Chico. Both Tejon and Mar- tinez are absent. The Claremont shale, the second formation of the Monterey group, lies almost in direct contact with the Chico, the Sobrante or basal sandstone of the Monterey group being represented by only a thin layer of 3'ellow incoherent sandstone, mapped with the Claremont. There are no conglom- erate beds at the base of the Monterey in this section, for here the Eocene rocks, which are more than 4000 feet thick 9 or 10 miles farther northeast, either were not deposited or, as seems more probable, had been completely removed in pre-Monterey time. One justification for the view that the Eocene rocks once extended over the region of the Berkeley Hills and were removed in the Eocene-Miocene interval is that this part of the Coast Ranges contains no representative of the San Lorenzo formation (Oligocene), which occurs to the thickness of 2500 feet in the Santa Cruz quadrangle. In San Lorenzo time the Berkeley Hills region was probably a zone of erosion. /o The fifth section in which the Monterey is exposed in super- position upon older rocks is at Selby, on San Pablo Bay, in the Napa quadrangle, a few miles north of the San Francisco quadrangle, where a well-defined unconformity is revealed in a cliff. The surface upon which the Monterey rocks rest is a marine wave-cut terrace, perforated by many holes made by boring mollusks. The strata in which this terrace is cut are soft black shales, which are probably Martinez in age, for Mar- tinez fossils have been found in the sandstones that adjoin them on the north. These shales have a southerly dip of about 70°, and the surface of the perforated terrace and the superimposed sandstones of the Monterey dip in the same direction at about 60°. It is evident that the shales were elevated above sea level and inclined at about 10° to the horizon when they were truncated to form the terrace. The sandstone of the Monterey contains a fossil fauna which, in the opinion of Prof. J. C. Merriam, is that of the middle Monterey. The Tejou is appar- ently absent here, although it is abundantly represented only a few miles to the southeast, along the strike of the rocks. The Monterey sea evidently did not extend over this part of the region until middle Monterey time, and therefore part of the erosion is referable to early Monterey time. In general, how- ever, the historical facts that are so clearly manifest at Selby are consistent with and support the interpretation of the sec- tion in the Berkeley Hills, where the Monterey rests directly upon the Chico. SUBDIVISIONS. In the Concord and San Francisco quadrangles the Monterey group is made up of sandstones and bituminous shales, indi- cating an alternation of deposition along a shore with deposi- tion in the deeper water in which the nondetrital shale was laid down. In its most complete section the bituminous shale occurs in four distinct divisions with intervenine; divisions of sandstone. Sandstones occupy the top and bottom of the sec- tion, so that the Monterey here consists of nine stratigraphic divisions. This ninefold subdivision, however, is local and seems to represent a vertical oscillation of the coast in Monterey 76 time, whereby a region of shoal water, in which detrital mate- rial was accumulating so abundantly as to mask any volcanic or organic admixture, was four times depressed, permitting the accumulation of the deep-water sediments that were deposited off the coast at this time — sediments in which organic and vol- canic material were greatly in excess of terrigenous detritus. These oscillations in Monterey time doubtless also affected the deeper off-shore region but probably did not change the char- acter or interrupt the continuity of sedimentation, so that the sequence of nine subdivisions in the region east of the Bay of San Francisco may well be the chronologic equivalent of a solid, undivided columnar section of shale in Monterey County. In neighboring territory, however, the uppermost shale thins out toward the south and finally disappears, so that the seventh and ninth subdivisions, which consist of sandstone, come together, and as they are very much alike it is practically impossible to map the boundary between them. It thus becomes convenient on the map to treat the uppermost shale as a member and to represent the two sandstones as one forma- tion. A little farther east, in the Concord quadrangle and in parts of the Mount Diablo quadrangle, the absence of certain of the shales shows that the deeper water did not extend so far inland, except at times of maximum depression, the Monterey being there represented largely by sandy littoi'al deposits, and the record of oscillation is not sufficiently impressed upon the sediments to be clearly legible. Similar evidence of the oscillation of the sea bottom in Monterey time may be found in other portions of the Coast Ranges. In Santa Barbara County, for example, according to Fairbanks, the lower part of the Montere}' group consists chiefly of gypsiferous clays. These clays and gypsum beds doubtless represent shallow-water deposits in parts of the Pacific Ocean that became landlocked by uplift and were subsequently depressed, so that they received the accumu- lation of bituminous shale that rests upon the clays. In the bituminous shales of Santa Cruz County and Mendocino County the continuity of the shale is interrupted by strata of ordinary sandstone and even pebbly beds, indicating unmis- 77 takably the presence of shallow water. One such pebbly bed occurs in a shale formation in the San Francisco quadrangle. The subdivisions of the strata of the Monterey group in the San Francisco and Concord quadrangles represent events in the geologic history of the region that are sufficiently important to entitle each of them to receive a distinctive name. These names will flicilitate the discussion of the local geology and the correlation of the geologic record in this region with similar records of oscillation in other parts of the Coast Kanges ; but while the Monterey group may be thus divided, according to its purely physical features, into several parts, and each may receive a formation name, the group if considered paleontologi- cally can be divided into only three parts. The formations of the Monterey group, particularly the sandstones, are at many localities richly fossiliferous, and a study of the stratigraphic distribution of these fossils by Prof. J. C. Merriam shows that the group contains three fairly characteristic though not wholly distinct faunas. The names of the stratigraphic subdivisions; their relations to the paleontologic divisions, and the approxi- mate thickness of each as observed in a section across the Bear Creek anticline, in the Concord and San Francisco quadrangles, are given in the following table: Subdivisions of Monterey group. Paleontologic sub- divisions. stratigraphic subdivisions. Petrographic character. Thick- ness. Upper faunal zone. Briones sandstone Hercules shale mem- ber Briones sandstone Sandstone Bituminous shale Sandstone Feet. 800 500 1000 Rodeo shale Hatnbre sandstone Tice shale Bituminous shale Sandstone _ . 670 1200 Middle favinal zone. Bituminous shale Sandstone 460 Oursan .sandstone Claremont shale 600 Bituminous shale and chert _ 250 Lower faunal zone. Sobrante sandstone _ Sandstone . .. 400 Total thickne.s.s, 0880 feet. 78 SOBRANTE SANDSTOjSTE. The largest exposure of the Monterey group in the area here considered is in the broad belt that traverses the Concord quadrangle diagonally from northwest to southeast. The strata in this belt have been folded into a series of synclines and anticlines, locally complicated by faulting, the dissection of Avhich has revealed in their normal sequence all the forma- tions of the group. This belt also cuts across the northeast corner of the San Francisco quadrangle. In the axis of the Sobrante anticline the basal formation of the group, the Sobrante sandstone, named from Sobrante Ridge, rests on the Tejon formation, and it is exposed in another anticline in the valle}^ of Pinole Greek, where, however, the underlying Tejon is not revealed. The Sobrante sandstone is somewhat variable in character but is prevailingly fine grained and light colored, though it shows local ferruginous staining. Some of the beds, however, are gritty and some are flaggy. In this sandstone, near its base, is a bed of white rock that ranges in thickness from a few inches to 20 feet or more. On micro- scopic examination this rock proves to be a volcanic ash, made up cliiefl}^ of pumiceous glass and angular fragments of well- cemented quartz. FOSSILS OF LOWER FAUNAL ZONE. Some of the beds of the Sobrante sandstone are richly fossil- iferous, and b}^ means of these fossils the Sobrante can be separated from the rest of the Monterey group to form the lowest of its three paleontoloe-ic divisions. A list of the most characteristic fossils of the formation is given on page 79 ; the species were identified by Prof. J. C. Merriam and Dr. Bruce Clark. These fossils are typicid of the lower part of the Monterey of the Coast Kanges generally. 79 Common fossils of the Sobrante sandstone. Pelecypoda: Area devincta Conrad. Chioue luathewsoni Gabb. Chione n. sp. Dosinia wbitneyi Gabb. Glyeinieris .sp. Leda taphria Dall. Mytilus mathewsoni Gabb. Nucula (Acila) n. sp. (?) Ostrea titan Conrad. Pecten n. sp. Panopea generosa Gould. Phacoides acutilineatus Conrad. Psaminobia n. sp. Solen Curtis Gould. Spisulaoceidentalis Gabb. Tellina cf. congesta Conrad. Tivela n. sp. Tellina n. sp. Yoldia impressa Conrad. Thraeia cf. trapezoides Conrad. Gastropoda: Agasoma gravida Gabb. Ancillaria lisbii Gabb. Batliylouia keepi Arnold. Crepidula sp. cf. pra;rupta Con- rad. Calyptrsea cf. costellata Con- rad. Gastropoda — Continued. Calliostoma n. sp. Cancellaria n. sp. cf. heuipbilli. Cancellaria condoni Anderson. Fusus stanfordensis Arnold. Gyriniiiui niathew.soni Gabb. Molopborus biplicata Gabb. Neptunaea recurva Gabb. Ocinebra n. sp. Opalia n. sp. Poly n ices (Neverita) eallosa Gabb. Polynices n. sp. Scalaria n. sp. Terebra cooperi Anderson. Turritella aff. ocoyana Coni'ad. Turritella variata? Conrad. Tliais (Micella) prsecursor Dall. Scaphopoda: Dentaliura n. sp. Cirripedia: Balanus sp. Cephalopoda: Aturia n. sp. Antbozoa: Reef coral sp. Vertebrata : Fish scales and bones. This fauna is so different from that of the underlying Tejon that the two formations are probably unconformable. The failure to recognize a structural discordance at the base of the Sobrante sandstone does not preclude the existence of such a discordance, for the exposures of the underlying Tejon do not fully reveal the structural relations of these rocks. CLAREMONT SHALE. The Claremont shale, which is named from Claremont Creek, in the Concord quadrangle, represents the earliest appearance of the bituminous shale in the Montere}^ group. It occurs not only in the Sobrante and Pinole Valley anticlines but appears in bold and persistent outcro])S along the crest and northeastern edge of the Berkeley Hills. In the Sobrante anticline it is in part soft and distinctly shaly or chalky and in places contains a large admixture of line detrital material, but in the Berkeley 80 Hills it is notably cherty, consistiDg of beds of hard, flinty chert alternating at regular intervals with partings of shale. (See PI. V.) This belt in the Berkeley Hills extends in unbroken continuity across the southwestern part of the Concord quad- rangle and across the northeast corner of the Haywards quadrangle. The bituminous Claremont shale in the Berke- ley Hills rests upon the Chico formation, no Eocene strata intervening, and it has a thickness of about 1000 feet. The Sobrante sandstone, if present, is represented only by a thin bed of yellowish friable sandstone, difficult to observe in the field and too slight a feature of the stratigraphy to be repre- sented on the map. The stratigraphic relations indicate a well-marked unconformity between the Monterey in general and the older rocks. OURSAN SA>'DSTOJJE. The Oursan sandstone, named from Oursan Kidge, in the Concord quadrangle, is a rather fine grained rock. It outcrops parallel to the Claremont shale in the Sobrante and Pinole Valley anticlines and flanks the Claremont belt in the southern part of the Berkeley Hills, having been removed by erosion from the northern part prior to the deposition of the next higher series of rocks. It persists southeastward as a belt traceable witli some uncertainty across the northeast corner of the Hay- wards quadrangle. In its northwestern exposures it flanks the anticline that extends from the Concord into the San Francisco fpiadrangle on tlie north side of Pinole Valley. TICE SHALK. The bituminous shale to which the name Tice shale is here applied parallels the Oursan sandstone in both the Sobrante and the Pinole Valley anticlines along the northeast flank of the Berkeley Hills, in the southern part of the Concord quad- rangle and the northwestern part of the Haywards quadrangle, and in the vicinity of the town of Walnut Creek. It is a per- sistent formation of bituminous shale, prevailingly chalky, in some places whitish, in others pinkish or yellowish, and has been named the Tice shale from its exposures along Tice Creek, in the Concord (juadrangle. 81 HAMBRE SANDSTONE. The Hambre sandstone, named from the Arroyo del Hambre, in the Concord quadrangle, has a wider distribution. Besides Hanking the Sobrante and Pinole Valley anticlines in parallel outcrop to the lower formations, it has an extensive outcrop along the southwest flank of the Franklin anticline, where it is the lowest formation of the Monterey group and lies next to the Martinez and Tejon formations. The Hambre sandstone appears also in the heart of the sharply appressed anticline of Las Trampas Ridge and in the axis of an anticline that lies north of Lafayette Ridge. It occurs also in tlie Walnut Creek syn- cline. The formation everywhere consists of medium-textured, slightly ferruginous sandstones with some sandy shales. RODEO SHALE. The bituminous shale to which the name Rodeo shale is here applied occurs at a strongly marked stratigraphic horizon in the northwest quarter of the Concord quadrangle and is also exposed in the adjoining part of the San Francisco quadrangle. Tlie formation is named from Rodeo Creek, in the Concord quadrangle. The shale is mostly chalky and more or less stained with oxide of iron but is locally cherty. Its chief exposures are on the flanks of the Sobrante and Pinole Valley anticlines and on the southwest limb of the Franklin anticline. It occurs also in a narrow^ belt, difficult to trace continuously, wdiich incloses the Hambre sandstone in the Las Trampas anticline and appears also on the nortiieast flank of the Berkeley Hills in their extension across the Haywards quadrangle. FOSSILS OP MIDDLE FAUNAL ZONE. The three formations of bituminous shale, the Claremont, the Tice, and tlie Rodeo, the stratigraphic position and distribu- tion of which have been briefly indicated, together with tlie two separating sandstone formations, the Oursan and the Hambre, have sufficient paleontologic community to warrant their segregation as a faunal zone distinct from the Monterey formations above and below. The fossils which thus distin- guish these formations in the middle zone of the Monterey San Francisco— 6 82 group are chiefly the following, the determiDations having been made by Prof. J. C. Merriam : Fossils of the middle faunal zone. Pelecypoda : Area devincta Conrad. Chione mathewsoni Grabb. Chione aff. securus Shumard. Chione n. sp Corbicula cf. duniblei Anderson. Leda coolata Hinds. Hemimactra lenticularis Gabb. Mactra n. sp. Macoma nasuta Conrad. Marcia n. sp. Modiolus rectus Conrad. Panopea generosa Gould. Pandora scapha Gabb. Pecten peckhami Gabb. Pecten andersoni Arnold. Pecten n. sp. Pelecypoda — Continued. Phacoides acutilineatus Conrad. Psammobia n. sp. Ostrea titan Conrad. Tellina oregonensis Conrad. Tellina n. sp. Thracia cf. trapezoidea Conrad. Solen Curtis Conrad. Tirela n. sp. Yoldia submontereyana Arnold. Gastropoda: Agasonia gravida (?) Gabb. Agasoma sanctacruzana Arnold n. var. Cancellaria condoni Anderson. Crepidula sp. indet. Lunatia n. sp. BRIONES SANDSTONE. Character and distribution. — The Briones sandstone, so named from the Briones Hills, in the Concord quadrangle, is the most widely distributed formation of the Monterey group. It is prevailingly a light-colored to whitish well-washed sandstone, in some places pebbly or conglomeratic and in general of coarser texture than the lower sandstones. Many of its strata are abundantly fossiliferous and some of them are veritable shell beds. As it is a cleanly washed quartzose sandstone it yields only a light and generally a thin soil. It is one of the chief geologic features of the Concord quadrangle, forming a belt that extends diagonally across it from northwest to southeast. It flanks the combined Bear Creek and Pinole Valley anticlines and is the most conspicuous formation of Las Trampas Ridge, several of its beds being hard and resistant and standing out as prominent ridges or as ribs on the walls of the transverse canyons. This sandstone forms tlie crest and upper slopes of Kocky Ridge, one of the boldest features of relief in the Con- cord quadrangle. Hercules shale member. — The continuity of the deposition of the Briones sands was interrupted in the northern part of the 83 quadrangle by the deposition of sediments such as make up the bituminous shale. As this particular deposit of shale is nonpersistent it is mapped not as a distinct formation but as a shale member of the Briones sandstone and is named the Her- cules shale, from Hercules station, on San Pablo Bay. FOSSILS OF UPPER FAUNAL ZONE. The fossils listed below, whicli were identified by Prof. J. C. Merriam and Dr. Bruce Clark, are common in the Briones sandstone or the upper faunal zone of the Monterey group: Fossils of the upper faunal zone. Echinodermata : Scutella breweriana R^mond. Pelecypoda : Area trilineata Conrad. Cardium quadrigenarium Con- rad. Cardiuni corbis Martin. Chione securus Shumard n. var. Cryptomya ovalis Conrad. Diplodonta harfordi Anderson. Dosinia n. sp.? Dosinia cf. -vvhitneyi Gabb. Marcia oregonensis Conrad. Metis alta Conrad n. var. Macoma secta Conrad. Macoma n. sp. Modiolus directus Dall {''.). Modiolus rectus Conrad. Modiolus n. sp. Mulinia cf. densata Conrad. Nucula sp. Pandora scapho Gabb. Panopea genero.sa Gould. Pecten crassicardo Conrad. Pecten acutilineatus Conrad. Pelecypoda — Continued. Ostrea bourgeoisii R6mond. Saxidomus nuttalli Conrad. Schizothserus nuttalli Conrad. Solen sicarius Gould. Solen curtis Conrad. Siliqua lucida Conrad. Spisula albaria Conrad. Spisula catilliformis Conrad. Tivela n sp. Tellina 3 sp. Tellina oregonensis Conrad. Yoldia cooperi Gabb. Gastropoda: Calyptrsea filosa Gabb. Cancellaria vestusta Gabb. Cancellaria 2 sp. Chrysodomus 3 sp. Crepidula princeps Conrad. Nassa n. sp. Neverita recluziana Petit u. vj Polynices n. sp. Trophon ponderosum Gabb. Trophon n. sp. PARTLY DIFFERENTIATED MONTEREY STRATA IN THE CONCORD QUADRANGLE. In the northeastern part of the Concord quadrangle there are two other belts of strata which belong to the Monterey group and which appear to represent more nearly persistent shallow- water deposition. The bituminous shale in these belts is meager in amount and prevailingly sandy, so that in this area the group can not be subdivided into formations by its petrography as easily as in the more central portion of the quadrangle. 84 One of these belts, which is flanked on both sides by the Tejon formation, lies in a synclinal trough between Martinez Ridge and Pacheco. The fossils collected in this belt indi- cate that it includes both upper and lower Monterey. The lower and upper faunal zones are well represented, but the middle zone is represented only by a comparatively thin sandy bituminous shale, which may be the equivalent of the entire fivefold alternation of sandstone and bifuminous shale found a few miles farther west or any part of it but which is mapped as the Tice shale. The second belt forms the crest of Shell Ridge, a northwest- ern spur of Mount Diablo, constituting the northeastern limb of an overturned syncline. This, like the belt near Pacheco, also apparently represents all the deposits of Monterey time but is deficient in bituminous shale, including only one shale for- mation. The character of the rocks in this belt indicates that this part of the basin of deposition was only once removed by subsidence far enough from the shore to permit the deeper- water sedimentation represented by the bituminous shale. This body of bituminous shale has been mapped as the Tice formation. UNDIFFERENTIATED MONTEREY STRATA IN THE TAMALPAIS QUADRANGLE. The only other Monterey strata that remain to be noted form tiie thick body of bituminous shale that occupies the peninsular ridge west of Bolinas Lagoon and Olema Creek, in the Tamal- pais quadrangle. Neither the bottom nor the top of the Mon- terey group is here exposed, the bottom being deeply buried and outcropping only in the quadrangle to the north, on the Point Reyes Peninsula, and the top having been completely removed by erosion. Moreover, so far as has been observed, the shale here includes no intercalated sandstones. The rocks consist of fine-grained soft or chalky shales, which are locally strongly bituminous, the bituminous matter being at some ])laces so abundant that it colors the rocks deep brown or even black. In other places the shales are prevailingly of a faded piirplisli or creamy color. The lower part of the section con- tains evenly laminated cherty varieties of shale, and the section 85 includes also small concretionary masses of impure limestone, the largest 2 feet in diameter. The strike of the beds is pre- vailingly that of the ridge but is locally complicated by folding, which makes it difficult to estimate the thickness of the rocks in sections normal to the general strike. The southern end of the ridge is in general anticlinal, the beds on the north- east side of the ridge dipping toward Bolinas Bay and those on the southwest side dipping toward the Pacific. The total revealed thickness is probably not less than 2000 feet. If this body of bituminous shale represents all the shale formations of the middle part of the Monterey group in the Concord quadrangle, as it probably does, the conditions that prevailed in this part of the basin of deposition were the com- plement of those that prevailed in the belts near Pacheco and at Shell Ridge, in the Concord quadrangle, for the sea floor here in the middle of Monterey time was apparently never brought close enough to the shore by uplift to permit an influx of littoral sands. The continuity of bituminous-shale deposi- tion in this region was thus unbroken and produced a single formation, which corresponds probably to five or more forma- tions in the Concord quadrangle. BASALT. Within the Santa Cruz quadrangle there are several masses of basalt and diabase, which have been closely studied by Haehl and Arnold,"' who have determined that these rocks were the products of a volcanic eruption that occurred in Miocene time. Small masses of similar basalts occur also at and near the south- ern boundary of the San Mateo quadrangle. These may be erosional residuals of flows that rest u])on the worn surface of the Franciscan rocks, but the relations shown in structure sections F-F and G-G suggest that they may be intrusive. They are of post-Franciscan age and are doubtless the products of volcanic activity of the period to which were assigned the rocks in the Santa Cruz quadrangle, so that they are probably "Haehl, H. L., and Arnold, Ralph, The Miocene diabase of the Santa Cruz Mountains in San Mateo County, Cal. : Am. Philos. Soc. Proc, vol. 43, No. 175, p. 16. 190-4. 86 of Miocene age. These basalts and diabases are fully describ(id by Haehl and Arnold, who present minute details as to their petrographic character and geologic relations in the Santa Cruz quadrangle. SAN PABLO FORMATION. General features. — In the area mapped in this folio the San Pablo formation occurs only in the Concord and Haywards quadrangles. The rocks composing the formation are of marine origin and consist chiefly of medium-grained sand- stones which, where unoxidized are of a pronounced blue color. This blue sandstone occurs generally in massive beds that show only obscure traces of bedding and weathers typi- cally in very rugged outcrops that have a more or less cavernous appearance. In some parts of the formation the sandstone is admixed with volcanic tuff", and at a few localities thin beds of tuff are intercalated with the sandstones. Certain beds of the blue sandstone are richly fossiliferous and yield a fiuina that is different from that of the Monterey, on which the formation rests, and from that of the distinctly Pliocene formations of the region. The same assemblage of fossils occurs in other beds of sandstone that is regularly stratified, that is not blue, and that can not easil}^ be distinguished from the sandstones of the Monterey group. Disiribution. — The San Pablo formation is rather widely distributed in the Concord quadrangle, lying in several belts. One of these belts is in a synclinal trough east and southeast of Walnut Creek, a second is in a syncline that extends north- westward from Walnut Creek to the northern limits of the quadrangle and thence to San Pablo Bay, and a third runs diagonally across the central part of the quadrangle, on the northeast side of the large synclinal trough that contains the Orinda formation. This tliird belt splits into three parts toward the southeast, owing to overthrust faulting, one part following the southeast flank of Las Trampas Kidge, the second lying along the southwest flank of Rocky Kidge, and the third passing northeast of Las Trampas Ridge. In the vicinity of Walnut Creek the syncline in which the San Pablo formation lies is sharply appressed and overturned, 87 so that the San Pablo strata on the northeastern limb of the fold dip under the older Monterey beds at angles ranging generally from 40° to 60° but in some places as low as 30°. The average dip is about 45°. The tliickness of the forma- tion on this limb of the syncline averages about 1400 feet. The outcrop along this limb forms the southwestern crest of Shell Ridge, a spur of Mount Diablo, and is the north- ern limit of a large outcrop that encircles Mount Diablo on the south. The western limb of the syncline outcrops in a belt about half a mile wdde along Walnut Creek, extending from the lower end of San Ramon Valley to Ygnacio Valley. Here beds of the San Pablo dip away from the beds of the Monterey group at lower angles than those displayed by the Monterey beds. In the town of Walnut Creek a bed of fine- textured white volcanic ash, from 1 to 2 feet thick, occurs in the lower part of the San Pablo formation. In the northwestern part of the quadrangle the San Pablo formation occurs as a series of outlying synclinal patches, of low dip, which possibly lie unconformably upon the Briones sand- stone. These are outliers of a Avell-defined synclinal trough of the San Pablo which is well exposed on the shores of San Pablo Bay, in the Napa quadrangle. The largest exposure of the San Pablo formation forms a belt that lies between the Monterey and the Orinda, on the northeast side of the dominant syncline of the quadrangle. Southeast of Lafayette this belt broadens and is synelinally folded. Its distribution is determined by a series of feults, which are shown on the map. Thichiess. — The thickness of the San Pablo formation, which may be best determined in the sections exposed on the shores of San Pablo Bay, in the Napa quadrangle, is about 1700 feet. On the west flank of Las Trampas Ridge it seems to have about the same thickness. In the Walnut Creek syn- cline about 1400 feet of strata are exposed. At the north end of Las Trampas Ridge and in the northwest corner of the Concord quadrangle only remnants of synclinal troughs are preserved. Age. — The age of the San Pablo formation is determined partly by its superposition upon the Monterey in relations that 88 indicate unconformity and partly by the fossils it contains. Although the structural discordance indicated is apparent at only a few places in the area here discussed, it is clearly evident in other parts of the Coast Ranges, where the Monterey forma- tions were disturbed and eroded and the configuration of the basins of deposition was radically changed Ijefore the sediments that formed the San Pablo were laid down. The San Pablo fauna has been considered Miocene by nearly all paleontolo- gists who have studied it, but C. E. Weaver, wlio several years ago studied the fauna in the middle Coast Ranges, referred it to the Pliocene because he identified with living forms 41 species, or 56 per cent, of the 73 species in the fauna examined by him. It is interesting to note that he found twice as many San Pablo forms that persist to the present day as have been discovered in the Merced, or later Pliocene — a fact that throws light on the vicissitudes of the region in Tertiary time. Bruce Clark's more recent and still unpublished investigations of the paleontology of the San Pablo formation have, however, greatly reduced the percentage of living forms and indicate a Miocene rather than Pliocene age for its fauna. The Santa oMargarita formation in the Santa Cruz quad- rangle and the more southern part of the Coast Ranges is probably the equivalent of the San Pablo, for it contains practically the same fauna and its stratigraphic relations are the same. F'ossih. — The following list of fossils has been revised by Dr. Bruce Clark, who refers the fauna to the Miocene and con- siders it more closely related to the Monterey than to the Merced fauna : Fossils of the San Fab lo formation. Echinodermata: : Pelecypoda— Continued. Astrodapsis turaidus R6mond. | Caidiuiu corbis Martyn. Astrodapsis whitneyi R(''iiiond. Chama pellucida Conrad. Scntella gabbi R6niond. Chione 2 n. sp. Astrodapsis 4 n. sp. i Dosinia 3 n. sp. Asteris remondi Gabb. ' Diplodonta orbella Gould. Pelecypoda: Amiantie 2 n. sp. Cardium quadrigenariuni Con- Macoiiia 4 n. sp. Macroeallista newcombiana Car- penter. rad. j Mactra n. sp. 89 Fossils of the San Pablo foj-mation— Continued. Pelecypoda — Continued. Modiolus n. sp. Mulinia densata Conrad. Miiliiiia n. sp. Mytilus 3 n. sp. Paphia stauiinea Conrad. Pecten pabloensis Conrad. Peeten crassicardo Conrad. Pecten estrellanus Conrad n. var. Pecten 5 n. sp. Petricola n. sp. Phacoides richthofeni Gabb. Phacoides tenuisculpta Carpen- ter. Pitaria 2 n. sp. Nucula conradi Dall. Platyodon cancellatus Conrad. Sanguinolaria alata Gabb. Sanguinolaria nuttalli Gabb. Solen Curtis Conrad. Solen sicarius Gould. Spisula catilliformis Conrad. Spisala abscissa Gabb. Spisula cf. falcata Gould. Siliqua lucida Conrad. Pelecypoda — Continued. Yoldia n. sp. Zirphaea dentata Gabb. Gastropoda: Calliostoma 2 n. sp. Calyptriea filosa Gabb. Calyptriea inornata Gabb. Calyptrsea 2 n. sp. Crepidula onyx Sowerby. Crepidula n. sp. Polynices (Euspira) 2 n. sp. Polynices (ISTeverita) recluziana Petit n. var. Bittium 3 sp. Chrysodomus 4 n. sp. Heniifusus n. sp. Leptothyra n. sp. Littorina remondi Gabb. Littorina n. sp. Ocinebra 2 n. sp. Priene n. sp. Ranella n. sp. Tegula 3 n. sp. Trophon carisaensis Anderson. Trophon [)onderosum Gabb. PLIOCENE SERIES. LEONA RHYOLITE. Distribution and character. — The formation for -which the name Leona rhyolite is here nsed is a lava that forms a dis- continuous helt along the west front of the Berkeley Hills, in the Concord and Hay wards quadrangles, from Hamilton Gulch, in Berkeley, nearly to Decoto, a distance of 21 miles. It reaches its maximum width a little south of Leona Heights, in the Concord quadrangle, where it is about a mile and a half wide. The formation is named from Leona Heights. The rock varies in appearance from place to place but is in general an acidic or rhyolitic lava, though it includes local masses of darker, more basic rock, which in the field can not easily be segregated from the general mass. When fresh or unweathered the rhyolite is a light bluish-green compact rock, feebly porphyritic and studded with minute crystals of pyrite, and contains apparently no ferromagnesian silicates. It appears very massive and generally shows no flow structure, 90 but in its larger exposures it is traversed by rather irregular joints and cracks. In certain local facies it is amygdaloidal and some of the vesicles are drawn out. In other local facies it is of coarser texture and has a granular structure. Under erosion this rock forms steep slopes that are resistant to degra- dation, so that its profiles present a contrast to those of the formations with which it is associated. Under the weather the rock disintegrates both mechanically and chemically. The mechanical alteration yields a soil charged with small, sharply angular fragments; the chemical alteration is due to the oxida- tion of the pyrite to limonite and hematite and to the decom- position of the silicates of the rock by the sulphuric acid formed by this oxidation. The general result is that the outcrop and soil are reddish or yellowish brown, but in many prominent outcrops where the iron oxides have been leached out the rock is yellowish white. A microscopic examination of typical speci- mens of the least decomposed facies of the rock shows that it consists of a microcrystalline aggregate of quartz and feldspar in which are embedded a few small phenocrysts of orthoclase, oligoclase and andesine and, more rarely, corroded phenocrysts of quartz. It includes also some polysomatic quartz that grades into and interlocks with the groundmass, but this may be sec- ondary. It contains a few slender prisms of apatite and crys- tals of zircon and, in addition to the pyrite, some small crystals of magnetite. The decomposition of the rock forms much white isotropic, nearly opaque earthy material in the groundmass, tlirough which chlorite is usually disseminated. This material is prob- ably residual amorphous silica. Tlie chlorite is not derived in place from preexisting ferromaguesian silicates but has migrated through the rock. Secondary quartz, calcite, and chlorite fill minute cracks. The abundant chlorite makes the rock dark. The feldspars are cloudy and in part silicified. Many of the crystals of pyrite, both the cubes and the pentag- onal dodecahedra, are surrounded by tlie white earthy mate- rial above mentioned. Some of tbe pyrite occurs partly in tlie feldspars, which bear allotriomorphic relations to them, indicating that the pyrite may perhaps he an original constit- 91 uent of the rock. Other occurrences of pyrite are, however, probably of secondary origin. Chemical composition. — Tlie chemical composition of the rock is shown in the following "analyses of samples selected from collections made near Laundry Farm: Analyses of Leona rhyolite. SiOe - TiO, . Al,03 FeO.- MnO. 71.00 CaO BaO MgO NagO K,0 P2O5 SO3 CO, FeS, CI H3O (at 110° C.) Ignition 12.70 .66 2.44 ,90 1.39 6.45 2.99 .28 1.56 100. 37 72.12 .17 11.49 2.77 2.30 .10 .94 Trace. .85 5.28 .86 1.15 .03 Trace. .06 .96 71.60 .12 11.93 3.00 3.40 .09 .52 1.33 4.62 1.90 ,62 ,08 .38 99.96 .07 ,55 100. 21 1. Analyzed by C. P. Richmond in the laboratory of the University of California. 2 and 3. Analyzed by Gf. E. Colby in the laboratory of the University of California. The high content of silica, the low content of lime, and the proportion of soda to potash indicate clearly that the rock is a soda rhyolite; but the fact that all the material is more or less altered precludes the possibility that these analyses may repre- sent the true composition of the original rock. Some of the ferric oxide in samples 2 and 3 is doubtless contained in hematite produced by the oxidation of pyrite. Sample 1 was practically free from pyrite. Chemically it closely resembles 92 the Northbrae rhyolite, to be described under the next heading, but reasons are there given why it is regarded as a separate lava flow. Field relations. — This rhyolite is a lava which lies indiffer- ently on the several formations of the Franciscan group, on the rocks intruded into it, and on the Knoxville formation. The rhyolite belt follows in a general way the line of the Knoxville- Franciscan contact. It is evident that before the extrusion of the lava the Cretaceous rocks had been deeply eroded and in part completely stripped from the underlying Franciscan. The linear disposition of the rock is probably due to the fact that the lava flowed along a valley that followed the contact. At some places the rhyolite lies on a pebbly conglomerate that probably represents the river gravels of the valley into which the lava flowed. The belt of rhyolite is in general coincident with the fault zone of the Haywards rift, and in this zone there are many minor or auxiliary faults, some of which traverse the rhyolite, so that in some places the boundaries of the formation are fault contacts. Several faults cut entirely across the rhyolite, particularly at Hamilton Gulch, in Berkeley, and near Temescal Lake, in the Concord quadrangle, and at San Leandro Can- yon, in the Haywards quadrangle. Age. — The age of the rhyolite can not be determined pre- cisely. Its superposition in certain places upon the Knoxville formation clearly indicates that it was laid down after a post- Cretaceous erosion and is therefore Tertiary or Quaternary. The western flanks of the rhyolite belt are covered by rather early Quaternary alluvium, so that the rhyolite is doubtless Tertiary in age, but to what division of the Tertiary it belongs is largely a matter of conjecture. Analogy with other localities would suggest that it is of late Tertiary age, probably Pliocene. Since it is of about the same age and of the same chemical composition as the Northbrae rhyolite, it might be regarded as part of the same lava flow if it were not for certain physical differences, to be described later. 93 NORTHBftAE RHYOLITK. Distribution. — On the western slope of the Berkeley Hills north of Berkeley there are numerous isolated patches of a white rhyolite lava, apparently the remnants of a flow, the greater part of which has been removed by erosion. The area covered by these patches is about 5 miles long and about 1 mile wide, and the thickness of the rock probably nowhere exceeds 100 feet. This rock is named the Northbrae rhyo- lite from the district of that name near Berkeley, in the San Francisco quadrangle, where it is most abundant, and should be distinguished from the Leona rhyolite, found southeast of Berkeley. It lies indifferently upon the worn surface of the Franciscan and of the Cretaceous formations. Dikes of similar rock that cut the Franciscan formations may have been com- posed of the same molten rock that formed this lava. The rhyolite is overlain by conglomerate, which is referred to the Campus formation, and its relation to the Orinda formation at the northern end of the area indicates that the Orinda also overlies it. It may safely be regarded as of pre-Orinda or early Orinda age. Correlation. — Its position suggested that the rhyolite might be the massive correlative of the pumiceous Pinole tuff, which farther north occurs at the base of the Orinda formation and in Sonoma County at the base of the Merced formation. An analysis by George E. Colby of a sample of the Pinole tuff obtained near Cordelia is given in the table on page 94 with analyses of the Northbrae rhyolite made by Charles Palache. The comparison does not support the suggestion afforded by stratigrapliic considerations. Pctrographic features. — The Northbrae rhyolite has been studied and described in detail by Palache." In some places it shows pronounced flow structure, locally marked by vesicu- lation and layers of small spherulites. In other places it con- tains hollow spherulites, the largest several inches in diameter, which make up a considerable part of the otherwise glassy rock. One facies of the rhyolite is holocrystalline and porphy- "Palaclie Charles, California Univ. JJept. Geology Bull., vol. 1, ISo. 2, pp. 61-73, 1893. 94 ritic. The groundmass is a fine aggregate of quartz and feld- spar and the phenocrysts are corroded quartz, orthoclase, and acidic plagioclase. Tiie rock contains no ferromagnesian sili- cates but in places includes a little magnetite. Comparison of analyses of Pinole tuff and Northhrae rhyoUte. SiOs TiO, AI2O3 FeoOa FeO MnO CaO MgO Na,0 K,0 CI H^OCatllO" C.) Ignition 65.40 .55 15.35 2.10 1.23 .05 1.12 .60 2.07 3.21 .01 1.18 7.00 100. 04 75.46 13.18 .91 .95 .10 6.88 1.09 ,93 99.50 69.85 13.34 .73 .87 Trace. 5.58 2.68 I 6. 15 99.20 1. Pinole tuff near Cordelia. 2. Nortlibrae rliyolite, spherulitic facies. 3. Northbrae rhyolite, glassy facies. The Northbrae rhyolite is chemically similar to the Leona rhyolite and is probably of about the same age. It differs physically, however, in showing flow structure, w^ith spheru- lites, in being glassy in certain facies, and in containing no pyrite, the presence of which is a striking feature of the Leona rock. This physical contrast indicates that the two rocks are not different parts of the same lava flow. PINOLE TUFF. General featiores. — The Pinole tuff occurs only in small exposures in the Concord and San Francisco quadrangles but is more extensively exposed in the Coast Ranges farther north. The formation is named from the town of Pinole, on San Pablo Bay, near which it is well exposed. Stratigrapliically it lies chiefly between the San Pablo formation and tlie Orinda forma- 95 tion, but it is in part interbedded with the basal sediments of the Orinda and therefore appears to have been associated in its deposition with the Orinda rather than with the San Pablo. The San Pablo is a marine formation, whereas the Pinole tuff on the shores of San Pablo Bay, immediately north of the San Francisco quadrangle, contains fresh-water fossils and the bones of terrestrial mammals, thus allying it with the Orinda and indicating that it represents the first deposits laid down in the basin that contained the Orinda lake. The same kind of tuff, moreover, occurs in the fresh-water Orinda formation in Sobrante Pidge, in the northern part of the San Francisco quadrangle and in the Mount Diablo quadrangle, indicating that the volcanic activity which gave rise to the materials of the tuff persisted into the early part of the Orinda epoch. The Pinole tuff, though occupying a well-defined stratigraphic posi- tion between the San Pablo and the Orinda in several sections, is not so persistent as either of these formations. On San Pablo Bay it has a thickness of about 1000 feet, but southeast of this bay, in the San Francisco and Concord quadrangles, its volume abruptly diminishes and it thins out and disappears in the eastern part of the Concord quadrangle. It was evidently laid down in a fresh-water basin which displaced, probably after an interval of erosion, the marine basin of San Pablo time, so that there is warrant for assuming disturbances at the close of the San Pablo epoch sufficient to cause an unconformity between the San Pablo and the Pinole formations, although no angular discordance between the tw^o formations has been detected. Petrograph'ic character. — In most of its deposits the Pinole tuff is distinctly stratified and appears to have been assorted by currents of water. It consists almost wholly of whitish or light-yellowish pumice, partly in fragments ranging in size from 1 to 50 millimeters and partly in fine dust, the pumiceous character of which can be observed only by means of the microscope. The general absence of quartz from the tuff and the fact that at some places it contains fragments of andesite indicate that the formation as a whole represents the froth of an andesitic magma wdiicli was scattered over a wide expanse of country by violent explosions from volcanoes whose site is 96 not yet known but which probably lay north of the Bay of San Francisco. Distribution. — The most Extensive exposures of the Pinole tuff in the Concord quadrangle are on the flanks of the over- turned syncline that lies east of Walnut Creek. Here the outcrop of the underlying San Pablo on the west side of the syncline is paralleled by a belt of the tuff for 5 miles, and that on the east side is paralleled by a similar belt for about Si- miles. The tuff on the west limb of the syncline dips away from the San Pablo in the vicinity of Walnut Creek at an angle of about 45°, and that on the east limb of the syncline dips under the San Pablo formation at an angle of 45^. The tuff on the west limb has a maximum thickness of probably 100 feet. At the north end of this exposure it is cut off by the alluvium of Ygnacio Valley, and at the south end it abuts abruptly against an east-west fault. Its maximum thickness on the east limb of the syncline is perhaps 150 feet, but it thins out and disappears before it reaches the eastern boundary of the quadrangle. Another deposit of the Pinole tuff lies at the northwest end of Las Trampas Ridge, where it forms a simple open synclinal trough, having the San Pablo beneath it and the Orinda above it. Around the edge of the syncline the rocks dip from 20° to 45^ toward the axis of the fold, which strikes northwest. The thickness of the tuff here does not exceed 50 feet. Between Grizzly Creek and Las Trampas Creek, on the west side of Las Trampas Ridge, the Pinole tuff crops out with a northerly strike and westerly dip and evidently represents the aiiticlinal correlative of the syncline just described, which, however, has been broken by an axial fault. In tlie northeast corner of the San Francisco quadrangle tliere are two areas of the Pinole tuff, one on each side of Sobrante Ridge. One of these is on the east side of Pinole Valley, where it presents a bold outcrop along a low ridge al)out a mile long. The strike of the tuff strata here is a little west of north and is transverse to the strike of the Monterey formations, Avhicb al)ut upon it. The tuff here has evidently been brought against the Monterey by a fault that coincides in 97 trend with Pinole Valley, so that the mass is an outlying remnant that has bf^en preserved from erosion. The outcrop on the southwest side of Sobrante Kidge is about 2 miles long, the strata having a northwesterly strike and a low dip to the southwest and being interstratified with the basal beds of the Orinda formation. The thickness of the tuff here prob- ably does not exceed 50 feet. Tt is remarkable that in neither of these localities is there any trace of the Ban Pablo formation between the Pinole tuff and the rocks of the Monterey group, although the San Pablo is represented by about 1500 feet of strata a few miles farther north, on the shores of San Pablo Bay, and is equally well represented farther southeast, in the Concord quadrangle. This absence of the San Pablo from its normal stratigraphic horizon indicates either that the uncon- formity between the Pinole tuff and the San Pablo formation represents a longer time than might be inferred from their relations in other localities in these quadrangles, the wdiole of the San Pablo having been removed by erosion before the tuff was deposited, or that the San Pablo was not deposited in the region of Sobrante Ridge because that region was a land area in San Pablo time. ORINDA FORMATION. General features. — The Orinda formation, which is confined to the Concord and San Francisco quadrangles, is named from Orinda, in the Concord quadrangle. It forms a broad belt that traverses the middle of the Concord quadrangle from southeast to northwest and extends across the northeast corner of. the San Francisco quadrangle. Another belt, broadening toward the southeast, wdiere it passes into the Mount Diablo quadrangle, lies in the overturned syncline east and southeast of Walnut Creek. The formation consists of a thick accumulation of fresh-water beds comprising (1) conglomerates that include waterworn polygenons pebbles, few larger than a man's fist, and in places strongly cemented; (2) light-colored sandstone; (3) blue, gray, and brown clay shales; (4) limestones; (5) some thin seams of lignite; and (6), at a few horizons, thin layers of brown decomposed volcanic tuff. Farther east, in the Mount San Francisfo— 7 98 Diablo quadrangle, the formation includes a few beds of pumiceous tuff similar to tlie Pinole tuff, in its lower part. Fresh-water ostracodes are found at certain horizons in the clay shale and the sandstone. The limestone is of two kinds. Certain beds are made up wholly of remains of ostracodes; in others the rock is dense or compact, is light gray or bluish in color, and, instead of ostracodes, contains fresh-water mol- lusks, such as Limn^ea, Physa, and Planorbis. This variety is generally cherty, and the chert may be irregularly dis- tributed through the limestone or may be interlaminated with it. Here and there the remains of mollusks are found in the chert. The limestone beds are thin and in the aggregate constitute probably less than 1 per cent of the formation. In San Pablo Ridge, in the San Francisco quadrangle, toward t]ie northwest end of the main belt, the formation is from 2000 to 2500 feet thick, but ferther southeast, near the south- ern boundary of the Concord quadrangle, its thickness increases to about 6000 feet. Stratigraphic relations. — The Orinda formation lies conform- ably upon the Pinole tuff, but southeast of Walnut Creek, where the tuff thins out and disappears, and elsewhere along the eastern border of the area of its exposure, it rests directly upon the San Pablo formation. In a large part of its outcrop in these quadrangles, however, it is underlain by neither the Pinole tuff nor the San Pablo formation, for it rests with marked angular discordance upon the worn edges of either Monterey or Franciscan strata. The general structure of the main belt of the Orinda is that of a great synclinal trough witli subordinate folds. One of these folds forms a syncline along San Pablo Kidge east of Berkeley, in the trough of which lies a later series of volcanic rocks. Between this syncline and the main trough of the Orinda there is a low anticline on the northeast flanks of San Pablo Ridge. Anotlier subordinate syncline, on the other side of the main trougli, determines the course of Pleasant Valley, northwest of Lafayette, and outlying remnants of this syncline of Orinda rest upon the Pinole tuff in small canoe-shaped troughs at the northwest end of Las Trampas Ridge. 99 The Pleasant Valley syncliiie is separated from the main trough by a broad anticline that pitches southeastward and can not be followed far. In the southern part of the Concord quadrangle, near the head of Las Trampas Creek, the main belt of the Orinda bifurcates at the end of an upthrust fault block of Monterey and San Pablo strata which constitutes the high and bold Rocky Ridge, one portion following the course of Bolinger Canyon to the eastern limit of the quadrangle and the other flanking Rocky Ridge on the southwest and extend- ing southeastward across the northeast corner of the Haywards quadrangle. The synclinal trough of the Orinda southeast of Walnut Creek is a simple fold, which, with the underlying strata already described, has been overturned by overthrust pressure from the northeast, so that the Orinda beds on the northeast flank of the syncline dip under the Pinole tuff at an angle of about ?§'. The strata in certain low gravel-strewn hills in the northeast corner of the Concord quadrangle are also referred to the Orinda as a result of studies made by C. E. Weaver in the adjoining Napa quadrangle, where their character is better displayed. Fossils. — Besides the ostracodes of the Orinda formation, the following fresh-water mollusks have been recorded by J. G. Cooper^' from the west side of San Pablo Creek, along the road from Berkeley to Lafayette: Anodonta uuttal liana Lea var. ! Limnsea contracosta Cooper, lignitiea Cooper. ! Planorbis pabloanus Cooper. These fossils were found in association with a thin seam of lignite. The first is a living species; the two others are extinct. Tlie following fossils, described by Cooper, were collected north of Livermore, from beds that are probably the extension of tiie Orinda formation across the Mount Diablo quadrangle into the Pleasanton quadrangle. "Cooper, J. G., On some Pliocene fresh-water fossils of California: California Acad. Sci. Proc, 2d ser., vol. 4, p. 169, 1894. 100 Bythinella binneyi Tryon. Carinifex newberryi Lea. Cochliopa rowelli? Tryon. Limnophysa palustris Linn6. Liiunophysa desidiosa Say. Menetus opercularis Grould. Gyraulus vermicularis Gould. Phy.sa diaphana Tryon. Helix californiensis Lea. j Pisidium occidentale NeAvberry. Limnophysa humilis Say. I Pompholopsis whitel Call. The following vertebrate remains, identified by Prof. J. C. Merriam, have been found in the Orinda formation near the upper part of Las Trampas Creek and in Bolinger Canyon, in the Concord quadrangle: Right superior molar of Hipparion speciosum. Left superior molar of Hipparion speciosum. Radius, ulna, carpus, and metacarpus of a camel. Left superior molar of a camel. Metapodial of undetermined form. Pelvis of undetermined form. Age. — The Orinda formation can with confidence be assigned to the Pliocene epoch, for it lies stratigraphically above the Pinole tuff, which is Pliocene. Its correlation with other Plio- cene formations of the Coast Ranges is, however, somewhat more diflficult. The mammalian fossils recorded above, frag- mentary as they are, indicate older Pliocene having affinities w^ith the Miocene, and their consideration has led to the opin- ion, expressed in earlier papers, that the Orinda antedates the Merced formation of the Pacific coast, Avhich has usually been held to represent later Pliocene, but the w^ork of Osmont" in the Coast Ranges north of the Bay of San Francisco has shown that the Sonoma tuff of that area is intercalated in the lower portion of the Merced. Work done by C. E. Weaver in the Napa quadrangle, the results of which are not yet published, and the writer's review of the whole field since Osmont's and Weaver's work Avas done show further that the Sonoma tuff is probably identical stratigraphicall}^ as well as petrographically with the Pinole tuff, which underlies the Orinda formation. If this is true the Orinda should obviously be correlated with the INIerced on grounds w^hich are independent of the evidence afforded by the fossils obtained from tlie two formations. "Osmont, V. C, A geological section of the Coast Ranges north of the Bay of San Francisco: California Univ. Dept. Geology Bull., vol. 4, No. 3, pp. 89-37, 1904. 101 According to this view the Orinda is the fresh-water equivalent of the Merced, laid down in an interior diastrophic trough and shut off from the sea by a ridge corresponding in position and trend with the present Berkeley Hills. MERGED FORMATION. General character and distribution. — The Merced formation, named for Lake Merced, comprises a thick accumulation of marine sediments, which occur chiefly on the San Francisco Peninsula, in the San Mateo quadrangle, but of which there is an outlying patch at and north of Bolinas, in the Tamalpais quadrangle, and a siuiilarly isolated deposit at Miramontes, northwest of Halfmoon Bay, in the San Mateo quadrangle. The Merced is well exposed in the section along the sea cliffs between San Francisco and Mussel Rock, where the maximum height of the cliffs is about 700 feet. The section here shows about 5800 feet of marine clays, sandy shales, sandstone, fine pebbly conglomerates, and shell beds, all of which dip prevail- ingly northeastward at angles ranging from 15° to 75°, though a small part dip more nearly eastward at lower angles, some as low as 5°. Stratigraphic relations. — The basement upon which the Merced formation rests in the vicinity of Mussel Rock is the worn surface of volcanic rocks of the Franciscan group. Between the basement volcanic rock and the basal beds of the Merced is a wedge of post-Franciscan and pre-Merced alluvium, now firmly cemented, composed almost wholly of fragments derived from the underlying volcanic rock. Upon this ancient alluvium lies a layer of forest material, less than a foot thick, comprising carbonized wood, bark, matted leaves, and pine cones of the species Finns insignis, and above this are the marine beds. Cones of the same species have been found in the marine beds a few hundred feet higher in the section, just north of the landslide near Mussel Rock. Still higher are cones of Psendotsuga douglasi. In one of the can- yons east of Mussel Rock, where a small stream has cut down into the formation, several trunks of coniferous trees are exposed, the wood and bark being excellently preserved. 102 These trunks lie in tlie Merced formation, and a sandstone bed a little above them contains numerous remains of marine raollusks. Thin lignitic seams also occur here and there in this section. Toward the upper part of the section there is a bed of white volcanic ash, which consists chiefly of minute fragments of pumiceous glass. The ash contains no quartz and is probably andesitic. It ranges in thickness from a few inches to perhaps 2 feet. On the clifls north of Mussel Rock the actual exposure of the inclined beds measures about 3f miles along the shore, or obliquely across the strike. From the cliffs the beds strilvc southeastward along the southwest side of Merced Valley and have been traced nearly as far as San Mateo. The breadth of the outcrop along this belt decreases steadily to the southeast by the disappearance of the upper ])eds. Everywhere, however, the base of the Merced rests upon the Franciscan. As a large part of the formation is composed of soft beds it is exposed at but few places along the northeastern side of the belt, and it can with difliculty be discriminated from the soft alluvial Quaternary formations and the sand dunes that occupy the adjoining territory. The belt of Merced is limited on the northeast by a fault that drops the formation not less than 7000 feet against the Franciscan rocks of San Bruno Mountain. The actual trace of this fault is, how- ever, obscured by an overlying mantle of Quaternary sands. Toward the northwest it passes beneath the waters of the Pacific, and toward the southeast it passes beneath the Bay of San P'rancisco. On the low coastal ridge known as Miramontes, northwest of Pillar Point, in the southwest corner of the San Mateo quadrangle, a remnant of the Merced forms a belt about 2 miles long and a quarter of a mile wide, striking northwest. The rocks of this belt are chiefly sandstones, sandy shales, and shell beds, in part gently folded in open anticlines and synclines, as may be seen clearly on the wave-cut terrace at low tide, and in part so closely appressed that they stand nearly vertical. At the north end of the belt, near Seal Cove, the basal beds of the Merced formation rest directly upon the granitic rocks that form the shore farther north. Here 103 all the phenomena of a bowldery beach may be seen at the base of the section. North of the Golden Gate, at Bolinas, in the Tamalpais quadrangle, another mass of the same formation, containing typ- ical Merced fossils, rests unconformably upon much-disturbed beds of the Monterey group and dips eastward beneath Bolinas Bay so as to abut upon the Franciscan, against whicli the Merced beds have been faulted by a northwestward extension of the San Bruno fault. Fossils. — The beds of the Merced formation are at several horizons abundantly fossiliferous and have yielded the forms named in the following list. The species were determined by B. M. Martin, who has recently made an exhaustive study of the fauna and who finds that 63 per cent of the forms in the lower part of the formation, constituting its greater part, are those of living species. Fossils of the Merced formation. Echinodermata : Scutella interlineata Stimpson. Peleeypoda : Area trilineata Conrad. Cardium meekianum Gabb. Cardium quadriginorum Con- rad. Cardium corbis Martyn. Chione succincta Valenciennes. Cryptomya californica Conrad. Macoina inquinata Deshayes. Macoma nasuta Conrad. Marcia oregonensis Conrad. Modiola recta Conrad. Mya japoniea Jay. Mytilus edulis Linn6. Nucula superstriata Carpenter. Ostrea lurida Carpenter. Pandora grandis Ball. Paphia staleyi Conrad. Paphia staminea Conrad. Paphia staminea var. diversa Sovverby. Paphia tenerrima Carpenter. Pecten sp. Phacoides annulatus Reeve. Cardium centifilosum Carpenter. Saxidonms giganteus Deshayes. Peleeypoda — Continued. Saxidomus nuttalli Conrad. Siliqua patula var. nuttalli Con- rad. Solen sicarius Gould. Spisula albaria Conrad. Spisula catilliformis Conrad. Schizothjerus nuttalli Conrad. Schizothserus pajaroanus Con- rad. Tellina bodegensis Hinds. Transennella tantilla Carpenter. Zirphsa gabbii Tryon. Gastropoda: Amphissa corrugata Reeve. Astyris californica Gaskoin. Astyris gausapata var. carinada Reeve. Astyris richthofeni Gabb. Bittium asperum Gabb. Cei'ithidea californica Hinds. Chrysodomus portolaensis? Arnold. Chrysodomus stantoni Arnold. Chrysodomus tabulatus Baird. Crepidula grandis ]\lidd. Crepidula onyx Sowerby. Crepidula princeps Conrad. 104 Fossils of the Merced formation — Continued. Gastropoda— Continued. Drillia inerniis Hinds. Drillia mercedensis n. sp. Lacuna compacta Conrad. Margarita pupilla Gould. Monoceros engonatum Conrad. Nassa fossata Conrad. Nassa mendica Gould. Nassa mendica var. cooperi Forbes. Natica clausa Broderip and Sowerby. Olivella biplicata Sowerby. Olivella pedroana Conrad. Gastropoda— Continued. Pachypoma sp. Pisania fortis Carpenter. Pisania fortis var. angulata Arnold. Ttiais canaliculata Duel. Thais crispata Martyn. Thais crispata var. septentrio- nalis Reeve. Tritonium sp. Trochita radians Lamarck. Trochita filosa Gabb. Balanus sp. These fossils establish the age of the formation as late Pliocene. Considered paleoutologically, the upper part of the formation might be regarded as Quaternary, as it is, indeed, by some writers. This upper part, in which the living species of mollusks predominate, shows perfect stratigraphic continuity with the lower part, which is clearly Pliocene, and the forma- tion as a whole antedates the diastrophic movements which deformed the region and ushered in the Quaternary throughout the greater part of California. It therefore seems best to use this diastrophic event as a line of demarcation between the Tertiary and the Quaternary and to class the formation as a whole as Pliocene. BERKELEY GROirp. GKJfEBAL DESCRIPTION A group of volcanic lavas and intercalated fluviatile and lacustral deposits lies on the southwest side of tlie main belt of the Orinda formation in the Concord and San Francisco quadrangles. This group, which was originally named the Berkeleyan, from the city of Berkeley, near which the forma- tions composing it are well exposed, originally included the Orinda (then Orindan) formation, which has now been sepa- rated from it, chiefly to simplify the statement of geologic jelations, the Berkeley group being largely volcanic and lying unconformably on the Orinda formation, which consists chiefly of fresh-water sediments. The l^erkeley group as here defined 105 comprises three formations, which, named in ascending order, are (1) the Moraga formation, consisting chiefly of hivas with subordinate sedimentary beds; (2) the Siesta formation, com- posed of hicustral deposits; and (3) the Bald Peak basalt, which includes very subordinate sedimentary intercalations. MORAGA FORMATION. The Moraga formation, so named from Moraga Valley, in the Concord quadrangle, consists chiefly of flows of andesite and basalt, with which are associated some basic tuffs and beds of well-cemented rhyolite tuff. Between these volcanic rocks lie lenticular beds of conglomerate, clay, and limestone. One of these limestone beds, which contains fresh-water fossils, is 30 feet thick and has lavas above and below it. This bed is well exposed on the northeast side of Siesta Valley, the outcrop extending to Eureka Peak. One of the lenses of conglomerate attains in places a maximum thickness of about 200 feet. The earliest flow of the formation is a rather basic amygdaloidal andesite, which shows great constancy in character and strong persistence in occurrence and which has a uniform thickness of about 50 to 60 feet. The lavas that lie above this flow are less persistent in occurrence and thickness and are therefore more lenticular in form. Bather long intervals evidently elapsed between the successive flows of these lavas, as may be judged by the presence of brick-red laterite at the top of several of the lavas, indicating considerable exposure to the atmosphere before they were buried by later flows. One of these intervals was unusually long, and during its continuation the region was subject to notable degradation, so that when later lavas were poured out upon the surface thus modified they filled the ravines and are therefore discordant with the rocks below them. The first of the flows after this interval was the andesite of Grizzly and Buin peaks. Its discordant relation to the lavas and conglomei'ate of the lower part of the formation is well exposed below Buin Peak, on the south- west side of San Pablo Bidge. The total thickness of the Moraga formation differs from section to section, but its maxi- mum is about 1200 feet. 106 SIESTA FORMATIO]N'. The lavas of the Moraga formation that have just been described appear to have so greatly interfered with the drain- age ways that they formed an extensive lake basin, which lay partly on the surface of the lava and partly on older rocks. In this lake basin were deposited the beds of the Siesta forma- tion, which in places has a thickness of 200 feet. The for- mation was originally named Siestan, from Siesta Valley, in the Berkeley Hills. It comprises fresh-water sandstones that grade into conglomerates, clay shales with seams of lignite, cherty limestones containing abundant fresh-water fossils, and layers of volcanic tuff. In the clays have been found the skull of a beaver {Sigmogompkius lecontei), the teeth of a species of Lepus, and the dentary bone of a species of Lacerta. The same beds contain remains of fresh-water shells belonging to the genera Limnsea, Helix, and Ancylus. In the limestone beds of the Siesta, which in places attain a thickness of 10 to 20 feet, species of Planorbis, Limnsea, and Pisidium are common. Some of these fossils are found in the chert as well as in the limestone. BALD PEAK BASALT. The Siesta lacustral epoch was brought to a close by the flooding of the lake basin with basaltic lavas in a succession of flows that are well exposed on Bald Peak, from which they have been named. These lavas have a maximum aggregate thickness of 300 to 350 feet and constitute the summit of the Berkeley group, but as they are an erosional residual it is impossible to say how much thicker they may have been. Between two of these flows of basalt at the head of Siesta Valley is a short lens of fresh-water limestone. RELATIONS OF THE GROUP TO ADJACENT FORMATIONS. After the volcanic and lacustral rocks of the Berkeley group had accumulated to a thickness somewhat exceeding their present known mass the region was disturbed by a sharp orogenic movement, which bent the Berkeley and underlying strata into a well-marked syncline and a subordinate anticline. 107 Practically all the Berkeley group that is known to-day is the remnant of this synclinal trough, which is sunk into the Orinda and forms a belt extending from Moraga Valley to Wildcat Canyon, a distance of about 9 miles, nearly all of it in the Concord quadrangle. This belt, b}^ reason of its hard volcanic rocks, which adjoin the softer Orinda formation, makes the boldest and highest ridges of the Berkeley Hills. The two limbs of the syncline form parallel ridges, composed of lavas of the Moraga formation, and between them lies a long subsequent valley, bottomed for the most part by the clays of the overlying Siesta formation. Some remnants of rocks of the Berkeley group are also found in the subordinate anticline on the northeastern flanks of San Pablo Ridge. The position of the Berkeley trough on the southwest side of the Orinda belt and the fact that the Orinda is much thicker and more varied in its strata on the northeast side than on the southwest indicate that the Orinda was affected by crustal movements before the lavas of the Moraga formation were poured out and that the Orinda may therefore be separated from the Berkeley by an unconformity. TERTIARY AND QUATERNARY DEPOSITS. SANTA CLARA FORMATION. On the southwest side of Crystal Springs Lake, in the San Mateo quadrangle, is an embankment of ancient alluvium, which is well exposed on the wagon road to Halfmoon Bay. The north end of this embankment lies a few hundred yards northwest of the road along the side of the lake. This appears to be the northern part of a large area of alluvium that has been mapped in the Santa Cruz folio as the Santa Clara for- mation. The embankment at Crystal Springs Lake, which is probably 300 feet thick, is a rudely stratified deposit of angular fragments of rock and waterworn pebbles, few of them larger than a hen's egg. The Santa Clara formation has been referred in part on very slender evidence to the late Pliocene and its beds in the Santa Cruz quadrangle are regarded as a terrestrial chronologic equivalent of the Merced. The correlation with the Merced 108 of some of the deposits mapped in the Santa Cruz folio as the Santa Clara formation is questionable, however, for they consist of Quaternary alluvium and other distinctly fluviatile and lacustral beds of undetermined age. The deposit at Crys- tal Springs Lake closely resembles the San Antonio forma- tion at East Oakland, and parts of the Santa Clara formation that lie farther south are still more like the San Antonio beds, but the data necessary for the geologic correlation of the Santa Clara with other deposits are lacking. QUATERNARY SYSTEM. PLEISTOCENE SERIES. CAMPUS FORMATION. The Campus formation occurs on the ridge of the Berkeley Hills which separates Strawberry Canyon and the head of Wildcat Canyon from the valley of the Bay of San Francisco near Berkeley, in the San Francisco and Concord quadrangles. The formation extends from Strawberry Canyon northwest- ward along this ridge into Wildcat Canyon for a distance of about 3 miles. It is a mile and a quarter wide just northeast of North Berkeley, and from this width it tapers to a point at each end. The formation is made up of fresh-water deposits, fluviatile and lacustral, and various lavas, tuffs, and agglomerates. The basal part of the formation, which has the widest distribution, consists of conglomerates mixed with tuff and sandstones with intercalated beds of clay shale and lenses of limestone. This accumulation is interrupted here and tliere by sheets of andes- ite and small but relatively thick layers of tuff. Above these there is a series of beds consisting largely of tuff but contain- ing conglomerate, lenses of limestone, and small patches of basalt. Above these beds is a tliick series of basalt flows, and lastly, resting on the basalt, there are remnants of a once extensive deposit of rhyolitic tuffs and agglomerates, now well cemented. The formation occupies a trougli wJiich trends about northwest and wdiich lies athwart the northwest end of the 109 older syncline of the Berkeley group — a trough that is formed in part by a synclinal fold and in part by a fault, the fault having dropped the northeast side of the trough against formations of the Berkeley and Monterey groups. This fault- ing appears, however, to have affected only the lower mem- bers of the Campus formation, since the volcanic rocks of the up]^er part of the formation are spread out over the trace of the fault in the upper part of Wildcat Canyon, in the Concord quadrangle, and the displacement must have occurred during the deposition of the formation. The fault in general, how- ever, determines the northeastern boundary of the trough. On its southwestern boundary the basal beds of the Campus formation rest successively upon the Chico, the Knoxville, the Franciscan, and the Orinda. Although the lower part of the Campus formation is faulted down against the Berkeley group, it is apparent from the map that the Berkeley strata had been folded and in large measure degraded before the fresh-water basin in which the Campus formation accumulated had been formed and that an unconformity therefore exists between the Berkeley and the Campus. At the northwest end of the trough the conglomerates of the Campus rest on the Orinda, and as both formations are of much the same character and break down readily into soils it is difficult to discriminate the tw^o, so that the mapping is somewhat doubtful. The Campus trou'gh has been dislocated by a number of faults besides the one that forms its northeastern boundary. The Campus formation is regarded as Pleistocene in age because it lies unconformably upon the Moraga and Siesta formations of the Berkeley group, which is later than the Orinda. The Orinda has been correlated in time wdth the Merced and is therefore regarded as Pliocene. The Campus formation Avas originally named Campan, from the campus of the University of California, within wdiich portions of the for- mation occur. ALAMEDA FORMATION. Beneath the alluvial deposits of Oakland and Berkeley there is a formation of yellow sandy clay, of very uniform fine 110 texture, which has been revealed in numerous excavations and well borings in these cities and is exposed in the trench of Diamond Creek. Without much change in character it passes into beds that carry marine shells, and intercalated with these marine deposits are nonpersistent beds of gravel of fluviatile origin, the conditions indicating delta formation alternating with marine or estuarine deposition. Three wells sunk at the corner of Twenty-eighth Street and Thirteenth Avenue, East Oakland, after passing through the yellow sandy clay with some layers of sand and gravel, struck blue clay at a depth of about 142 feet. This blue clay was 20 feet thick, and at its base were fragments of marine shells. Below this bed was a layer of blue sand, 10 feet thick, and below the sand was clay that extended to the bottom of the well, 190 feet. The same well borer found shells in the same formation at a depth of 110 feet on Eleventh Avenue and "clam" shells at a depth of 125 feet on Fifth Street near Clay Street. In the artesian wells at Roberts Landing, wdiich pierce chiefly clays and sands and some layers of gravel, marine shells were found 133 feet, 148 feet, and 317 feet below the marsh, which stands very close to the level of high tide. The same formation is exposed in the cliffs at the water front of West Berkeley, where it is a blue sandy clay, super- ficially and irregularly oxidized brown. This sandy clay extends nearly to the base of the Berkeley Hills and underlies the alluvium of the slope upon which Berkeley is situated. In a well sunk on the property of the late J. F. Sims, on Prospect Street, Berkeley, immediately at the l>ase of the hills, this clay was 40 feet thick. A light-yellowish sand, doubt- less a phase of the same formation, was exposed to a thick- ness of about 12 feet some years ago in excavations made when the Mechanics Building was erected on the University campus. From the facts narrated it appears that practically every- where beneath the alluvial slopes of Berkeley and Oakland there* is a Quaternary formation, predominantly of marine origin, having a thickness of several hundred feet. This formation is here called the Alameda formation, from the city of Alameda, Ill where it is well developed. The contrast between the Alameda formation and the overlying alluvium indicates an important event in the geologic history of the region. The fine sandy clay which everywhere forms the upper part of the Alameda formation extends close to the foot of the steep front of the Berkeley Hills. It is evident that this steep front and there- fore the Berkeley Hills themselves were not in existence when the sandy clay was deposited, for such a slope would be sub- ject to rapid degradation immediately after its uplift and no products of this degradation appear in the Alameda formation. Between Lake Merced and the Pacific Ocean in the San Mateo quadrangle there is a deposit of light-yellow sands, about 200 feet thick, which probably lies unconformably upon the Merced and is therefore of Quaternary age and may be the correlative of the Alameda formation. These beds are but slightly disturbed but can not easily be distinguished from the Merced and therefore have not been separately mapped. They probably underlie a considerable part of the valley southwest of 8an Bruno Mountain. SAN ANTONIO FORMATION. At the foot of the steep face of the Berkeley Hills betw^een East Oakland and Berkele}'^ there is a great series of alluvial fans built up by streams that emerge from the hills. This alluvial deposit, which records a significant chapter in the Quaternary history of the region, is here called the San Antonio formation, from the township of that name. The alluvium is divisible into two parts, an older and a younger part, which are mapped separately. The older gravel forms a belt that lies closer to the foot of the range and is the product of stream work done in the adjoining hills before the streams had cut back very far into the upland. It consists only of rock fragments derived from the front of the hills, for it contains no debris from the prominent band of chert, of Monterey age, which traverses the range a short distance back of its front slope. The later or upper part of the alluvium contains abundant fragments of the chert and so re])resents a later stage of the dissection of the range, at a period when 112 the streams had cut into the belt of Monterey rocks. This younger deposit is referred to as the chert-gravel member of the formation. Since its accumulation this deposit has been thoroughly dissected and terraced. One of its highest points is in the vicinity of Laundry Farm, where its surface stands 250 feet above sea level. Farther northwest a broad terrace is cut out of the alluvium at an altitude of about 175 feet above sea level, and below this is a flat-bottomed valley or stream terrace, the upper part of which stands at an altitude of 125 feet. This lower terrace, which is between 500 and 600 feet wide, is fur- ther dissected by the sharp, narrow trench of Diamond Creek, about 30 feet deep. The San Antonio formation, particularly that part of it which lies north of the San Francisco quadrangle, contains the bones of extinct vertebrates, the following having been identified : Morotherium gigas Marsh. I Equus sp. Bison antiquus Leidy. Camelid. Elephas sp. Large carnivore, genus indet. Mastodon americanus Kerr. ^chmothorus occidentalis. Equus paeificus Leidy. I A tidewater canal dug to a depth of 18 feet a few years ago at the east end of Alameda exposed 13 feet of sand resting on 5 feet or more of the San Antonio formation, in which were found the femur and pelvis of a ground sloth {Morotherium gigas). MKRRITT SAND. The outer edge of the terraced alluvial embankment of the San Antonio formation in Oakland has a prevailingly steep front, which is evidently not its original front but is a cut cliff, probably a sea cliff formed when the east side of the valley of the Bay of San Francisco was 60 or 70 feet lower than it is now. The lower terrace of Diamond Creek was the graded flood plain of the stream when the base of this cliff was ai sea level, the terrace at the mouth of the valley where it emerges from the low gravel hills having the same altitude as the base of the cliffs. Marine sediments deposited at the time 113 of this depression now form the sand underlying Oakland and Alameda, here named the Merritt sand, from its occurrence on Lake Merritt, in the city of Oakland. In a well sunk on the property of Prof. W. J. Raymond, at the corner of Grove and Sixteenth streets, in Oakland, this sand has a maximum thick- ness of 44 feet and rests upon blue and yellow clay having a thickness of 45 feet. Below^ this clay lies gravel, which was pierced to a depth of 6 feet and yielded a flow of water. The Merritt sand is well exposed in Alameda, wdiere it is probably 43 feet thick, an estimate made by assuming that the highest part of Alameda stands about 30 feet above high tide. TERRACE GRAVEL. A small area of gravel on the slope of Buriburi Ridge, in the San Mateo quadrangle, near the head of Crystal Springs Lake, is one of several such small remnants of stream gravels left on terraces of extinct streams. Its high elevation indicates that it is of Pleistocene rather than Recent age. RECENT SERIES. TEMESCAL FORMATION. The Merritt sand would naturally grade into a beach deposit at the base of the cliffs cut into the San Antonio formation, but the base of the cliffs is now everywhere buried under sec- ondary alluvium, which was derived from the alluvial embank- ment of the San Antonio formation in the course of its degradation and which consists of the same kind of material — fragments of the Mesozoic and Tertiary rocks of the Berkeley Hills. The superposition of this secondary alluvium upon the marine Merritt sand w^as well exposed a few years ago b}^ a deep trench dug on Telegraph A\'^ie in Oakland. Just south of Hobart Street the trench was cut wholly in clean sand. Nearer Hobart Street the trench disclosed the feather edge of an alluvial deposit wdiicli tliickened toward the north and between Hobart and Twenty-second streets rested directly on the sand. At Twenty-second Street it w^as 11 feet thick, and a little beyond it showed a thickness of 13 feet, the full depth of the trench. This alluvium extends up to the foot San Francisco — 8 114 of the old sea cliffs and conceals the underlying beach deposit. It is called the Temescal formation, from the creek of that name in the San Francisco quadrangle, where it is well developed. The alluvial deposits at the base of the steep slopes of the San Francisco, Haywards, and San Mateo quadrangles appear to be the chronologic equivalents of the Temescal formation and are so mapped. OTHER RECENT DEPOSITS. Terrace deposits. — A wave-cut terrace at Bolinas carries patches of marine sands, which still remain on divides between the ravines and gullies that have been cut into the terrace since its uplift but which are not mapped. Travertine. — On the western slope of the Berkeley Hills north of Berkeley there are sheets of travertine, which were deposited by springs of calcareous water. Most of these deposits have been more or less affected by landslides, some having been carried far down the slope, and they are therefore not shown on the geologic map. Larger and thicker deposits of travertine, which are mapped, occur on Lime Ridge, a spur that projects into the Concord quadrangle from Mount Diablo near the town of Concord. Dunes. — Wind-blown sands cover large areas in the city of San Francisco. These sands drift in from the ocean beach south of the entrance to the Golden Gate and naturally take the form of dunes. Similar dunes occur at intervals along the coast in the San Mateo quadrangle but are nowhere so exten- sive as in San Francisco. Most of these dunes are of very recent origin, but several other eolian deposits in the area are evidently much older and may be Quaternary — such deposits, for example, as those which form part of the cliffs on the south side of Hunter Point. Salt-marsh deposits. — The only other deposits that remain to be mentioned are the clays and silts now accumulating in the salt marshes that fringe the Bay of San Francisco. The deposits are gradually encroaching upon the bay and tend to restrict its area. 115 STRUCTURE. GENERAL FEATURES. The dominant structure of the region about the Bay of Ban Francisco is expressed in three long orographic blocks that extend from northwest to southeast, each tilted northeastward, with its crest on the southwest side, as shown in figures 2 (p. 10) and 3. These are (1) the Montara block, culminating in Montara iZZ%s' 122°30' SAN MATEO y^z'lb' HAYWARDS 122-bd Scale Figure 3.— Outline map of the Tainalpais, San Francisco, Concord, San Mateo, and Hay wards quadrangles, showing the limits of the great fault blocks, the larger faults, and the axes of folds. The folding and faultiug occurred at intervals from the Cretaceous into the Quaternary period. The limits of the major fault blocks are shown by heavy daslied lines; faults by continu- ous lighter lines, except in the ocean, where they are broken ; anticlinal axes by dashed lines ; synclinal axes by dotted lines. T indicates the thrust side of an overthrust fault ; u the upraised side of a fault block ; D the downthrown side of a fault block. Mountain and extending northward to I the foot of San Bruno Mountain; (2) the San Francisco-Marin block, dissected super- ficially at the Golden Gate and culminating south of it in San Bruno Mountain and north of it in Mount Tamalpais; and 116 (3) the Berkeley Hills block, overlooking the bay from the east. The outlines of the form and structure of these three earth blocks are sho^Yn in figure 3. MONTARA BLOCK. FAULTS. The southwest and south sides of Montara Mountain form a bold, precipitous slope, which is incisively scarred by steep ravines. The main part of this slope is on the quartz diorite ("Montara granite"), but at the base of the slope south of the San Mateo quadrangle lie sedimentary beds that are tilted at high angles. These beds are coarse sandstones, which rest upon the quartz diorite and lie directly beneath a thick body of typical bituminous shales of the Monterey group. It was once suggested that these sandstones might be Tejon in age,*^ but it now seems more likely that they are the basal beds of the Monterey. The attitude of these beds, which are thus steeply inclined against the massive plutonic rock, suggests that a fault plane lies at the base of the mountain along which the strata on the north side have been raised and those on the south side have been dropped, and this is doubtless the move- ment that gave the northern mass its asymmetric profile — a profile characteristic of slightly degraded tilted blocks. The beds of the Monterey group that now lie on the lower soutliern flank of tlie mountain are remnants of deposits which at one time extended well over the area of the mountain but which have been eroded away on its uplifted side. A large part of this erosion, however, occurred before the movement that gave the block its present profile. From the crest of Montara Mountain northward the general profile of the tilted block descends rather gently and by steps to the Bay of San Fian- cisco, or to the base of the southwest front of San Bruno Mountain, which is the degraded scarp of the San Bruno fault. The entire Merced formation, which occupies the northeastern part of the Montara block, abuts upon this fault plane, as "Jjawson, A. C, Sketch of the geology of the San Francisco Peninsula: U. S. Geol. Survey Fifteenth Ann. Rept., p. 458, 1895. 117 shown in sections A-A and B-B, and the amount of differ- ential displacement that has occurred on it is therefore estimated at not less than 7000 feet. The San Bruno fault is paralleled by another, which lies closer to the face of the mountain and is regarded as auxiliary to the main fault, probably converging downward toward it and joining it far below the surface. The trace of this fault where it crosses the buttress-like shoulders of San Bruno Mountain is marked by fault breccia in the saddles. The chief structural feature within the Montara block is the San Andreas fault, which finds expression at the surface in the well-defined San Andreas rift valley, show^n in Plate IX. This is a long, straight, narrow valley, which trends about N. 34° W. In this rift valley lies the trace of the San Andreas fault, on w^hich repeated movements have taken place, the last notable movement occurring on A]iril 18, 1906, when there wns a differential displacement of such a nature that the country northeast of the fault moved horizontally southeast- ward and the country southwest of the fault moved horizontally northwestward, the maximum displacement in this region hav- ing been 12 to 16 feet. In its prolongation northwestward beneath the sea, outside of tlie Golden Gate, this fault con- verges upon the San Bruno fault, and the two appear to be coincident in the rift valley between Bolinas Lagoon and Tomales Bay. The rift produced during the movement in 1906 along this fault is shown in Plate X. The fault that parallels the San Andreas fault on the west slope of the Cahil Ridge, in the Santa Cruz quadrangle, prob- ably follows Pilarcitos Canyon, in the San Mateo quadrangle, and passes through the length of the southwest arm of Pilar- citos Lake. It is traceable over Whiting Ridge to the head of San Pedro Valley, beyond which, to the coast, it is concealed by the alluvium of the valley bottom. From a point neai- the southern border of the San Mateo quadrangle to a point within a mile of Pilarcitos Lake this fault forms the boundary between the Franciscan and the quartz diorite of Montara Mountain, and from the point last named to the coast it forms the boundary between the Franciscan and the wedge-shaped 118 area of Eocene beds that flank the north side of the mountain. The movement here was probably a thrust, by which the rock mass northeast of the fault overrode the rock mass southwest of it, thereby cutting away the northeast limb of the syncline of Eocene rocks in the vicinit}^ of Pilarcitos Lake. Another fault appears to follow the canyon that lies between Sawyer Ridge on one side and Cahil and Fifield ridges on the other. There are also other minor faults in the Montara block, but their structural relations have not been clearly deciphered. Along some of the fault planes in the region about San Francisco Bay earth movements still occur and are accom- panied by earthquakes. The importance of giving due con- sideration to these faults in connection with engineering projects is considered under the heading "Economic geology." FOLDS. In the Franciscan group of the Montara block the folding- is obscure and diflicult to decipher in detail, partly because the prevailing sandstone has but feebly marked stratification and is everywhere very much the same in appearance in its weathered outcrops and partly because the strata include irregular sheets of basic igneous rock, some contempora- neous and some intrusive, and are also much broken and dis- located in a minor way. In general, however, the folds of the Franciscan rocks are notably open, the strata dipping at low angles. Sawyer Ridge appears to be a very flat syncline pitching southeastward. (See section E-E.) Cahil Ridge is a slightly more appressed syncline, with no perceptible pitch, but near its south end, almost at the southern border of the quadrangle, it has a rather complicated twist. This synchne, with its sub- ordinate folds, probably extends through to the coast at Calera Valley. The sandstone of Sweeny Ridge is probably anti- clinally related to the basalts that flank it on both sides. Tn tlie area just northeast of the San Andreas rift valley, near Belmont, Sausalito chert lies on the Cahil sandstone in a 119 flat but broken syncline, which, on Belmont Hill, locally takes the form of a sharply overturned fold. (See section G-G.) The structure of the sedimentary rocks of Buriburi and Pulgas ridges is masked by abundant igneous rocks, but where the stratification is exposed the beds in general appear to stand much steeper than in the area southwest of the San Andreas rift. A marked structural feature of these two ridges is their slightly inclined, gently undulating surfaces, which were determined by a laccolithic sheet of peridotite, now ser- pentinized. Associated with this laccolithic sheet are dikes of the same material. A notable fold of the .INIontara block is that in wdiicli lies the tapering belt of Eocene rocks betw^een the Franciscan rocks and the quartz diorite of Montara Mountain, extending from San Pedro Point beyond Pilarcitos Lake. These beds were evidently laid down across the line of contact of the Franciscan and the quartz diorite and have since been buckled up in an irregular syncline by movements antedating the for- mation of the Pilarcitos fault, an overthrust which has cut off the northeast limb of the syncline at the southeast end of the belt, as shown in section D-D. The rocks in this general trough show a double syncline on the coast section and are traversed by a great many small faults. It is evident that these strata yielded much more readily under compressive stress than the stronger rocks on either side of the trough. Another notable fold within the Montara block is a syncline on its northeast margin, which involves the Merced formation. The southwest limb of this syncline is well exposed in the sea cliffs between Merced Lake and Mussel Rock, where the Merced strata dip uniformly to the southeast at angles ranging from 15° to 75° but are broken by numerous small normal faults, which have throws ranging from a few inches to a few" feet. The strata are over a mile thick in the measured section of the sea cliffs, and the dip of this great volume of sedi- mentary beds indicates that they abut upon tlie San Bruno fault beneath Merced Valley, as shown in sections A-A and B-B. If the Merced strata were folded before the San Bruno fault was formed, as seems probable, then the nortlieast Hmb 120 of the fold lias been carried up by the fault and removed by erosion. The Merced strata are similarly preserved in the same syncline in the rift valley northwest of Bolinas Lagoon, in the Tamalpais quadrangle. (See section A-A.) The underlying bituminous shales of the Monterey group that occupy the south end of the Point Keyes Peninsula are much broken and confused in structure, but the outcrops of these shales on the shore between Bolinas and Duxbury Point display part of an anticline in which the strata dip prevailingly toward the northwest. The relation of these beds to the San Bruno and San Andreas faults is further described under the heading "San Francisco-Marin block." Some interesting folding of Merced strata occurs at Mira- montes Ridge and on the wave-cut terrace at its base, in the southwest corner of the San Mateo quadrangle. This area of Merced lies beyond the southwestern limit of the Montara block, but its structural features may conveniently be men- tioned here. The Merced formation rests directly upon the surface of the quartz diorite, its basal deposits being those of a bowlder and pebble beach and containing abundant n)arine fossils of beach habitat. These beach deposits are firmly cemented, but the beds immediately above them show notable differences in resistance to marine corrasion, some being easily worn away and others remaining as salient reefs. The strata are folded into a series of small synclines and anticlines, and in the development of the wave-cut terrace the arches or domes of many of these small anticlines have been truncated. The hard beds of these truncated anticlines form circular or oval reefs that inclose depressions out of which the softer under- lying beds have been washed by the w^aves. Between these circular reefs lie the sinuous depressions of the intervening synclines, from whicli soft beds that lay above the hard reefs have been similarly scoured away. At low tide there is thus presented to view over the broad terrace a remarkable model of the folded structure. The structure is, however, even more remarkable than it apy^ears superficially, for the folded strata rest upon an early Merced, relatively smooth wave-cut surface 121 of the quartz diorite, as shown in sections C-C and D-D, and it is difficult to picture in the mind the adjustments that have taken phice within these Merced beds. It seems ahnost neces- sary to assume that they had been crowded horizontally over the flat quartz diorite surface upon which they rest, and that the forces that thus folded them had not greatly deformed the underlying massive rock. SAN FRANCISCO-MARIN BLOCK. FAULTS. That part of the San Francisco-Marin block which lies in the peninsula south of the Golden Gate is bounded on the southwest by the San Bruno fjuilt. The general features of the degraded San Bruno scarp are continued in the Marin Peninsula by the steep slope that overlooks the Pacific and the rift valley between Bolinas Lagoon and Tomales Bay. It is therefore probable that the San Bruno fault extends across the sea floor outside the Golden Gate and follows the rift val- ley from Bolinas Lagoon northw^estward. (See fig. 3.) This interpretation is supported by the fact that at Bolinas the Merced strata dip northeastward and appear to abut upon the feult, as shown in section A-A, just as they do on the San Francisco Peninsula. But on entering the rift valley the San Bruno ftiult becomes coincident with the zone of the San Andreas fault, which is probably a later feature of the struc- ture of the region and w^hich locally followed the line of weakness and rupture already established by the San Bruno fault. The history of displacement along the zone of faulting in the rift valley of Marin County is long and complicated. The rocks on the two sides of the fault zone are very different and owe their juxtaposition to the faulting. On the northeast side lies a great thickness of Franciscan strata, with which are associated igneous rocks. On the southwest side there are no Franciscan rocks, but in the area northwest of the Tamalpais quadrangle there is an extensive body of pre-Franciscan granitic 122 rock, which is overlain by strata of the Monterey group," and these are in turn unconforraably overlain by Merced strata. It would therefore seem probable that the earlier movements on this fault zone were pre-Miocene and that they caused a rela- tive upthrow on the southwest side of the fault, in consequence of which the Franciscan rocks were lifted into the zone of erosion and stripped off the underlying granitic rock. This erosion may have taken place in any part or during the whole of Cretaceous and Eocene time. In the southern part of the Point Reyes Peninsula there is a great thickness of bituminous shale of the Monterey group. The shore line of the sea in which these bituminous shales were deposited must have lain far east of Bolinas Pidge, for we can not regard the beds at the western base of the ridge as in any sense littoral. It follows that the Monterey beds were laid doAvn not only over the area of the Point Reyes Peninsula but also over a large part of the territory farther northeast, and that they were therefore spread over the trace of the old fault. In post-Miocene time there was probably a recurrence of movement on the fault plane at the time of the deformation of the Monterey strata and their uplift into the zone of erosion, but the effect of this movement can not be satisfactorily differ- entiated from that of a later post-Pliocene displacement. After the erosion of part of the folded and crushed Monterey strata the region was again depressed and received the Merced deposits, and at the close of the Pliocene epoch there was a great displacement on the San Bruno fault. It was this move- ment which raised the southwest margin of the San Francisco- Marin block and depressed the northeast margin of the Montara block. In consequence of this uplift the Merced strata on the northeast side of the fault zone were completely removed by later erosion, and whatever Monterey strata remained over that region after the post-Monterey period of erosion were also removed. The facts thus stated and partic- ularly the similar relation of the Merced strata to the San Bruno fault on l)oth sides of the Golden Gate show that the "Anderson, F. M., The geology of the Point Reyes Peninsula: California Univ. Dept. (ieology Bull., vol. 2, No. 5, 1899. Point Reyes Peninsula is orogenically a part of the Montara block. In later Quaternary time, subsequent to the large displace- ments that are represented by the San Bruno fault, there began the movements which find expression in the San Andreas fault — movements which are still in progress but are as yet relatively small and are characterized by a great excess of their horizontal over their vertical component. In the Tamalpais quadrangle the trace of the San Andreas fault is coincident with that of the San Bruno fault, but in the area south of the Golden Gate the line of the San Andreas fault is separate and divergent from the older line of dislocation. Within the San Francisco-Marin block there are many minor faults, some of which are indicated on the geologic maps. One of these is well exposed on the sea cliffs about three-quarters of a mile south of Fort Point, San Francisco. By this fault a band of radiolarian chert of the Franciscan group (probably Ingleside) is brought against sandstone of the same group (probably Marin). The fault doubtless extends southeastward across the city of San Francisco, but back from the shore its trace is obscure. A number of faults in the southern part of the Marin Peninsula have been recognized by dislocations which they cause in the Franciscan strata. In the main portion of the Franciscan group, however, where the rocks are prevailingly sandstone, similar faults are dif- ficult to detect, and even if a fault is observed at one place it is difficult to follow and map. The region therefore prob- ably contains many more faults than are indicated on the geologic maps. FOLDS. In general, the folds of the Franciscan rocks in the San Francisco-Marin block show remarkably little appression, par- ticularly in their larger features, but in parts of the Franciscan terrane where the rocks are thin bedded and therefore incom-, petent to transmit pressure they show the results of great crushing, the beds having been broken and crumbled. The strike of the folded beds is in general inconstant, even where 124 the strata are highly inclined, and the axes of such folds as can be made out are also very diverse in direction. This irregularity of structure is doubtless due to the fact that the folding is the result of a succession of earth movements having different directions. The irregular folding in the Franciscan strata presents a striking contrast to the folding in the later sedimentary beds of the Coast Ranges, which is simpler and more like the Appalachian type. The prevailing dip of the Franciscan rocks on the San Fran- cisco Peninsula is to the northeast. The San Miguel Hills, on the southwest edge of the city, lie in an undulating syn- cline, the sandstone of the southwest flank of the hills passing under the cherts of the summits at a low angle. The sand- stones at the northwest end of San Bruno Mountain, near Ocean View, dip to the southwest, their attitude indicating that they lie on the southwest limb of an anticline, but the strata here differ from the the usual sandstones of the Fran- ciscan and may possibly belong to the Cretaceous system. The structure northeast of the San Miguel syncline is obscured by bodies of serpentine and by an extensive mantle of dune sand. The sandstones south of Fort Point are very evenly stratified and dip to the northeast, and as those on the south side of Hunter Point, although intensely contorted, appear to have the same general dip, their attitude suggests that they lie in the northern limb of an anticline. On Pincon Hill, above Rincon Point, the prevailing dip is similarly to the northeast. In the San Miguel Hills and in the neighboring liills the sequence of the Franciscan formations and the character of the folding can be made out fairly well, but the structure elsewhere in the city of San Francisco is obscure. In many exposures in street cuts and other excavations even the local dip can not be determined, owing to the mashing of the softer strata, the complications due to intrusive rocks, the depth of rock decomposition, and the surftice creep. The general struc- ture across tlie southeastern part of the city, however, is shown in section E-E. On tlie Marin and Tiburon peninsulas certain broad features of the folding can be made out, but a detailed interpretation of 125 tlie structure is possible only locally. On the northeast sides of Tihuron Peninsula and Angel Island the dips are southwest, but at Belvedere and on the southwest side of Angel Island the dips are northeast. The structure of Tiburon Peninsula and Angel Island is therefore synclinal, though complicated by numerous intrusions. (See sections B-B and C-C.) On the hills northwest of Sausalito the general dip is to the southwest or south, so that Richardson Bay and Strawberry Point lie in an anticline. At the south end of the Marin Peninsula, west of Sausalito, the strike swings from northwest to west and the dip from southwest to south. The general structure is that of an open syneline of low pitch to the southwest. It is noteworthy that much of the stratification at this end of the peninsula is transverse to the San Bruno fault and probably abuts against it farther west, under the ocean. From Frank Valley northwestward to the northwest corner of the Tamalpais quadrangle the strike becomes parallel to the trend of Bolinas Ridge, and the prevailing dip from the base of the ridge to its summit is northeastward at moderate angles. This would indicate that Mount Tamalpais occupies the axis of a broad syneline (see section A-A), perhaps the same s\^n- cline that is recognized on the Tiburon Peninsula. The strata north and northeast of San Rafael dip northward and the ridge that terminates in Point San Quentin would therefore appear to be an anticline lying between the Tiburon Peninsula syneline and another synclinal fold on the northern border of the Tamalpais quadrangle. In figure 3 these are named the San Quentin anticline and San Rafael syneline. The prevail- ing dip on the Potrero San Pablo Peninsula is southwestward, though this dip is at several places locally reversed. The sandstones on Goat Island dip northeastward but show local small folds, some of which are broken and overthrust. BERKELEY HILLS BLOCK. FAULTS. The Berkele}'' Hills orogenic block is bounded on its south- west or uplifted margin by a zone of acute deformation, which extends through the San Francisco, Concord, and Havwards 126 quadrangles. At several places in this zone a fault, named the Haywards fauh, is manifested by geomorphic features similar to those of the Ban Andreas rift, and in the same zone there are numerous subordinate faults, many of them more or less oblique to the general trend of the rift, and some of them even transverse to it. Notable manifestations of rift geomorphy occur in all three quadrangles, but the faults that caused them are not mapped because it has not been possible to determine their locations exactly. Perhaps the most striking manifestation is that in the Concord quadrangle, between Claremont Creek and the Arroyo Viejo. Along the foothills between these points lies a long, narrow interrupted valley which is parallel to the trend of the range. The drainage lines of the w^est slope of the Berkeley Hills are consequent upon the surface resulting from the uplift of this margin of the block and run down the steep slope transverse to the trend of the range. This narrow longitudinal valley is therefore a strikingly excep- tional feature of the slope. Most of the transverse conse- quent streams whose valleys intersect the longitudinal valley in passing down the slope are diverted for short distances along the longitudinal valley before they pass southwest- ward through breaches in its wall. (See fig. 4.) Kohler Creek and Hayes Creek appear to represent an originally continuous consequent drainageway that has been divided into two parts by the longitudinal valley, along which the upper stretch of the stream has been diverted. The Arroyo Viejo, similarly, may once have flowed through a notch in the ridge which now bars its path to the southwest but was deflected to the north along the longitudinal valley. A typical trans- verse consequent stream, however, Shepard Creek and its continuation as Diamond Creek, shows very little deflection, and the «;ori'e in which it flows is continuous on both sides of the longitudinal valley. It would thus appear that the consequent drainage of the southwest slope of the Berkeley Hills, though still immature, nevertheless antedates the longitudinal valley, which is prob- ably in part diastrophic in origin and was in part formed by 127 the more rapid erosion of subsequent drainage along a line of exceptional weakness due to the faulting. Therefore, as the consequent drainage was begun by the uptilt of the Berkeley Scale 2 3 Miles Figure 4.— Outline map of the western slope of the Berkeley Hills in the southwestern part of the Concord quadrangle and adjacent parts of the San Francisco and Haywards quadrangles, showing the deflection of streams by the longitudinal rift valley of the Haywards fault zone. Several of the southwestward-flowing cousequent streams follow the longitudinal valley for a short distance before continuing their southwestward courses. Hills block, it follows that the Haywards fault, which finds its chief expression in the longitudinal rift valley, occurred subse- quent to the development of the main fault zone between the 128 Berkeley Hills block and the San Francisco-jNIarin block. In this respect the relation of the Havwards fault to this fault zone is analogous to that of the San Andreas fault to the earlier San Bruno fault. The Hay wards fault may therefore mark the advent of an earth movement that was unrelated to and differ- ent from that which displaced the large blocks, just as the San Andreas fault appears to have been unrelated to the movement that displaced the Montara and the San Francisco-Marin blocks, although it is in part of its course coincident with the San Bruno fault. The rift valley which is so well developed in the Concord quadrangle extends only a short distance southward, into the Haywards quadrangle, as a continuation of the northwest- southeast depression in which a part of the Arroyo Viejo flows. Beyond this point toward Haywards, however, the trace of the Haywards fault may be followed southeastward through a line of sags and saddles in 'the hills west of Lake Chabot, until, at Haywards, it passes out to the base of the hills. The faults mapped between Haywards and the edge of the quadrangle, east of Decoto, and those shown in the vicinity of Lake Chabot are features of the general zone of deformation that bounds the Berkeley Hills on the southwest and probably represent the earlier movement, which defined the Berkeley Hills block, rather than that which produced the Haywards fault. The zone of deformation on the southwest flank of the Berkeley Hills is marked in the San Francisco quadrangle by a series of step faults, the effects of which are still clearly defined in the profile of the slope between Strawberry Creek and Cordonices Creek. Beyond Cordonices Creek similar faults are indicated by stratigraphic displacements, although they are only feebly expressed in the geomorphic profile. In the district known as Cragmont, east of Northbrae, one of the fiults is marked bv abundant coarse fault breccia, in a silicified facies of the soda rhyolite which outcrops as a bold knob on the slope. On the crest of the northwest end of the ridge that separates Wildcat Canyon from the valley of the Bay of San Francisco there is a saddle-like depression, which lies parallel to the crest, ;ind in the line of this depression there are several 129 sinks or undrained ponds, features that have their counterpart in certain places along the San Andreas rift and are believed to have had a similar seismic origin. The more notable transverse faults that cross the zone of deformation are at Cordonices Creek, Strawberry Creek, Hamilton Gulch, Temescal Creek, and San Leandro Creek. Of the numerous other faults in the Berkeley Mills block only the more important will be mentioned here. They are shown in figure 3. A large and well-defined displacement on a nearly vertical fault plane having a general northwest strike traverses Strawberry and Claremont canyons. The downthrow is on the southwest side and amounts to several hundred feet. This fault is traceable southeastward beyond the head of Temescal Canyon. In Strawberry Canyon it intei'sects tw^o of the transverse faults, and in the triangle between them a block of soft Quaternary beds (Campus for- mation) has been dropped against the Cretaceous rocks. The fault is traceable northwestward beyond Strawberry Canyon for several miles along the northeast side of Wildcat Can- yon and may therefore be called the Wildcat fault. In one part of the canyon, on its southwest side, the Orinda strata, dipping to the northeast, appear to abut upon the Franciscan rocks, exposed on the northeast side, but because of the small- ness of the exposure of the Franciscan beds they are not so mapped. A notable fault enters the San Francisco quadrangle from the north in the valley of Pinole Creek. The downthrow is on the west side, which brings the tuff of the basal part of the Orinda formation against several diiTerent divisions of the Monterey. This fault may be referred to as the Pinole fault. About a mile and a half south of the northern limit of the quadrangle the line of faulting leaves Pinole Creek and, pass- ing through the hills on its south side, enters the Concord quadrangle near the summit of Sobrante Ridge, which it fol- lows as far as Bear Creek. Here it turns southward and changes from a longitudinal or strike fault to a transverse fjuilt, cutting across the trend of the folded strata. In the southern part of its course the fault may be followed down San rraufisco~9 130 Bear Creek, across San Pablo Creek, and thence to the crest of San Pablo Ridge and into Siesta Valley, where it appears to die out. This fault traverses strata that had been previously folded, and the discordance due to displacement is very strik- ingly shown in many sections along its course, the downthrow being uniformly on the southwest or west side. It is appar- ently due to overthrust from the northeast (see section F-F on the San Francisco map), and at its maximum displacement it brings the Tejon formation against the upper part of the Monterey. A less persistent fault enters the same group of hills from San Pablo V^alle}'' at a point about 3 miles east of the town of San Pablo and, with a southeast strike, conv^erges toward the Pinole fault for a distance of about 3 miles. Its strike is parallel to that of the strata, but its structural effect is not altogether clear. Another remarkabl}^ curved fault line appears at the south- east end of the Bear Creek anticline, in the northwestern part of the Concord quadrangle. This fold is broken and over- thrust bv a fault havino- a northwest-southeast strike. The fault follows Briones Valley for 2^ miles from Bear Creek and then turns sharply to the north, across the Briones Hills, and crosses Vaca Canyon to a shoulder of Franklin Ridge, where it is cut off by another fault, which extends along the south- west side of Franklin Ridge and which will be referred to later. This curving fault, which may be called the Briones fault, also cuts strata that had been previously folded, and the apparent horizontal displacement of the faulted formations is very marked and easily mapped. The curvature of the fault trace and the character of the displacements of the surface out- crops suggest that the fault movement was an overthrust on a fiat-lying plane, involving a slight rotation of the overthrust slab on a vertical axis. A prominent fault, here named the Franklin fault, enters the Concord quadrangle near the west end of the Franklin Canyon tunnel, on the Santa Fe Railway. It is an overthrust fault by which the Cretaceous rocks on the northeast have been caused to override the Monterey rocks on the southwest. 131 The thrust plane passes through the tunnel with a low dip to the northeast, and the sliales of the Monterey group, which in the tunnel lie l)eneath the Cretaceous sandstones tliat outcrop at the surface, have been reduced by the movement to a clayey mass which when wet flows into the tunnel, so that the rail- way engineers have had great trouble in keeping it open. West of the trace of the fault there are isolated patches of Cretaceous strata, which lie upon the Monterey. Three of these patches are shown near the top of the map. These are residual parts of a former extension of the overthrust Creta- ceous terrane. For nearly a mile and a half south of tlie northern limit of the Concord quadrangle the trace of this flat thrust is the boundary between the Cretaceous and the ISIon- terey. Farther south it is the boundary between the Martinez and the Monterey for a little over 3^ miles, and beyond this, still farther south, it is the boundary between the Tejon and the Monterey for about 4 miles. At the end of this stretch the thrust is dislocated by a transverse fault and its trace is offset over an eighth of a mile, the horizontal dis]~»lacement being to the east on the south side of the transverse fault. South of this point the fault plane is distinctly traceable for more than 5h miles as the boundary between the Tejon and the upper beds of the Monterey group and the San Pablo formation. Its strike is the same as that of the strata of the overthrust block. For the next mile and a quarter it is not so clearly traceable, but it probably continues southward and joins obliquely anotlier thrust fault on the northeast flank of Las Trampas Ridge, at a point about a mile west of Danville, at the eastern edge of the quadrangle. The thrust fault last mentioned may be called the Las Trampas fault, since it crosses the ridge of that name obliquel3\ It extends from the vicinity of Lafayette to a point about a mile south of Danville, at the eastern edge of the Concord quadrangle, beyond wliich it is lost in tlie alluviated floor of Ban Ramon Valley. It brings several horizons of the Monterey against the San Pablo along the main portion of its course, and its strike is in general parallel to that of the strata of the overthrust block on the southwest. The movement on tliis 132 fault was from the southwest, in a direction opposite to that of the Franklin fault, so that where the two faults meet, west of Danville, the fault planes intersect like the blades of a pair of shears, the plane of the Las Trampas fault being the upper blade and so cutting oat the Franklin feult at the surface, as shown in section E-E. On the southwest side of Bolinger Canyon, well up toward the crest of Rocky Ridge, there is another notable thrust fault, which may be called the Bolinger fault. For a considerable part of its course its trace is roughly parallel to that of the Las Trampas fault, and the displacement is in the same direc- tion in the two thrusts — that is, the rocks southwest of the fault line have overridden those northeast of it. As a result of this thrust the Monterey rocks of the crest of Rocky Ridge overlie and dip away from the Orinda formation on the southwest side of Bolinger Canyon. The trace of this fault farther north, beyond the head of Bolinger Canyon, swings westward down Las Trampas Creek and can not be found in the alluviated valleys of the Orinda formation. It is probable, however, that the thrust movement was taken up by the overturned syncline that extends northwestward from Moraga Valley. This over- turned syncline becomes more and more symmetrical in cross section toward the northwest and finally, east of Berkeley, becomes a simple open syncline. A transverse fault whose downthrow is on the southeast side extends from the Las Trampas fault to the Bolinger fault, passing just north of Las Trampas Peak. This transverse fault appears to have antedated both the thrust faults between which it lies. Near the south end of Rocky Ridge two paral- lel transverse faults are cut at nearly right angles by the Bolinger fault, A narrow mass of rock between these two faults appears to have dropped in the manner of a graben. Farther south, at the end of the ridge, is another transverse fault, with larger throw, the trace of which lies in the line of Crow Creek. The displacement of the strata on this fault is to tlie southwest on the south side of the fault, indicating an upthrow on this side. FOLDS. The folding of the strata in the Berkeley Hills block is strongly marked and for the most part may be easily deciphered, owing to the contrast between the different formations and the abundant fossils that many of them contain. The general structure between the southwest front of the Berkeley Hills and Mount Diablo is that of a great synclinoriura. The basal rocks having this general structure are Franciscan. They appear on the lower flanks of the southwest front of the Berkele}^ Hills and again in Mount Diablo, in the country adjoining the Concord quadrangle on the east. Within this general synclinorium are many folds, some simple and open, others appressed and overturned. The folds are complicated by numerous faults, already described, most of which appear to have been developed after the folding of the country. The axes of the folds and the faults are shown in figure 3. The most persistent of the folds is the synclinal trough in which lies a broad belt of the Oiinda formation, extending from Sobrante, in the San Francisco quadrangle, to the southeast corner of the Concord quadrangle, and called the San Pablo syncline. (See section F-F, San Francisco map, and sections D-D and E-E, Concord map.) Southwest of the San Pablo syncline, on the slopes of San Pablo Valley above the town of Orinda, is an anticline, which fades out rapidly toward the northwest but which persists for several miles toward the southeast. Southwest of this anticline is a well-marked syncline, the axis of which extends from the head of Wildcat Canyon through Siesta Valley to Moraga Valley and possibly beyond. It may be called the Siesta syncline. This fold affects not only the Orinda formation but also the volcanic and fresh-water formations of the Berkeley group, as shown in section B-B. In the hills back of Berkeley the fold becomes an open syncline, symmetric in so far as the dips of the strata are concerned, but stratigraphically asymmetric, owing to the lenticular form of the beds involved. Southeastward from Bald Peak toward Moraga Valley the Siesta syncline becomes gradually more closely appressed and 134 asymmetric and finally passes into a strongly overturned syncline with isoclinal dips toward the southwest on both limbs of the fold. It is evident that southeastward from Bald Peak the compression becomes more and more intense, involv- ing overthrust to the northeast or underthrust to the southwest. Still farther southeast the compression probably failed to find relief in folding alone but expressed itself in the great over- thrust fault which has been called the Bolinger fault. If this is true, then the overthrust faulting is referable to the same earth movements that induced the latest folding of the region, though, as already stated, it appears generall}^ to be subsequent to the folding. Northeast of the Pinole fault is a notable anticline, known as the Bear Creek anticline. In the center of this anticline lie the sandstones and shales of the Tejon formation, which are flanked on the northeast by the Monterey strata, in normal sequence. (See section A-A.) The southwest limb of this anticline has, however, been sliced off by the displacement along the overthrust of the Pinole fault, which brings the Tejon against several horizons of the folded Monterey strata. Tlie northwest end of the Bear Creek anticline is truncated by the Pinole fault, and its southeast end is broken and crushed by the Briones overthrust fault. Northeast of Pinole Valley and north of the end of the Bear Creek anticline is the well-defined Pinole Ridge anticline, which is also truncated and offset by the Pinole fault. The axis of this fold is well marked for about 3 miles southeast of the Pinole fault, but beyond this stretch the fold dies out. The Pinole Ridge anticline extends west of the Pinole fault for another mile, to the place where it passes beneath the unconformably overlying Orinda formation. The axis of the fold is, however, distinctly offset by the fault. Between this anticline and the northern part of the Bear Creek anticline lies a synclinal fold, which is also transected by the Pinole fault. Between the Bear Creek and Pinole Ridge anticlines on the southwest and the Franklin fault on the northeast there is a broad syncline of Monterey formations, in the center of which 135 lie irregularly shaped remnants of the San Pablo formation. (See section A-A.) This syncline persists halfway across the Concord quadrangle, although it is traversed by the Briones fault, wliich offsets its constituent formations. It may be appropriately named the Briones Hills syncline. The fold terminates against the Franklin thrust fault near the town of Walnut Creek, after being offset again by a later transverse fault. Northeast of the Franklin fault, in the northern part of the Concord quadrangle, is a massive anticline, known as the Martinez anticline, which exposes the Chico formation for a breadth across the strike of about 2 miles. The Chico rocks are flanked in normal sequence by the Martinez and Tejon strata, but on the southwest flank of the fold these are succes- sively, from south to north, sliced away by the Franklin thrust, so that finally the Chico rocks near the Santa Fe tunnel rest directly upon the Monterey. The complementary syn- cline northeast of the Martinez anticline is known as the Pacheco syncline, a simple fold in the middle of which lie the upper beds of the Monterey group. Its axis has a straight northwest course extending from Ygnacio Valley beyond the northern limits of the quadrangle. (See sections A-A and B-B.) At the south end of Ygnacio Valley, near the village of Walnut Creek, is another deep synclinal fold, known as the Walnut Creek syncline. This trough lies between the Frank- lin overthrust fault and the Arroyo de las Nueces y Bolbones. It is an overturned syncline, the outcrops on its limbs diverg- ing southeastward in the direction of the pitch of the fold. In the middle of the trough lies the Orinda formation and outward from this on either flank lie the other formations in normal sequence down to and including the Tejon. The syn- cline is thus stratigraphically symmetric but structurally asym- metric, owing to overturning to the southwest. (See section D-D.) The axial plane of the fold dips northeastward and the axis pitches southeastward. On the southwest limb of the fold the formations are in their normal superposition, with northeasterly dip, but on the northeast limb, the San Pablo 136 rests upon the Orinda, the Monterey upon the San Pablo, and the Tejon upon the Monterey, the reversed dip being com- monly about 45° but in places as low as 30°. This over- turned syncline extends far beyond the Concord quadrangle and is one of the dominant structural features of the southwest flank of Mount Diablo. Beneath this overturned fold lies the thrust plane of the Franklin fault. It is evident that the great pressure, which found partial relief first in the folding of the strata and next in the overturning of the fold, found further and more complete relief in the rupture and over- thrusting manifested in the Franklin thrust fault. Between the Franklin fault and Las Trampas fault is an extended syncline, which passes through Lafayette. This is truncated at its northwest end, on Bear Creek, by the Pinole fault and at its southeast end by the Franklin fault. It involves the Monterey, San Pablo, and Orinda strata, as shown in section D-D. This, the Lafayette syncline, is flanked on the north by a rather sharp anticline, which extends from the Franklin fault near Walnut Creek through Reesley Valley to the Briones fault and for some distance beyond. This, the Reesley Valley anticline, appears to have a low pitch to the southeast. On the south side of the Lafayette syncline is a more open anticline, which extends southeastward from the Pinole fault at Bear Creek and pitches rather steeply in that direction. Between the Reesley Valley anticline and the south end of the Bear Creek anticline is a short but well-defined syncline pitching southeastward toward and abutting upon the Briones fault at its sharp bend. There are also a minor syn- cline and anticline between the Bear Creek anticline and the San Pablo syncline on the southwest flank of Sobrante Ridge. South of the Bolinger fault is a large anticline which has been overthrust northeastward upon a syncline of the Orinda formation that occupies Bolinger Canyon. The northwest limb of this syncline has been cut off by the fault, and the northeast limb of the anticline has been buried by the overriding mass. In general, the folding and the faulting in the Berkeley Hills block are related in the sense that they are both mani- festations of the same compressive forces, but most of the fault- 137 ing was later than the folding and marked the culmination of the movement caused by the compressive forces. The most noteworthy general fact is that the overthrusting, whether mani- fested in overturned folds or in thrust faults, is not all in the same direction. The great thrust movement which appears in the Walnut Creek syncline and, in more intense form, in the Franklin thrust fault is clearly due to a force that acted from the northeast toward the southwest. Along this general line of deformation Mount Diablo has been shoved to the south- west, the older rocks having been pushed up over the younger. It is equally clear that the overturning of the Siesta syncline between Bald Peak and Moraga Valley and the overthrusting on the Bolinger and Las Trampas thrust faults are due to similar forces acting from the southwest. The date of the folding and associated faulting may be determined approximately by the fact that not only the Orinda formation but also the overlying volcanic rocks and fresh-water beds of the Berkeley group have participated in these move- ments, even in their most acute manifestations. The correla- tion of the Orinda with the Merced epoch indicates clearly that the principal deformation of the region as expressed in folds and certain associated faults of the Tertiary formations was post-Pliocene in age and may therefore be referred with confidence to the general epoch of mountain making which ended the Tertiary and began the Quaternary throughout the California region. The faulting that tilted and separated the great orogenic blocks, however, occurred later, in Quaternary time. It should be noted that there is evidence of an earlier and milder deformation of the Berkeley Hills region in the unconformity between the Orinda and the San Pablo. In the normal sequence the Orinda rests upon the San Pablo, but over considerable areas the San Pablo had been removed by erosion before the deposition of the Orinda, which therefore rests upon the upturned edges of the several divisions of the Monterey. Inasmuch as the structural discordance between the San Pablo and the Monterey is not pronounced, it seems probable that the pre-Orinda folding was post-San Pablo. 138 There is also clear evidence of uplift and erosion of the Eocene formations prior to the deposition of the Monterey, but the angular discordance is not great. Similarly, there was but slight deformation in the interval between Cretaceous and Eocene time. In the interval between Franciscan and Creta- ceous time, however, there was notable and widespread dis- turbance, uplift, and erosion. It therefore appears that of the numerous and various earth movements that have affected the region since Franciscan time the movement at the close of the Tertiary period was the most intensely deformational in its effects. GEOLOGIC HISTORY. PRE-FRANCISCAN TIME. The geologic history of the area described in this folio begins with the story told by the Gavilan limestone of Pilarcitos Canyon. This limestone is included in the quartz diorite and is but a fragment of a series of marbles, quartzites, and crystal- line schists that occur more abundantly in other parts of the Coast Ranges, particularly farther south. These rocks are the highly metamorphosed representatives of sediments of unknown age, which were deposited in this region long before the advent of the quartz diorite batholith that now forms a large part of the southern Coast Ranges. The intrusion of the quartz diorite into these ancient sedi- mentary rocks was doubtless the culminating event of a series of profound disturbances that formed a high mountain chain. Both the quartz diorite and the rocks of the sedimentary series into which it is intruded extend without material change of character or apparent structural break from the Coast Ranges to the Sierra Nevada. If followed northward into the Sierra Nevada they appear to be continuous witli the granite and the metamorphic rocks, respectively, of the parts of that range wliich are described in several folios of the Geologic Atlas of the United States." In these folios the granitic rocks have "U. S. Geol. Survey Geol. Atlas, folios 8, 5, 11, 15, 17, 18, 29, 31, 37, 89, 41, 43, 51, 63, 66, and 138. 139 been shown to be post-Jurassic in age and the invaded rocks to be in part Upper Jurassic. It therefore appears that the bedrock complex of the Sierra Nevada extends into the Coast Ranges, and that if the granitic rocks of the Sierra Nevada are post-Jurassic the granitic rocks of the Coast Ranges are also presumably post-Jurassic. FRANCISCAN EPOCH. The high mountain chain that was formed in the region of the Coast Ranges by the disturbances that culminated in the intrusion of the granitic rocks was subjected to long-continued erosion and was greatly degraded before it was submerged to receive the Franciscan sediments. The character of the forma- tions of the Franciscan group indicates a migration to and fro of the shore line of the Franciscan sea. The foraminiferal limestones and the radiolarian cherts were probably deposited on a sea floor at places so far from the shore as to preclude large admixtures of terrigenous sediments, whereas the sand- stones of the same series are, if marine at all, clearly littoral deposits. Tiie migration of the shore line thus indicated by the alternation of sandstone with either limestone or radiola- rian chert is only an expression of a vertical oscillation of the land in respect to sea level if the sea level was practically stationary, and its deviation from a stationary position was probably not great enough to weaken the conclusion that in Franciscan time the region was subject to alternating uplift and depression. As the Franciscan group includes at least three nonterrigenous formations that are separated by detrital formations the stratigraphy indicates at least three notable depressions of the region in Franciscan time and three move- ments of uplift. Certain sheets of amygdaloidal lava that are interstratified with the sandstones of the Franciscan group show that during part of Franciscan time volcanoes were active and that their lavas were poured out on the area of deposition on which the sandstones were accumulating. The Franciscan epoch of sedimentation was brought to a close by the uplift and deformation of the region. A feature 140 of tlie disturbance was the intrusion into the Franciscan strata at many places of irregular bodies of basalt and diabase which show ellipsoidal or spheroidal structure. This intrusion was followed by the intrusion of peridotites in the form of lacco- lithic lenses and dikes. These peridotites have since been altered to the serpentine which is so common in the Franciscan rocks of the Coast Ranges. Closely associated with these intru- sive bodies of ellipsoidal basalt and serpentinized peridotite are belts and irregular areas of glaucophane schists and other metamorphic rocks. These schists are metamorphosed igneous and sedimentary rocks, and their mode of occurrence indicates that they are a product not of dynamic metamorphism but of contact action. This metamorphism took place in the Fran- ciscan rocks before the deposition of the Knoxville formation and, indeed, before the erosion ijiterval between Franciscan and Knoxville time. From this it appears that the intrusions of the peridotite and ellipsoidal basalt must have occurred so soon after the deposition of the Franciscan strata that they may with propriety, as well as convenience, be referred to Franciscan time. KNOXVILLE EPOCH. The erosion which followed the uplift of the Franciscan rocks appears nowhere to have entirely removed them. The degradation and removal of the uplifted Franciscan was inter- rupted by the depression that ushered in the Cretaceous period of sedimentation. The basal formation of the Cretaceous is the Knoxville, which in this area is composed largely of shale. The occurrence of only small deposits of conglomerate or other coarse detrital beds at the base of the Knoxville and the entire absence of such beds in many sections show that peculiar con- ditions attended the beginning of the Cretaceous depression. The coarse sandstones and conglomerates which are usually spread over a sinking continental slope by a transgressing sea are here but very feebly if at all represented. The facts observed indicate that the profile of the coast of the Knoxville sea was very flat and that the streams which flowed into that sea were of low grade, carrying chiefly fine sediment. A slight 141 subsidence of a coast so low would produce broad expanses of shallow lagoons and swamps devoid of powerful currents. In such shallow, rehitively stagnant water the Knoxville appears to have been deposited. But if the land surface that subsided and received the Knoxville deposits was low and flat, the low relief must have been due to peneplanation in post-Franciscan time, so that the interval between Franciscan and Knoxville time was long. Further, the post-Franciscan disturbance could not have involved any large vertical displacements, for if such dis- placements had occurred the peneplanation of the region would have completely removed the Franciscan from the uplifted blocks, so that the Knoxville would have been deposited upon pre-Franciscan rocks, but the Knoxville generally rests upon the Franciscan. The fact that the Knoxville beds attain a very considerable thickness in neighboring portions of the Coast Ranges indicates that the sea floor continued to subside steadily during Knoxville time while the adjoining land, from which the sediments were drawn, suffered no marked uplift or steepening of its gentle slopes. CHICO EPOCH. In most of the area mapped the Knoxville formation is succeeded by the Oakland conglomerate member of the Chico formation. This member reaches a maximum thickness of about 1000 feet and in places is rather coarse, containing waterworn bowlders 6 to 10 inches in diameter, although the dimensions of most of the pebbles are, of course, nuich less than these, the smaller ones being of the sizes of marbles and peas. This conglomerate is interesting and significant and would ordinarily be regarded as indicating an unconformity, but this member appears to rest concordantly upon the Knoxville for- mation, the relation showing that the sea floor upon which the Knoxville beds were accumulating was probably not uplifted at the beginning of the deposition of the conglomerate. The thickness of the conglomerate indicates that during its accumu- lation there was progressive subsidence to an amount measured by that tlr3knes3. But if the subsidence of the Knoxville sea 142 floor continued into Oakland time without reversal the sudden change in the character of the sediments shows that the region from which these sediments were drawn was changed at the close of Knoxville time from a low-l^^ing country, with low- grade streams, to an upland dissected by actively eroding- torrents. The Knoxville subsidence was widespread and may be regarded as an epeirogenic movement, and the Oakland conglomerate may be regarded as the depositional record of an orogenic movement which deformed the coast of the Creta- ceous sea but which only slightly affected the adjacent sea bottom. The Oakland conglomerate member is succeeded by a great thickness of sandstones and shales, with scattered lenses of conglomerate, which comprise the rest of the Chico formation. The prevalence of fine-grained sandstones, the notable dearth of limestones, and the scarcity of conglomerates clearl}^ indicate tliat the abrupt profiles which characterized the continental margin in Oakland time had been greatly subdued and that the later Chico sea remained shallow though steadil}- subsiding. MARTINEZ EPOCH. The advent of the Tertiary appears to have been marked by no violent orogenic movements, but there is evidence of general warping and uplift of the floor of the Chico sea and of an interval of erosion. In the quadrangles described in this folio no angular discordance between the early Eocene strata and the underlying Chico has been detected, but in the adjoining Mount Diablo quadrangle there is a well-defined unconformity between the Martinez and the Chico," and this unconformity doubtless prevails over the Concord quadrangle, although conditions there are adverse to its discovery. The Martinez rocks of San Pedro Point, in the San Mateo quadrangle, rest directl}^ upon a pre-Knoxville surface, yet on the coast a little farther south, in the Santa Cruz quadrangle, thei'e are several thousand feet of Chico strata. It seems probable that the Chico beds once extended over the San Francisco Peninsula and were "Dickerson, R. E., California Univ. Dept. Geology Bull., vol. 6, No. 8, 1911. I4;j thus continuous on both sides of the Bay of San Francisco. If they were so deposited there must have been a long period of degradation in post-Chico time to permit the deposition of the Martinez beds of San Pedro Point upon a pre-Chico surface. This presumption of a considerable interval of erosion between the Cretaceous and the Tertiary is consistent with the abrupt change of the fossil fauna from one terrane to the other. The fact that the Martinez formation is not so widely distrib- uted as the Cliico indicates that the marine basin of sedimenta- tion was smaller in early Tertiary time than it was before the uplift at the close of the Chico. TEJON EPOCH. After more than 2000 feet of sandstones and shales had accumulated in Martinez time the region again suffered dis- turbance and uplift. The denudation begun by this movement appears to have been moderate, for although, as Dickerson" has shown, an unconformity exists between the Martinez and the overlying Tejon in the Mount Diablo quadrangle, no discord- ance at this contact has been detected in the Concord quadran- gle except in so far as it may be inferred from the conglomerate at the base of the Tejon northwest of Walnut Creek. There is, however, a pronounced contrast in the fossil faunas of the two formations. The Tejon subsidence reestablished in a gen- eral way the conditions that had prevailed in Martinez time, but the Tejon area of sedimentation was probably larger than that of the Martinez. The sea was shallow and the subsidence permitted the deposition of about 2000 feet of sandstones and shales. MONTEREY ErOCII. The Tejon subsidence and sedimentation was brought to a close by earth movement and uplift, and, after erosion, a wide- spread submergence ushered in the period of deposition of the Monterey group. The interval between the Tejon and the Monterey was probably a notable period in the geologic history "Op. eit. 144 of the region. The fects presented under the heading "Mon- terey group" indicate that large areas of pre-Monterey rocks were raised above the sea and somewhat fokled and faulted by the uplift and that these areas were extensively eroded before the Montere}^ submergence began, so that the Monterey rocks lie unconformably upon the Tejon and older formations, the discordance in dip being in places considerable. The fact of chief historic interest connected with the Mon- terey is the record of crustal oscillation revealed by its con- stituent formations. Even in the Concord quadrangle the sequence of the Monterey formations is not uniform. The most complete section, containing not only the greatest thick- ness of strata but also the largest number of petrographically distinct formations, is found in the large sj^ncline that lies between the Bear Creek anticline and the Franklin overthrust fault. Here five divisions of sandstone alternate with four divisions of bituminous shale, as described in detail under the heading "Monterey group." In their purer facies the shales are nonterrigenous deposits, though in places they include silt or even fine sand, and they were evidently deposited rather far from the shore, but the sandstones between the shales and those at the bottom and top of the Monterey group are littoral deposits. The migration of the shore line to and fro implied by this alternation in conditions of sedimentation is interpreted to signify a vertical oscillation of the region. Apparently four movements of depression and four of uplift occurred during Monterey time, but the net result of these upward and down- ward movements was a depression, which corresponded approx- imately in amount to tlie entire thickness of the Monterey group in this region. Although this oscillation so greatly affected the depth of tlie water and tlie position of the shore tliat it raiHcally clianged the conditions of deposition in the region indicated, it may not have affected these conditions in the neighboring parts of the sea floor that lay farther from tlie continental margin, so that the deposition of bituminous shales there may have proceeded continuously. Thus a uni- form body of bituminous shales would become the stratigraphic 145 and chronologic equivalent of a series of alternating sandstones and shales nearer shore. Conversely, the margin of the basin might have remained a littoral region notwithstanding depres- sions that affected chiefly the rest of the -basin, so that sand- stones may have been continuously deposited along the shore as the stratigraphic equivalent of the alternating sandstones and shales laid down farther out. Conditions favoring the continuous deposition of bituminous shale after the transgressional basal sandstone and conglomerate had been formed were probably realized in the Monterey of the Point Reyes Peninsula and probably also in the Monterey of Monterey County. The second condition — that of continu- ous littoral sedimentation — is in some measure realized in the Monterey of the Pacheco syncline and Shell Ridge. At both of these places we find the lower and upper portions of the Monterey, but the deep-water sediments of the middle Monterey are represented by a single comparatively thin and impure formation of bituminous shale. SAN PABLO EPOCH. Monterey time was brought to a close by uplift and disturb- ance of the Coast Range region. The disturbance was great in Santa Cruz County, where, as W. F. Jones" has sbown, the San Pablo formation rests upon the worn edges of the nearly vertical Monterey strata and, moreover, has a basal conglom- erate. The discordance, though not so pronounced, is still well marked in the Mount Diablo quadrangle.'' There is also dis- tinct indication of discordance between the San Pablo and Monterey in the distribution of the San Pablo along the middle of the large syncline in the northwestern part of tlie Concord quadrangle. Elsewhere in the Concord quadrangle, however, the San Pablo follows the Monterey without appreciable dis- cordance. ■ The subsidence that permitted the deposition of the San Pablo beds appears to have formed a much smaller basin than that which existed in Monterey time. The prevailingly "California Univ. Dept. Geology Bull., vol. 0, No. 3, 1911. ^Clark, B. L., The Neocene section at Kirker Pass, on the north side of Mount Diablo: Idem, vol. 7, No. 4, pp. 47-60, 1912. San Fraucisc'o — lO 146 thick bedding and rather coarse grain of the sandstones of tlie San Pablo and their general blue color tend to mark oflf this formation from the underlying commonly light-colored or rusty-yellow sandstones of the Monterey and indicate a change in the conditions of deposition. The absence of the bitumi- nous shales that so strongl}^ characterize the Monterey also points to such a change. MERCED EPOCH. The superposition of the Merced formation upon the San Pablo has nowhere been observed in the area here mapped, but such superposition, with evidence of unconformity, occurs near Chittenden, in Santa Cruz County, and has been described by W. F. Jones." The absence of the San Pablo below the Merced in the San Mateo and Tamalpais quadrangles itself suggests an unconformity between the two, unless the San Pablo basin was so small that it did not cover this part of the field. The depression wdiich permitted the accumulation of Merced strata to the thickness of over a mile on the site of the present San Francisco Peninsula appears to have been local and to have formed a sinking trough in which there accumulated marine clays, sands, and conglomerates. The land surface that was thus gradually -depressed moi'e than a mile below sea level had supported a forest growth, represented by abundant pine cones, which occur in the basal bed of the series at Mussel Rock. The fresh-water Orinda formation, which is partly fluviatile and partly lacustrine, lies in a similar geosynclinal trough east of the Berkeley Hills — a trough that must have subsided as the formation was deposited. These two deposits were probably synchronous in origin, for the basal beds of the marine Merced fornjation in Sonoma County, described by Osmont,'' contain abundant white tuff, which extends southward to the Bay of San Francisco and is probably identical with the Pinole tuff. This tuff occurs also in the basal beds of the Orinda formation in the northeastern part of the San Francisco quadrangle and is thus common to the marine Merced and the fresh-water "California Univ. Dept. Geology Bull., vol. 6, No. 3, 1911. ''Ideni, vol. 4, No. 3, 1904. 147 Orinda. The barrier between the marine trough and the fresh- water trough thus formed appears to have been nearly coinci- dent with the present Kne of the Berkeley Hills but may have lain a little west of it. Volcanic ash occurs in beds high up in both the Merced and Orinda and was probably ejected from distant volcanoes that were intermittently active. The ash in the Merced appears to be a fresh white andesitic pumiceous glass, but that in the Orinda is thoroughly decomposed. BERKELEY EPOCH. At the close of Orinda time a slight depression probal)ly occurred on the southwest side of the structural trough in which the fresh- water sediments of that epoch accumulated, and into this depression there was poured a succession of andesitic and basaltic lavas and occasional showers of ashes. Between these volcanic flows and showers there were times W'hen fluvia- tile and lacustral conditions recurred and other times in which the lava was exposed to atmospheric oxidation or to erosion. The beds of tuff or ash that are intercalated with the lavas include rhyolitic tuff but no flows of massive rhyolite. This accumulation of volcanic and interstratified fresh-water beds forms the Berkeley group, DEFORMATION AT CLOSE OF TERTIARY PERIOD. At the close of Berkeley time, Avliich marked also the end of the Tertiary period, the Merced, Orinda, and Berkeley strata became involved with all the older rocks in the great earth movements w4iich deformed the region. The basement upon which the Merced had been deposited and which, as we have seen, had been depressed more than a mile below sea level, was lifted far above sea level, as may be plainly seen at the exposure near INIussel Kock, in the San Mateo quadrangle. The extent of this uplift shows the magnitude of the orogenic movements that closed the Tertiary period. The overturning of the syncline involving the Berkeley group near Moraga Valley and the overturning of the Orinda beds in the oppo- site direction in the Walnut Creek svncline indicate the 148 intensity of the action. Most of tlie folding and faulting shown on the structure-section sheets of this folio occurred at this time. CAMPUS EPOCH. After a partial degradation of the Siesta syncline another fresh-water basin was formed across the eroded edges of the upturned lavas of the Berkeley group, and in this apparently rather small basin accumulated the gravels, clays, tuffs, and lavas of the Campus formation. ALAMEDA EPOCH. After the Campus basin had been filled but before the Berkeley Hills had been uplifted the degradation of the uplands continued, and in the depressions there accumulated the Ala- meda formation, which is chiefly a sandy clay with interca- lated delta gravels derived from Alameda Creek and a few beds of sand containing marine shells, all indicating that parts of the region which are now sharply accentuated were then of moderate relief. POST-ALAMEDA DIASTEOPHISM. After the deposition of the Alameda formation a movement occurred that separated the San Francisco-Marin block from the Berkeley Hills block and probably also from the Montara block. This movement consisted chiefly of faulting and mono- clinal tilting, one result of which was the outlining of the valley system of the Bay of San Francisco. A considerable interval had tlius elapsed between the diastrophism which closed the Tertiary, exemplified in the folding and faulting of the Merced and Orinda, and this later movement, which brought the tilted blocks into their present positions and left the Campus rocks in the form of steps on the steep west front of the Berkeley Hills block. This final uplift left the fine Alameda sediments close to the base of the same steep front, a situation in which they evidently could not have been originally deposited. 149 SAN ANTONIO EPOCH. In the fault valley of the Bay of San Francisco were deposited later Quaternary sediments. The earliest of these is the San iVntonio formation, a series of fans of coarse alluvium spread out at the base of the Berkeley Hills and made up of detritus derived from the newly formed steep front of the Berkeley Hills block. That the San Antonio formation is the depositional record of the uplift of the Berkeley Hills block is clearly shown by the fact that for a considerable portion of its extent it is separable into two parts. The lower part is made up of detritus brought down in the early stages of the degra- dation of the new mountain front, before the canyons had cut back to the chert of the Monterey group, and therefore includes none of this chert. The upper part abounds in chert frag- ments derived from the Monterey and represents the degrada- tion after the canyons had cut back to the belt of chert which marks for some distance the present crest of the uplifted block. As Alameda Creek in its transverse passage through the Berkeley Hills in the gorge near Niles, just east of the Hay- wards quadrangle, is an antecedent stream, formed before the uplift of the Berkeley Hills, the lower beds of its delta or alluvial fan probably correspond to the iVlameda formation and the upper beds to the San Antonio formation, deposition having been continuous from one epoch into the other. The remains of a notable vertebrate fauna, now extinct, in the San Antonio formation, particularly in its occurrence north of the San Francisco quadrangle, represent the life of this epoch. MERRITT AND TEMESCAL EPOCH. The abrupt cliff-like edge of the alluvial embankment of the San Antonio formation in the city of Oakland indicates that the region was depressed and attacked by the waves, so that a wave-cut terrace and sea cliff were here carved out of it. During this period of depression the Merritt sand of Oakland and Alameda was deposited. Since then the dissection of the main embankment of the San Antonio formation has given it a 150 prononiiced terraced effect and lias supplied the materials of the Teinescal formation, which rests upon the Merritt sand in the city of Oakland. Since the deposition of the Temescal formation the region has been slightly uplifted and shallow valleys have been cut in it and then submerged by a later subsidence, which produced tlie drowned effects in the topog- raphy about Lake Merritt and in the channel between Oakland and Alameda, but these later movements may have been local. It is noteworthy that the last depression appears to have con- tinued down to the time of the occupation of the region by man. The base of a large shell mound near Emery, on the Oakland shore of the bay, is more than 2 feet below sea level. The bases of other Indian mounds on the shores of the bay are also ])elow^ sea level, and as they were all probably started on dry land, their present position indicates very recent subsidence." EECENT UPLIFT AND DEPEJ:SSION. The record of uplift and depression which is read in the Quaternary deposits of the east side of the Bay of San Fran- cisco and in their dissection is not matched on the west side. The alluvium which extends from the base of the steep slopes to the edge of the salt marshes of the bay is the equivalent of the Temescal formation, and the Alameda, San Antonio, and Merritt formations are absent. The dentate contour of the shore — a series of embay men ts separated by rocky promon- tories — which characterizes the bay side of the Marin Peninsula and of the San Francisco Peninsula as far south as San Bruno Point clearly points to a subsidence much more pronounced than that which occurred on the east side of the bay. If the missing formations were ever deposited on the west shore, and it seems probable that some of them were, they have been entirely submerged by the greater subsidence of this part of the bay. "Ulile, JIux, The Kmeryville shell mound: California Univ. Pub. Am. Archeology and Ethnology, vol. 7, No. 1, 1907. Nelson, N. C, The shell jiiounds of the San Franoiseo Bay region: Idem, No. 4, 1909. 151 The heavy bank of old alluvium on the west side of Crystal Springs Lake, at the southern border of the San Mateo quad- rangle, which is the northern extension of the Santa Clara for- mation of the Santa Cruz quadrangle, is the product of degra- dation and fluviatile deposition during a period whose geologic age is in doubt. Outside of the Golden Gate the striking wave-cut terrace at Bolinas affords unequivocal evidence of uplift, the minimum measure of which is the elevation of the terrace where it abuts upon its now degraded sea cliff. This elevation is about 250 feet. The uplift thus clearly indicated is extremely interesting in view of the absence of evidence of uplift on the shore of the Marin Peninsula northeast of the San Andreas rift and in view of the positive evidence of depression and the entire absence of evidence of uplift on the bay side of the Marin Peninsula. Similarly uplifted shore lines are absent on the Pacific side of the San Francisco Peninsula between the Golden Gate and Point San Pedro. South of this point, however, partic- ularly south of Halfmoon Bay as for as Santa Cruz, elevated strands are prominent features of the coastal profile. It is noteworthy also that on the coast north of Tomales Bay elevated strands again become conspicuous, and on the east side of Tomales Bay itself there are raised shell beds, which stand 20 to 30 feet above the present shore line. Since the uplift of the Bolinas terrace there has been a slight depression of the Point Reyes Peninsula, as may be readily inferred from the drowning of the small streams flowing into Drake's Bay, near Point Reyes light. The evidence thus presented shows that the diastrophic movements of the region about the Bay of San Francisco in Quaternary time have been complex and uneven. The Marin Peninsula proper between Bolinas and the Bay of San Francisco has undergone marked depression, and with this depression may be associated the invasion of the Golden Gate by the sea. The east side of the bay in the vicinity of Oakland and Berkeley has been alternately lowered and raised, the net result of the movements being an uplift. The Point Reyes Peninsula has certainly l)een uplifted more than 250 feet and this uplift has been followed 152 by a slight depression. On both coasts of the San Francisco Peninsula the evidence, though not so clear as that presented on the Marin Peninsula, indicates subsidence in late Quater- nary time. CHANGES IN DRAINAGE. The courses of many of the streams in these quadrangles were modified during Quaternary time, and it would be interesting to trace their history in detail, but only a few of the more important changes will be mentioned. The most notable event in the modification of the drainage of the region was the conversion of the valley lying between the Berkeley Hills and the higher part of the San Francisco- Marin Mountain block into an inlet of the ocean by the sink- ing of the coast. The drainage from the Great Valley of California and from the Coast Ranges north and south of San Francisco converged upon this valley and flowed through a short transverse canyon at the Golden Gate. The submergence admitted the sea to the valley and so created the great harbor of San Francisco Bay. By this change the seat of deposition of the sediments brought down by the rivers from the interior was transferred from a delta outside of the coast line to San Francisco Bay, which is in consequence gradually filling up. Another remarkable change in the drainage of the region was the deflection to Alameda Creek of the waters that once flowed from Livermore Valley through San Ramon and Ygnacio valleys to Suisun Bay. The divide between the head of San Ramon Creek and the h3^drographic basin of Alameda Creek, in the countr^'^ east of tlie Concord quadrangle, is situ- ated on the nearly level floor of Livermore Valley, which was tlie flood plain of the much larger predecessor of San Ramon Creek before it was beheaded, somewhat modified l)y recent alhiviation. Tliis change explains the dispro])ortion between the large San Ramon Valley and the present small creek that flows through it. San Pablo Creek once probably had a straight course across tlie western flank of Sobrante Ridge to San Pablo Bay in the vicinity of Pinole, just north of the San Francisco quadrangle. 153 The Quaternary alluvium at Pinole, so rich in the bones of extinct animals, is doubtless in part the flood-plain deposit of the lower part of San Pablo Creek, carried there before its course was changed. The deflection of the stream's course from northwest to west near San Pablo is probably due to its capture by a small stream, consequent upon the uplift of the Berkeley Hills — a stream that eroded back rapidly in the soft clays of the Orinda formation. Alameda Creek, which passes through the southern exten- sion of the Berkeley Hills in a deep, narrow canyon and flows across the alluvial plain in the southern part of the Haywards quadrangle, has an interesting relation to the uplift of the Berkeley Hills. It heads on the slopes of Mount Hamilton and Mount Diablo and drains broad, low-lying valleys east of the Berkeley Hills. Its course across these hills between Sunol and Niles, in the Pleasanton quadrangle, was evidently estab- lished before their uplift, and the stream has persisted across the rising mountain block, cutting its rugged canyon deeper and deeper as the block rose. It is an excellent example of an antecedent stream. During most of the time in which it was thus entrenching itself across the rising mountain block Alameda Creek was much smaller than it is now, for the drainage of Livermore Valley, which once flowed nortli by way of San Ramon Valley, in the Concord quadrangle, and is now tributary to Alameda Creek, was captured by that creek late in Quaternary time. By this capture the hydro- graphic basin of Alameda Creek was more than doubled. The convergence of a large part of the drainage of the Haywards and Concord quadrangles at Castro Valley, near Haywards, has probably been in large measure determined by a structural sag in the Berkeley Hills block, formed at the time of its uplift. The drainage of the southwest slope of tlie Berkeley Hills has been described under the heading "Structure" and illus- trated in part in figure 4. The streams on this slope origi- nall}^ flowed with steep gradients across a zone of marked deformation and faulting and were relatively straight. Later faulting along the old fault zone apparently offset to a small 15i extent some of these streams, and other streams were deflected along the line of the fault in a longitudinal rift valle}^ that was formed in }3art directly by faulting but chiefly by excessive erosion of the zone of crushed and mashed rock produced b}^ faulting. The trench of Shepard and Diamond creeks is but slightly offset at the rift valley, whereas Viejo Creek has been deflected nearly a mile. Kohler Creek, which was probably once continuous with Hayes Creek, is now entirely separated from it and flows along the rift valley to Temescal Creek, a mile distant. Other streams in this vicinity have been more or less affected in a similar manner. The remarkably straight drainage line of the San Andreas rift in the San Mateo quadrangle was begun probably in Quaternary time by the creation of a belt of soft, mashed rock on a zone of recurrent faulting, but the more profound rift val- ley extending from Bolinas Lagoon to Tomales Bay, with its equally straight drainage lines, is an inheritance from a much older geologic period, as may be inferred from the discussion of the faulting at that place under the heading "Structure." ECONOMIC GEOLOGY. AVAILABLE RESOURCES. The mineral resources of the San Francisco district are chiefly building materials, such as clay, shale, limestone, rock suitable for crushing, gravel, and sand, although small quanti- ties of gold, lead, copper, manganese, asbestos, chromite, tale, magnesite, and quicksilver are found, and manganese ore and pyrite have been mined on a small scale. Volcanic ash has been quarried for use as polishing material and large quanti- ties of salt are extracted from the sea water. The most valuable natural resource in the region, however, is water. WATER. Surface water, impounded by dams in the valleys of streams, has been extensively used to supply the cities on the shores of San Francisco Bay, and the conversion of valleys into reservoirs for the storage of rain water is likely to be greatly 155 extended in the near future, particularly on the east side of tlie bay. Such utilization requires no special discussion here, thoui»;h knowledge of geologic conditions can assist in deter- mining dam sites that are suitable as to foundation and as free as possible from danger of disturbance by earthquake. A knowledge of the nature of the underlying formations and of the distribution of recent f^iults may also minimize the danger to projected pipe lines and tunnels, a fact that has been generally recognized since the earthquake of 1906. The rain water that is stored in the pores and voids of open- textured formations, such as gravels, sands, and sandstones, is also a source of water supply. Sandstone or other porous rock that is exposed directly to the weather becomes filled with water until it is saturated up to a certain level or plane, which in tlie wet season may coincide locally with the surface of the ground but is generally beneath the surface. This level is known as the ground-water plane. The underground storage in this region is well exemplified in the hill lands, which are very generally composed of sandstone and hold water up to a large but variable proportion of their volume. This ground water tends to escape in springs at the lower levels of the hill slopes, and owing to this leakage the ground-water plane becomes lower and lower during the dry season; but as the frictional resistance to flow^ through the minute spaces in the rocks greatly retards the escape of the water the supply may endure through the dry season till the leakage is made good by replenishment during the following rainy season. The ground water in the hill lands of this region is most commonly tapped not by wells but by tunnels driven into the hillsides, and it is a valuable source of local supply where the consump- tion is small. Many porous rocks that are well adapted for the storage of water are in large pai-t buried by clays and other rehitively impervious strata, but in places these pervious formations out- crop and surface water or rainfall at this outcropping edge sinks into and fills the open-textured rock. In valleys underlain by such formations it is generally possible to reach the saturated strata by boring through a mantle of impervious material. If 156 the porous outcrop at which the water enters the rock is not- ably higher than the top of the bore hole the water will flow from the well, and if the water does not quite reach the surface, owing to the slight difference of level between the point of intake and the valley floor, it may be lifted by pumps. Under- ground water supply of this kind, whether the w^ater overflows at the surface or not, is classed as artesian. The Bay of San Francisco is a sunken valley which as it sank gradually became filled with detritus washed in by streams from the surrounding hills, the coarser and more pervious deposits having been from time to time buried by finer and less pervious silts and clays, part of them fluviatile or delta deposits and part of them marine clays or sands. The valley in which the bay lies is therefore an artesian basin. The irregular tongues of gravel that grow wider and thicker away from the mouths of the canyons are separated by sheets of clay and have been buried by impervious deposits, leaving the gravels exposed only near the mouths of the canyons, where the present streams flow over them. These buried gravel tongues are thickest and most numerous in the old deltas of the larger streams, but it is probable that in many places the gravels at the several stages of the infilling coalesce in the axis of the buried valley, so that the water which tlie}^ contain is intercommunicating. The largest stream that enters the valley fi"om the surround- ing hills, except of course the streams in the Great Valley, is Alameda Creek. It has also the largest buried delta, and this delta is the source of the greatest artesian-water supply in the valley. Alameda Creek drains the northern slopes of Mount Hamilton and the southern slopes of Mount Diablo, its drain- age basin including about 600 square miles. It emerges from its gorge through the southward extension of Berkeley Hills at Niles, at an elevation of ept. Geology Bull., vol. 7, No. 4^ Berkeley. 1912. 180 Mkrriam, J. C, Vertebi'ate fauna of the Oriudan and Siestan beds in Middle California: California Uuiv. Dept. Geology Bull., vol. 7, No. 19, Berkeley. 1913. LouDERBACK, G. D., The Monterey series in California: California Univ. Dept. Geology Bull., vol. 7, pp. 177-241, Berkeley, 1913. DicKKRSOX, R. E., The fauna of the Martinez Eocene of California: Cali- fornia Univ. Dept. Geology Bull., vol. 8, No. 6, Berkeley, 1914. 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