DESCRIPTION OF THE GOLD BELT. GEOGRAPHIC RELATIONS. shore was apparently somewhat west of the Oro and the Mariposa slates, are found only on post-Juratrias upheaval, and that their age, present crest, and the sea extending westward the western flank of the . During therefore, is early Cretaceous. The principal gold belt of includes received Paleozoic sediments which now consti­ the earlier part of the Juratrias period portions SUPERJACENT SERIES. a portion of the Sierra Nevada lying between the tute a large part of the central portion of the of the Great Basin were under water, as is shown parallels of 37° 30' and 40° north latitude. It is range. by the fossiliferous beds of that age in Eldorado OEETACEOUS PEEIOD. bounded on the west by the Sacramento and San At the close of the Carboniferous the Paleozoic Canyon south of Virginia City and in the Hurn- Since no beds of early Cretaceous age are Joaquin valleys, and on the east by a diagonal land area of western Nevada subsided, and dur­ boldt Mountains, but nowhere from the foothills known in the Sierra Nevada, it is presumed that line extending from about longitude 120° 40' in ing the larger part of the Juratrias period it was of the Sierra Nevada to the east base of the during the early Cretaceous all of the present the neighborhood of the fortieth parallel to lon­ at least partly covered by the sea. At the close Wasatch, if we except certain beds near Genesee range was above water. gitude 119° 40' in the neighborhood of parallel of the Juratrias the Sierra Nevada was upheaved Valley, are any deposits known which are of late During the late Cretaceous the range subsided 37° 30'. There are other gold-bearing regions in as a great mountain range, the disturbance being Jurassic age. to some extent, allowing the deposition of sedi­ the State, both to the north and south of this accompanied by the intrusion of large amounts The following formations have been recognized ments in the lower foothill region. These belt, but by far the largest quantity of gold is of granitic rock. on the Gold Belt maps: deposits are known as the Chico formation, and produced within these limits. The area thus The Auriferous slate series comprises all of the (1) The Mariposa formation, which occurs in consist of sandstone with some conglomerate. denned contains approximately 9000 square sedimentary rocks that entered into the composi­ narrow bands along the western base of the In the area covered by the Gold Belt maps this miles. At the northern limit the gold deposits tion of this old range of Juratrias time. Forma­ range. The strata are prevailingly clay-slates, formation is exposed only near Folsom on the are scattered over nearly the entire width of the tions representing the Algonkian and all of the which are locally sandy and contain pebbles of up to an elevation of 400 feet, range, while to the south the productive region Paleozoic and Juratrias may therefore form part rocks from the Calaveras formation. Tuffs from and in the Chico district at elevations of from narrows to small dimensions, continuing as a very of the Auriferous slate series. contemporaneous porphyrite eruptions also occur 500 to 600 feet. Since their deposition these narrow strip for some distance south of latitude Fossils of Carboniferous age have been found in them. The fossils of these beds, such as strata have been but slightly disturbed from their 37° 30'. The whole southern part of the range in a number of places, and the presence of Silu­ Aucella and Perisphinctes, have their nearest original approximately horizontal position, but the is comparatively barren. North of the fortieth rian beds at the northern end of the range, north of analogues in Russia, and indicate a very late larger part of them has been eroded or covered parallel the range is not without deposits, but the fortieth parallel, has been determined. A Jurassic age. by later sediments. the country is flooded with lavas which effectu­ conglomerate occurs in the foothills of Amador (2) The Monte de Oro formation, occurring to Auriferous gravels are found to some extent ally bury the larger part of them. and Calaveras counties, interbedded with slates the northeast of Oroville. This consists of clay- in the Chico formation for instance, near Fol­ containing Carboniferous limestone; this con­ slate and conglomerate containing plant remains som showing that the gold-quartz veins had GENERAL GEOLOGY. glomerate is therefore presumably of Carbonif­ of late Jurassic age. already been formed before its deposition. The rocks of the Sierra Nevada are of many erous age. The conglomerate is evidence of a (3) The Sailor Canyon formation, which kinds and occur in very complex associations. shore, since it contains pebbles of quartzite, horn- appears well up toward the summit of the range, EOCENE PEEIOD. They have been formed in part by deposition blende-porphyrite, and other rocks, which have and consists of clay-slates, altered sandstones, and In consequence of slow changes of level with­ beneath the sea and in part by intrusion as been rounded by the action of waves. The tuffs. It is separated from the Mariposa forma­ out marked disturbance of the Chico formation, igneous.masses, as well as by eruption from vol­ presence of lava pebbles in the conglomerate tion by a broad belt of the Calaveras formation. a later deposit formed, differing from it some­ canoes. All of them except the latest have been shows that volcanic eruptions began at a very The fossils indicate that the period of its deposi­ what in extent and character. The formation more or less metamorphosed. early date in the formation of the range, for the tion covered both the later part of Triassic and has been called the Tejon (Tay-hone'). It appears The northern part of the range, west of longi­ hornblende-porphyrite pebbles represent lavas the earlier part of Jurassic time. in the Gold Belt region at the Marysville Buttes, tude 120° 30', consists prevailingly of clay-slates similar to the hornblende-andesites of later age. (4) The Milton formation, which has thus far in the lower foothills of the Sonora district, and and of schists, the latter having been produced by The great mass of the Paleozoic sediments of afforded no fossils; it is lithologically similar to it is extensively developed in the southern and the metamorphism of both ancient sediments and the Gold Belt consists of quartzite, mica-schist, a portion of the Sailor Canyon series, and future western portion of the Great Valley of California. igneous rocks. The trend of the bands of altered sandstone, and clay-slate, with occasional lime­ research may show that it really was deposited During the Eocene the Sierra Nevada remained sediments and of the schistose structure is gener­ stone lenses. On the maps of the Gold Belt at the same time. a separate, low mountain range, erosion continu­ ally from northwest to southeast, parallel to the these sediments are grouped under two forma­ ing with moderate rapidity but no great masses trend of the range, but great masses of granite tions : THE POST-JUEATEIAS UPHEAVAL. of gravels accumulating. and other igneous rocks have been intruded among (1) The Robinson formation, comprising sedi­ Soon after the Mariposa formation had been these schists, forming irregular bodies which ments and trachytic tuffs. This contains fossils deposited the region underwent uplift and com­ NEOCENE PEEIOD. interrupt the regular structure and which are showing the age to be upper Carboniferous. The pression. The result of uplift was the develop­ The Miocene and Pliocene periods, forming generally bordered each by a zone of greater formation is known on the Gold Belt series of ment of a mountain range along the line of the the later part of the Tertiary, have in this atlas metamorphism. These slates and schists and maps only in the Downieville quadrangle, a short Sierra Nevada. The Coast Range also was prob­ been united under the name of the Neocene their associated igneous masses form the older of distance south of the fortieth parallel. ably raised at this time. The action of the forces period. During the Neocene a large part of the two great groups of rocks recognized in the Sierra (2) The Calaveras formation, comprising by was such as to turn the -Mariposa strata into a Great Valley of California seems to have been Nevada. This group is generally called the Bed­ far the largest portion of the Paleozoic sediments nearly vertical position, and to fold them and under water, forming perhaps a gulf connected rock series. of the Gold Belt. Rounded crinoid stems, corals other Juratrias beds in with the older Paleozoic with the sea by one or more sounds across the Along the western base of the Sierra occur beds (Lithostrotion and Clisiophyllum), Foraminifera strata. The Juratrias clay-shales, in consequence Coast Ranges. Along the eastern side of this gulf of sandstone and clay, some of which contain thin (Fusulina), and bivalves have been found in the of pressure, now have a slaty structure, which was deposited during the earlier part of the Neo­ coal seams. These are much younger than the limestone lenses, and indicate that a considerable appears to coincide in most cases with the bed­ cene period a series of clays and sands to which the mass of the range and have not shared the met­ portion at least of this formation belongs to the ding. This epoch was one of intense eruptive name lone formation has been given. It follows amorphism of the older rocks. They dip gently middle or lower Carboniferous. In extensive activity. The Mariposa and other Juratrias and the Tejon, and appears to have been laid down westward beneath later deposits, which were areas of the Calaveras formation no fossils have, older beds were injected with granite and other upon it, without an interval of disturbance or spread in the waters of a shallow bay occupying however, been found, and older rocks may be intrusive rocks. There is evidence that igneous erosion. Marine deposits of the age of the lone the Valley of California and portions of which present in these. It is not likely that post-Car­ rocks were intruded in varying quantities at dif­ formation are known within the Gold Belt only have been buried beneath recent river alluvium. boniferous rocks are present in these non-fossil- ferent times; but that the intrusion of the great at the Marysville Buttes. Along the eastern Streams flowing down the western slope of the iferous areas. mass of the igneous rocks accompanied or imme­ shore of the gulf the Sierra Nevada, at least Sierra in the past distributed another formation diately followed the upheavals is reasonably cer­ south of the fortieth parallel, during the whole POST-CAEBONIFEEOUS UPHEAVAL. of great importance the Auriferous gravels. tain. Those beds that now form the surface of the Neocene formed a low range drained by The valleys of these streams served also as After the close of the Carboniferous and before were then deeply buried in the foundations of numerous rivers. The shore-line at its highest channels for the descent of lavas which poured the deposition of at least the later Juratrias beds the range. position was several hundred feet above the out from volcanoes near the summit. Occupying (Sailor Canyon, Mariposa, and Monte de Oro for­ The disturbance following the deposition of present level of the sea, but it may have fluctu­ the valleys, the lavas buried the gold-bearing mations), an upheaval took place by which the the Mariposa beds was the last of the move­ ated somewhat during the Neocene period. The gravels and forced the streams to seek new Carboniferous and older sediments under the ments which compressed and folded the Aurifer­ lone formation appears along this shore-line as a channels. These have been worn down below then retiring sea were raised above water level, ous slate series. The strata of succeeding epochs, brackish-water deposit of clays and sands, fre­ the levels of the old valleys, and the lava beds, forming part of a mountain range. The beds lying nearly horizontal or at low angles, prove quently containing beds of lignite. with the gravels which they protect, have been were folded and compressed and thus rendered that since they were accumulated the rock mass The Sierra Nevada during this period was a isolated on the summits of ridges. Thus the schistose. Smaller masses of granite and other of the Sierra Nevada has not undergone much range with comparatively low relief. The drain­ Auriferous gravels are preserved in association igneous rocks were intruded at this time. compression. But the fact that these beds now age system during the Neocene had its sources with lavas along lines which descend from north­ occur above sea-level is evidence that the range near the modern crest of the range, but the east toward southwest, across the trend of the JUEATEIAS PEEIOD. has undergone elevation in more recent time. channels by no means coincided with those of the range. The nearly horizontal strata along the The areas of land and sea which existed during present time. Erosion gradually declined in western base, together with the Auriferous the earlier part of this period are scarcely known. THE GOLD-QUARTZ VEINS. intensity and auriferous gravels accumulated in gravels and later lavas, constitute the second Fossiliferous strata showing the former presence The extent of the gold deposits has been indi­ the lower reaches of these Neocene rivers, the group of rocks recognized in the Sierra Nevada. of the Juratrias sea have been recognized in the cated in the introduction to this description. In gold being derived from the croppings of veins. Compared with the first group, the Bed-rock southeastern portion of the range, at Mineral character they may be classed as primary, or Such gravels could accumulate only where the series, these may be called the Superjacent series. King, where the sediments are embedded in deposits formed by chemical agencies, and sec­ slope of the channel and the volume of water were sufficient to remove the silt while allowing BED-ROCK SERIES. intrusive granite; at Sailor Canyon, a tributary ondary, or those formed from the detritus pro­ of American River; in Plumas County at the duced by the erosion of the primary deposits. the coarser or heavier masses to sink to the PALEOZOIC EEA. north end of the range about Genesee Valley The primary deposits are chiefly gold-quartz bottom with the gold. During the Paleozoic era, which includes the and elsewhere; and in the foothill region from veins, fissures in the rock formed by mountain- During the latter part of the Neocene period periods from the end of the Algonkian to the end Butte to Mariposa counties in the slates of the making forces and filled with gold-bearing volcanic activity, long dormant, began again, of the Carboniferous, the State of Nevada west Mariposa nd Monte de Oro formations. quartz deposited by circulating waters. The and floods of lavas, 1 consisting of rhyolite, of longitude 117° 30' appears to have been a land The land mass that originated with the post- gold-quartz veins of the Sierra Nevada are found andesite, basalt, and plagioclastic glassy rocks area of unknown elevation. This land probably Carboniferous upheaval became by gradual eleva­ in irregular distribution chiefly in the Auriferous chemically allied to trachyte, were ejected from extended westward into the present State of tion very extensive toward the end of the Jura­ slates and associated greenstone-schists and por- volcanic vents, and these eruptions continued to California and included part of the area now trias period. This continental mass of late phyrites, but they also occur abundantly in the the end of the Neocene. These lavas occupy occupied by the Sierra Nevada. Its western Jurassic time probably reached eastward at least granitic rocks that form isolated areas in the slate 1 The term "lava" is here used to include not only such as far as the east base of the Wasatch Mountains. series. While some gold-quartz veins may ante­ material as issued from volcanic vents in a nearly anhydrous This conclusion is based on the fact that the date the Jurassic period, it is reasonably certain condition and at a very high temperature, but also tuff-flows *Jointly prepared by Geo. F. Becker, H. W. Turner, and and mud-flows, and, in short, all fluid or semifluid effusive Waldemar Lindgren, 1894. Revised January, 1897. latest Jurassic beds of California, the Monte de that most of them were formed shortly after the volcanic products. small and scattered areas in the southern part of maximum glaciation. In this interval most of the Sierra Nevada the diabases and porphyrites are between a granite and a quartz-diorite, and is in the Gold Belt, increasing in volume to the north deep canyons of the range were formed. Such, of pre-Eocene age, and contain in most cases fact closely related to the latter. The large areas until, north of the fortieth parallel, they cover for example, are the Yosemite Valley on the secondary minerals, such as epidote, zoisite, ura- occupied by it and the constancy of the type almost the entire country. They were extruded Merced Kiver, the great canyon of the Tuolumne, lite, and chlorite. The unaltered equivalents of justify the special name. mainly along the crest of the range, which still is and the canyon of the Mokelumne. The erosion these rocks basalt, andesite, dacite, and rhyo­ Granite. A granular intrusive rock composed % crowned by the remains of the Neocene volcanoes. of these gorges may have been facilitated by the lite are, in the Sierra Nevada, chiefly of Neo­ of quartz, alkali and soda-lime feldspars, mica, An addition to the gold deposits of the range, fissure system referred to above, for many of the cene or later age. and sometimes hornblende. in the form of gold-quartz veins and irregular rivers of the range appear to follow one or another Tuffs are volcanic ashes formed by explo­ Aplite (also called Granulite). A granitoid thermal impregnations, attended this period of set of parallel fissures for a long distance. sions accompanying the eruptions. Mixed with rock usually occurring as dikes, and consisting volcanic activity. At what point the limit between the Neocene water, such material forms mud flows; and when principally of quartz and alkali feldspar. When the lavas burst out they flowed down and the Pleistocene should be drawn is a some­ volcanic ashes fall into bodies of water they Syenite. A granular intrusive rock composed the river channels. The earlier flows were not what difficult question. On the maps of the become regularly stratified like sedimentary chiefly of alkali feldspars, usually with some sufficient to fill the streams, and became inter- Gold Belt the great andesitic flows are supposed rocks and may contain fossil shells. Breccias are soda-lime feldspars and hornblende or pyroxene. bedded with gravels. They are now represented to mark the close of the« Neocene,/ and this divi- formed by the shattering of igneous rocks into Amphibolite, amphibolite-schist. A massive or by layers of rhyolite and rhyolite-tuffs, sometimes sion is in fact the only one that can be made with­ irregular angular fragments. Tuffaceous breccias schistose rock composed principally of green altered to "pipe-clay." The later andesitic and out creating artificial distinctions. But it is not contain angular volcanic fragments cemented by a hornblende, with smaller amounts of quartz, basaltic eruptions were of great volume, and for positively known that this line corresponds consolidated mud of volcanic ashes. feldspar, epidote, and chlorite, and usually the most part completely choked the channels exactly to that drawn in other parts of the world derived by metamorphic processess from augite- GLOSSARY OF ROCK NAMES. into which they flowed. The rivers were thus between these periods. porphyrite, diabase, and other basic igneous obliged to seek new channels substantially The Sierra, from an elevation of about 5000 The sense in which the names applied to igne­ rocks. those in which they now flow. feet upward, was long buried under ice. The ous rocks have been employed by geologists has Augite-porphyrite. An intrusive or effusive Fossil leaves have been found in the pipe-clay, ice widened and extended the canyons of pre­ varied and is likely to continue to vary. The porphyritic rock with larger crystals of augite and in other fine sediments at numerous points. existing topography and removed enormous sense in which the names are employed in this and soda-lime feldspars in a finer groundmass Magnolias, laurels, figs, poplars, and oaks are amounts of loose material. It seems otherwise folio is as follows: composed of the same constituents. represented. The general character of the flora to have protected from erosion the area it covered Peridotite. A granular intrusive rock gener­ Hornblende-porphyrite. An intrusive or effu­ is thought to indicate a warm and humid climate, and to have accentuated the steepness of lower ally composed principally of olivine and pyroxene, sive porphyritic rock consisting of soda-lime and has been compared with the present flora of slopes. Small glaciers still exist in the Sierra. but sometimes of olivine alone. feldspars and brown hornblende in a fine ground- the South Atlantic Coast of the United States. During the earlier part of the Pleistocene Serpentine. A rock composed of the mineral mass. period the Great Valley was probably occupied serpentine, and often containing unaltered remains Quartz-porpJiyrite. An intrusive or effusive THE NEOCENE UPHEAVAL. for a time by a lake dammed by the post-Miocene of pyroxene or olivine. Serpentine is usually a porphyritic rock consisting of quartz and soda- In the latter part of the Neocene period a great uplift of the Coast Ranges. Later in the decomposition product of rocks of the peridotite lime feldspar, sometimes with a small amount of dislocation occurred along a zone of faulting at Pleistocene this lake evidently was drained and and pyroxenite series. hornblende or biotite. the eastern base of the Sierra Nevada, and the alluvial deposits were spread over the valley. Pyroxenite. A granular intrusive rock com­ Quartz-porphyry. An intrusive or effusive grade of the western slope of the range was There is no valid reason to believe that the posed principally of pyroxene. porphyritic rock, which differs from quartz-por- increased. These faults are sharply marked central and southern part of the Sierra has under­ Gabbro. A granular intrusive rock consisting phyrite in containing alkali feldspars in excess from Owens Lake up to Honey Lake. There gone any important dynamic disturbance during of soda-lime or lime feldspars and pyroxene, or of soda-lime feldspars. was also a series of faults formed apparently the Pleistocene period, but renewed faulting with more rarely hornblende. Hhyolite. An effusive rock of Tertiary or at the very close of the Neocene within the small throw has taken place along the eastern Diabase. An intrusive or effusive rock com­ later age. The essential constituents are alkali mass of the range in Plumas County. Near base of the range in very recent times. posed of soda-lime feldspar (often labradorite) feldspars and quartz, usually with a small amount the crest the Sierra Nevada is intersected by a and pyroxene (more rarely hornblende). The of biotite or hornblende in a groundmass, which system of fissures, often of striking regularity; it IGNEOUS ROCKS. feldspars are lath-shaped. The pyroxene is often is often glassy. is believed that these fissures originated during Rocks of igneous origin form a considerable partly or wholly converted into green, fibrous Andesite. An effusive porphyritic rock of the Neocene upheaval. part of the Sierra Nevada. The most abund­ hornblende or uralite. From this change, also Tertiary or later age. The essential constituents ant igneous rocks there found are of granitic frequent in gabbros, rocks result which are refer­ are soda-lime feldspars (chiefly oligoclase and PLEISTOCENE PERIOD. character. Rocks of the granitic series are red to as uralite-diabase or uralite-gabbro. andesine) and ferromagnesian silicates (horn­ During Cretaceous, Eocene, and Neocene times believed to have consolidated under great press­ Diorite. A granular intrusive rock consisting blende, pyroxene, or biotite), in a groundmass the Sierra Nevada had been reduced by erosion ure and to have been largely intruded into over­ principally of soda-lime feldspar (chiefly andesine of feldspar microlites and magnetite, usually with to a range with gentle slopes, and the andesitic lying formations at the time of great upheavals; or oligoclase) and hornblende or pyroxene (some­ some glass. The silica is ordinarily above 56 per eruptions had covered it with a deep mantle of they are thus deep-seated rocks, exposed only times also biotite). cent. When quartz is also present the rock is lava flows. The late Neocene upheaval increased after great erosion has taken place. Quartz-diorite. A granular intrusive rock com­ called a dacite. the grade of the western slope greatly, and the The rocks called diabase and augite-porphyrite posed of soda-lime feldspar and quartz, usually Basalt. An effusive rock of Tertiary or later rivers immediately after this disturbance found on the Gold Belt maps are not usually intrusive, with some hornblende and brown mica. age, containing basic soda-lime feldspars, much new channels and, rejuvenated, began the work but largely represent surface lavas which have Granodiorite. A granular intrusive rock hav­ pyroxene, and usually olivine. The silica content of cutting deep and sharply incised canyons in been folded in with the sedimentary rocks and ing the habitus of granite and carrying feldspar, is usually less than 56 per cent. It is often dis­ the uplifted crustal block. correspond to modern basalt and augite-andesite. quartz, biotite, and hornblende. The soda-lime tinguished from andesite by its structure. A period of considerable duration elapsed In like manner hornblende-porphyrite corresponds feldspars are usually considerably and to a vari­ Trachyte. An effusive rock of Tertiary or between the emission of the lava flows which to hornblende-andesite, quartz-porphyrite to able extent in excess of the alkali feldspars. This later age, composed of alkali and soda-lime feld­ displaced many of the rivers and the time of dacite, and quartz-porphyry to rhyolite. In the granitoid rock occupies a position intermediate spars, with biotite, pyroxene, or hornblende.

GENERALIZED SECTION OF THE FORMATIONS OF THE GOLD BELT.

FORMA­ PERIOD. FORMATION NAME. TION. COLUMNAR THICKNESS SYMBOL. SECTION. IN FEET. CHARACTER OF ROCKS.

&« Recent. Pal ^^ ~T3° ~T= = 1-100 Soil and gravel. . ,1,0J O River and shore gravels. Pgv 1-100 Sand, gravel, and conglomerate. River and shore gravels. Ng 10-400 Gravel, sandstone, and conglomerate. 10-100 Shale or clay rock. '!w?v^mm?®®$%m -,?».$t$$%$ii!i/f,%ffi(Mip3?M 10-100 Sandstone. W^w3^£$i . . Coal stratum. H ' :''&'S^A:i-'J&^ : ''''?\'*y'.$::-i SUPERJACENTSERIES 5 . 1 lone. Ni :jjjjjjj$^$M X 50-800 Clay and sand, with coal seams.

0 I K W Tejon. Et '^^&f*7+£i£&;'f^£'':~^''fj''^r 10-300 Sandstone and conglomerate. ^i^M''^?^®! Tawny sandstone and conglom­ 50-400 CRETACEOUS liiiPP^^P erate. Chico. Kc M^lifp^^^ GREAT UNCONFORMITY

Monte de Oro. Jo Black clay-slate, with interbedded JURATRIAS Mariposa. Jm I 1000 or greenstones and some conglom­ more erate. Milton. Jml Sailor Canyon. Js

T T TVf f~1 f\ TVT Tjl f\ TJ TIT T 'P "V g|. Intrusive granitic rocks. grd

BED-ROCKSERIES

CARBONIFEROUSOLDERAND Argillite,. limestone, quartzite, 4000 or chert, and mica-schist, with inter- Robinson. Crb more bedded greenstones. Calaveras. Cc

^I'^/l Intrusive granitic rocks. gr grd ^o^'A-rl DESCRIPTION OF THE PYRAMID PEAK QUADRANGLE.

TOPOGRAPHY. Basin. The latter two lakes have no outlet. Lake timber are usually covered by a dense and often River which furnishes water for a ditch lower Tahoe, one of the largest and most beautiful almost impenetrable growth of manzanita bushes down on the , supplying the An account of the relations of the mountains, pla­ mountain lakes in the United States, has a maxi­ and other varieties of chaparral. The higher lower part of Amador County. The Placerville teaus, rivers, and lakes within this quadrangle, mum length from north to south of 21 miles and elevations, from 6000 to 9000 feet, are character­ and the Georgetown ditches have each a capacity with descriptions of the aspects of the district. a maximum width of 12 miles. Its areal extent ized by various species of firs, and also by the of several thousand miners'. inches. The import­ The Pyramid Peak quadrangle includes the is approximately 190 square miles, only 20 square tamarack (Pinus contortd). The silver fir, a ant watershed of Silver Creek is not yet utilized. territory between the meridians of 120° and 120° miles, however, lying within the limits of this tree of great beauty, grows chiefly above an 30'west longitude and the parallels of 38° 30' quadrangle. The elevation of its surface is 6225 elevation of 8000 feet. The timber of the higher GEOLOGY. and 39° north latitude. The quadrangle is 34.5 feet. The few soundings available show that, belt is of much less desirable quality than that A classification, description, and statement of the miles long and 27 miles wide, and contains 931.5 except at the shoals at the southern end, due to from the lower elevations. occurrences of the several Mnds of rocks found square miles. It embraces portions of Placer, sediments brought down by the Upper Truckee, Within a zone bordered on the west by a line in the quadrangle. Eldorado, Amador, Calaveras, and Alpine counties, the shores slope, rapidly below water level to drawn from Loon Lake to Echo, Silver Lake, and eastern Eldorado County occupying the central depths of over 1000 feet, and that the maximum , and on the east by the foot of BED-ROCK SERIES. and main portion of it. depths, ranging from 1400 to 1645 feet, are found the escarpment facing and Lake This series consists of the sedimentary rocks Relief. The quadrangle includes a part of the along the central north-south line, the deepest Valley, glacial erosion has removed most of the which were uplifted and compressed during or summit region of the Sierra Nevada, southwest of soundings being recorded near the northern shore. soil; as a consequence, this summit area is char­ before the post-Jurassic mountain-building dis­ Lake Tahoe, the elevations ranging from 3000 The larger part of the drainage is toward the acterized by vast stretches of bare rock, over turbance, together with the igneous rocks asso­ feet above sea-level in the canyon of the South Pacific. Branches of the American River drain which only smaller patches of forest growth are ciated with or earlier than this upheaval. Fork of the American Kiver to 10,430 feet on the the northern portion, the waters finally finding scattered. In the timber belt proper there is a summit of Round Top Peak. The relief is of the their way to the Sacramento River, while the notable absence of grassy meadows; the ground SEDIMENTARY ROCKS. most varied character. The level meadows south southern portion is drained by the Cosumnes and is usually covered by a thick, olive-green carpet Details of character and distribution. of Lake Tahoe contrast with the rugged cliffs of Mokelumne rivers, both flowing into the San of tar weed. Above an elevation of 5500 feet the peaks surrounding them, and these again with Joaquin River. Of the branches of the Ameri­ there are, on the other hand, many small scat­ It should be first stated that no fossils have the forest-covered plateaus and deeply incised can, the Rubicon is the most northern; it heads a tered meadows with rich pasture, in which wild been found in any of the sedimentary rocks of canyons to the west. In grandeur and beauty short distance north of Pyramid Peak and flows flowers blossom during the short summer months. the Bed-rock series shown on this sheet, and that the region rivals any part of the High Sierra. in a north-northwesterly direction in a broad open The climate, though more severe in the higher therefore the assignment of ages to these as given In spite of all local irregularities produced by valley, and then, making a big bend north of the than in the lower altitudes, is generally character­ below is in all cases tentative only. The deter­ vigorous trenching, the part of the quadrangle boundary line, returns southwesterly in a deep ized by dry, warm summers, with only occasional minations are, however, based on a comparison west of a line drawn from Mokelumne Peak to and narrow canyon. The region between the thunder showers, and by a heavy snowfall during with formations of approximately known age in Tells Peak shows topographic features strongly Rubicon and the South Fork of the American is the winter; in severe winters the snow may adjoining districts, and therefore, in general, pos­ resembling those generally found in the Sierra drained by Silver Creek, which empties into accumulate to a depth of 20 feet on the level. sess a strong degree of probability. Nevada well up toward the divide. It may be the South Fork farther westward; the several The temperature at an elevation -of 6000 feet Calaveras formation. The rocks referred to considered as an irregular plateau with a gentle branches of Silver Creek spread out in fan shape may reach 90° F. in the summer, while during this formation occupy several areas along the westerly slope, above which hills, often level-top­ and receive nearly the entire westerly drainage of the winter the thermometer scarcely ever falls western and southwestern border of the quad­ ped, rise to a height of 500 or 1000 feet. Such Pyramid Peak Range. The grade of the Rubicon below 0° F. or 18° C. On the other hand, frosts rangle. They extend only a few miles eastward, are Robbs Peak, Alder Hill, and Leek Spring River within this quadrangle averages 143 feet to frequently occur during the summer at elevations the area in the southwestern corner reaching the Hill. This plateau is cut in two by the deep can­ the mile. The South Fork of the American River, above 6500 feet. The annual precipitation ranges farthest point in that direction; and they are yon of the South Fork of the American River, the which flows in a deep canyon almost due west, from 40 inches at elevations of about 3000 feet to separated by bays of granitic rock, some of which northerly portion being limited in the northwest receives no important tributaries from the north, a maximum of TO inches at an elevation of 7000 reach far out into the adjoining Placerville quad­ corner by the Rubicon River, the southerly in the and only two large ones from the south, Alder feet. Over the Lake Tahoe basin the precipita­ rangle. The very irregular and jagged contact southeast corner by the Mokelumne River. Both Creek and Silver Fork. The grade of the South tion is decidedly smaller than at a corresponding well indicates the tearing asunder of the older parts are extensively dissected by a large number Fork averages 168 feet to the mile, but is very altitude on the western slope. slate formation by the intrusive granitic magmas. of creeks, generally flowing in a westerly direc­ variable, ranging from 50 feet up. Attention is There are no cities or towns within the quad­ These schist masses form the eastern edge of tion in sharp V-shaped canyons; and where the called to the fact that the South Fork does not, rangle. In fact, the permanent population is that large body of more or less altered sedi­ smaller canyons are crowded the plateau char­ like a normal river, split up into branches which, probably less than 100, and is chiefly confined to mentary rock referred to the Calaveras formation acter becomes correspondingly less pronounced. with gradually increasing grade, head among the the southwestern corner. A few people remain and represented on the Placerville sheet. Imper­ The general elevation of the plateau in the south­ higher peaks of the range; its cany on is beheaded during the winter at the road stations along the fect fossils have been found in the Calaveras for­ west corner is nearly 4500 feet, while the highest at Audrain Lake by the precipitous trench of the South Fork, and a few winter at the southern mation indicating clearly a Paleozoic and prob­ point reached at the base of the Pyramid Peak Upper Truckee. shore of Lake Tahoe. During the summer, how­ ably a Carboniferous age. While it is probable Range is about 8000 feet. The Rubicon and The several forks of the , from ever, the region is populated by cattlemen that this series is of the same age throughout, American rivers have cut their canyons to an Camp Creek on the north to the South Fork of and sheepmen, who drive their herds and flocks this can not be considered proved, but on account average depth of 2000 feet; the Mokelumne Can­ the Cosumnes, drain the southwestern plateau, from the dry pastures of the foothills to the fresh of lithologic similarity the whole has been yon, south of Mokelumne Peak, is 4500 feet deep. and the more important among them head in the meadows of the mountains. The delightful included in the Calaveras formation, which is North of the American River the plateau is vicinity of Alder Hill, the elevation of which is summer climate also brings a great number of defined as consisting of Paleozoic beds whose ages terminated toward the east by the Pyramid Peak Y900 feet; they do not reach the summit of the visitors, for whose convenience there are several can not at present be further determined. Range, rising in rough and jagged outlines to range. The grade is generally above 100 feet to stopping-places at the southern end of the great The whole of the Calaveras formation in the elevations of from 9000 to 10,000 feet. The the mile, and sometimes reaches 500 feet. Finally, lake and along the main highways. eastern part of the Placerville quadrangle and in broad, glaciated valley of the Rubicon divides the southeastern portion of the quadrangle is The principal industries are cattle-raising and this quadrangle has a pronounced siliceous char­ the Pyramid Peak Range from a complex of sharp drained by the Mokelumne River and its several timber-cutting, the latter chiefly near the western acter; it consists of altered sandstones grading peaks separated by glacial valleys and rising at branches. In the deep Mokelumne Canyon the boundary and near Lake Tahoe. Gold-mining into quartzite, and clay-slates grading into mica­ the head of glacial amphitheaters; this complex grade ranges from 120 feet to 250 feet to the occupies a very subordinate position, though ceous schists. The nature of this metamorphism extends from Ralston Peak to the northern mile. there are some placer mines and quartz-prospects is partly regional, due to dynamic movements boundary of the quadrangle, and a dozen points Many beautiful glacial lakes are found along in the southwest corner. affecting a large part of the Sierra Nevada, chiefly attain an elevation above 9000 feet. An abrupt the summit region. The largest occur on the Three chief transmontane highways cross the occurring prior to the great granitic intrusions, escarpment descends eastward from these high slope toward Lake Tahoe, quadrangle. The road from Georgetown to Lake and partly local, due to heat and emanations from ridges and peaks to the level of Lake Tahoe. being 3 miles long, while Echo Lake and Cascade Tahoe crosses the northwest corner, from Uncle enormous masses of intruded granitic magmas. South of the American River the plateau may Lake are of smaller extent. The most notable Toms Cabin to Gerle Creek; the central and While the latter metamorphism is superimposed be considered to extend as far eastward as Bryans ones on the western slope are Loon Lake, near most important road, from Placerville to points upon the former and the phenomena resulting Ridge, south of Echo Lake, where it attains an the northern boundary of the quadrangle, and in Lake Valley and Nevada, follows the South from each are not always easy to discriminate, it elevation of over 8500 feet, but immediately Silver Lake, at the head of the Silver Fork. Fork of the American, crossing the divide by is clearly seen that the extremely altered sedi­ south of this there rise above it the irregular Vegetation and culture. In the larger and Johnson Pass; the third, from the lower part of ments are found only at contacts with the granitic mountain complexes around Twin Lakes and western part of the quadrangle the summits of the Amador County to points in Alpine County and rocks, and that the degree of metamorphism Silver Lake which culminate in Round Top ridges, and to a less extent the canyon-slopes, are Nevada, follows in general the divide between gradually decreases away from these contacts. Peak*. Again, near the southern boundary of covered with soil and support a luxuriant the Cosumnes and Mokelumne rivers, crossing The contact zones are here very wide, typical the quadrangle, there towers above the plateau, forest growth of coniferous trees. The abundant the divide at , just east of the eastern contact-metamorphic rocks often being found 2 the sharp and rocky Mokelumne Peak, from occurrence of deciduous trees is confined to the boundary line. Beside these, there" are a great miles from the contacts, or even farther in case of which a descent of 4500 feet in less than 2 miles western border, where oaks mingle with the number of roads and trails, so that travel on projecting masses of sedimentary rocks surrounded carries one down to the bottom of the magnificent conifers, and to occasional places along the water­ horseback at least is easy except in the higher, on all sides by granite. It does not appear prob­ chasm of the Mokelumne River. courses, where willows and alders flourish. glaciated regions. able that any of these rocks are of Archean age. Drainage. The narrow strip to the east of the Scrubby oak-brushes have, however, been noted The abundant water supply is utilized by Less altered rocks, the clastic character of main divide drains northward through the Upper up to an elevation of 7500 feet. The pines, firs, means of storage reservoirs and ditches for irriga­ which is clearly apparent, occur at a few places into Lake Tahoe, the southwestern and cedars attain the greatest dimensions and tion and mining at lower elevations. The princi­ near the western border of the quadrangle. They part of which falls within this quadrangle. Lake most perfect development in a belt ranging in pal ditches are the Georgetown Water Company's, are principally dark clay-slates and quartzitic Tahoe again drains through the Truckee River elevation from 2500 to 6000 feet, the most valu­ diverting the headwaters of Little Rubicon and rocks which under the microscope show their into Pyramid and Winnemucca lakes, in the Great able varieties being the yellow pine (Pinus Gerle creeks, with a reservoir at Loon Lake, and fragmental origin. Such rocks occur on Silver ponderosd), the sugar pine (Pinus lambertiana), the Placerville Water Company's, taking their Creek near the western boundary, on Sly Park *The high rounded mountain 2 miles northeast of Twin the Douglas fir (Abies douglasii), and the cedar supply from the South Fork and Silver Fork, Creek, at Fort Grizzly, and southeast of Tarrs Lakes was designated Round Top on the maps of the Wheeler Survey; in the work of the Coast and Geodetic Survey and (Libocedrus decurrens). A similar growth of with a reservoir at Silver Lake. There are sev­ Saw Mill. But the larger part of the Calaveras of the Geological Survey this name has been transferred excellent timber is also found on the lower slopes eral smaller ditches for mining purposes in the formation in this quadrangle is occupied by the though rather inappropriately to the sharp peak southeast contact-metamorphic schists. In places especially of Twin Lakes, the highest in the vicinity. adjoining Lake Tahoe. Smaller areas free from southwestern part, and a large reservoir at Bear exposed to the action of the granitic'magma, such altered near the contacts to gneissoid micaceous making their appearance. The different areas against the schists of the Calaveras formation, as the projecting points at Ellicott's bridge south schists. The contacts are usually sharp, extremely will be described in a more detailed manner. and on the east against granodiorite. Its out­ of Orelli's ranch, at Brockliss's bridge west of so at a point where the road crosses the western The areas west of Loon Lake are variable in crops are not so prominent as those of the other Bullion Bend, and in the isolated areas near area. At other places, as on the west side of composition and appearance. The one adjoining areas, and the resistance to weathering is not so Brownell's ranch, the rock is converted to normal Loon Lake, the contact is very ill defined, the Loon Lake is composed of a reddish, medium- great, as is shown by the smoother, crumbling fine-grained gneiss or mica-schist, and at these reddish granitic outcrops in the area marked grained granitic rock which contains many schist outcrops; nor is the color of the weathered rock places the contacts with the granite, usually dis­ "gr" being everywhere mixed with schistose fragments and is traversed by dikes of normal so distinctly reddish. The normal rock is coarse­ tinct and sharp, are liable to become indistinct. fragments. granodiorite. The area south of the schists grained and light yellowish-gray; the average Somewhat farther away the schists are finer- The long and narrow area west of Tells Peak occupies some rough and pointed hills and shows grain is about 5 mm., while larger quartz grains grained, and are generally of a brownish color is strongly metamorphosed and is composed of great variation. The principal rock is, however, and orthoclase crystals may reach 1 cm. and give from the biotite which they contain. They often gneissoid schists, quartzites, and mica-chlorite- a medium-grained granite, distinctly porphyritic the rock a somewhat porphyritic aspect. Scat­ carry andalusite, characteristic of contact rocks, andalusite-schists. by larger orthoclase crystals, but containing more tered foils of black mica attain 2 mm. in diameter. in well-developed crystals, and such rocks may be The largest area of supposed Juratrias lies in plagioclase than the normal granitite. In composition and structure the rock is almost found more than a mile distant from the contact. Kockbound Valley, between and The large granite area occupying the summits identical with that of Pyramid Peak, and its char­ A typical andesite-schist was collected on the the Pyramid Peak Range. It has a roughly of the Pyramid Peak Range and spreading north­ acter remains constant over large areas. hill 1 mile south of the canyon of the North triangular form and is distinguished by out­ ward across the valley of the Rubicon is, on the Near Hams station the rock exhibits some Fork of the Cosumnes, near the western bound­ crops of dull-gray or brown color. These consist whole, of uniform appearance and composition. variations. For 2 miles north and east there ary-of the quadrangle, 3 miles from the granite. of a series of clearly stratified black slates and The crags and pinnacles of the narrow ridges, occurs mixed with the normal rock a reddish, Excellent exposures are found in the deep white quartzitic rocks. Beautifully banded, hard along which it is difficult and dangerous to pass, aplitic variety, and dark dioritic patches occur in canyons of Silver Creek, Camp Creek, and the rocks, dark-gray and white, also occur in Rock- testify to its high resistance to erosion. The it near the junction of Cat Creek with the Middle North Fork of the Cosumnes, but they are not bound Valley. The normal strike appears to be strong jointing traversing the rock and dipping Fork of the Cosumnes. South of Hams, adjoin­ easily accessible. The schistosity is indicated on north-northwest, with a dip of about 45° to the westward has caused a more moderate slope in ing the schist area, the light-colored granite con­ the outcrops by lines straight on the whole but east, which is very much less than the normal that direction, while precipitous cliffs often mark tains large foils of black mica and is mixed with delicately wavy in detail; heavy benches alternate dip of the sedimentary rocks on the western the eastern face. The top of Pyramid Peak is aplitic and dioritic rocks. with streaks in which the lamination is very fine. slope. In the western part of the area the rocks made up of a pile of enormous blocks, evidently On Camp Creek for 3 miles east of the diorite Nodules and nests of apparently segregated quartz are disturbed and dip in different directions. In produced by the collapse of jointed pinnacles. contact the granite contains hornblende and more are common. On the ridges and slopes distinct the vicinity of Suzy Lake white quartzitic rocks The rock is coarse-grained, yellowish or reddish, plagioclase than usual, forming an intermediate outcrops are rarely seen, as the rock there outcrop, less clearly stratified, often, indeed, and has a decided tendency toward a roughly type between granite and granodiorite. Fine­ weathers to a dark-red soil. appearing massive. The microscope shows that porphyritic structure; the orthoclase appears as grained or porphyritic variations often occur near The area south of Hams, in the southwestern the banded rocks from Rockbound Valley and large grains and imperfect prisms, of reddish- the diorite contacts, which are thereby made corner of the quadrangle, is also intensely altered; from, the Suzy Lake region are porphyrite-tuffs, gray color, up to 2 cm. long; the quartz is very indistinct and ill defined. Thus a granite with micaceous schists and a striped green and white probably deposited contemporaneously with the prominent in dark-gray rounded grains up to 1 porphyritic orthoclase crystals in a groundmass schist, consisting of pyroxene, quartz, feldspar, eruption of the large porphyrite mass of Mount cm. in diameter, while the black mica and smaller with abundant mica and hornblende occurs one- and wollastonite, evidently a product of contact- Tallac. Dikes of typical diabase - porphyrite feldspar and quartz grains lie between these fourth mile northwest of Bryants Saw Mill. metamorphic action on limestone, appear in this were noted on the western shore of Suzy Lake. larger constituents. A specimen from the south­ Finer-grained, reddish, aplitic granite occurs near vicinity. On the western slope of Rockbound Valley ern base of the bowlder pyramid of Pyramid Slippery Ford, along the indistinct granodiorite The stratification can be observed beyond uralite-porphyrites appear, which would seem to Peak contains 73 per cent quartz, 1,2 per cent contact. doubt only in rare conditions, as for instance lie conformably'in the sedimentary series and lime, 5.4 per cent potassa, and 3 per cent soda. The area southeast of the Ice House consists of where quartzite and black clay-slate alternate, which have been made somewhat schistose by North of Tells Peak the rock contains some horn­ a reddish, medium-grained granite, with scattered but, where distinguished, the stratification in pressure. The contact with the granite and grano­ blende and plagioclase, forming a type somewhat foils of black mica. Near Brownell is a dike of most cases approximately coincides with the diorite is generally sharp, and dikes of both approaching granodiorite, but in the Rubicon aplitic granite made up of orthoclase, albite, and superimposed schistosity. In the northern por­ rocks are found in the sedimentary series near Valley it again becomes normal. The contacts quartz; below, along the road in the canyon, it tion of the quadrangle the strike of the schis­ the contact. The metamorphosing influence of with the schist are well defined, as are those with splits up into several smaller dikes breaking tosity is generally due north and the dip either the granitic rocks is clearly marked for a dis­ the granodiorite in many places, though in others through the diorite. about vertical or westward at a steep angle, this tance of at least one-half mile from the contacts, they become very indistinct and the rocks appear Along the crest of the range, south of latitude being contrary to the general rule farther down the sediments being more or less schistose and to grade over one into another, as along the con­ 38° 30', a very coarse and distinctly porphyritic the slope. South of the South Fork of the micaceous. Smaller masses, included in the tact crossing the Rubicon Valley. Even where granite occurs, characterized by very large, well- American the strike is more irregular, but gener­ granite, are converted to gneissoid schists. The the contact is comparatively well defined, it is developed orthoclase crystals. This variety does ally east-west, while the dip is always within 20° large point running out in diorite south of Glen extremely common to find dikes and masses of not appear within the Pyramid Peak quadrangle, of the perpendicular and usually to the north. Alpine Springs is especially metamorphosed to a pegmatitic, aplitic, and dioritic character along but a few large glacial bowlders of this rock Thus, in the canyon of the North Fork of the high degree, and consists of schistose amphibolite the line of junction. The occurrences in the were found in the upper Truckee Valley. Cosumnes, one-quarter mile from the contact, a with many streaks of epidote and tourmaline. glaciated cirques at the head of Blakeley Creek It has become evident, from more detailed strike of east-west with dip 75° N. was observed. The contact of the sedimentary rocks with the and of : the Middle Fork of Silver Creek are studies, that a few of the areas indicated as grano­ This dip symbol has been omitted on the map. porphyrites of Mount Tallac is not very distinct, especially interesting. Bays of normal granodio­ diorite on the Placerville sheet, adjoining to the An examination of the southeastern part of the tuffs, breccias, and dikes occurring along it north­ rite here reach up to the summit of the range west, should rather be classed as granite. These Placerville map and the northeastern part of the west of Glen Alpine Springs. The slate area to from the east, and at the western end of these are the area of Sand Mountain, the area about Jackson map will show that the series in these the east of Mount Tallac consists of clay-slates bays lie dark, dioritic rocks traversed by light- Crystal mine, and the two small masses east of regions also has an abnormal east-west strike. and quartzites; near the porphyrite a striped, colored, mostly pegmatitic dikes. In Blakeley this, at the eastern boundary of the Placerville The cause may possibly be sought in the tuffaceous slate occurs, similar to that of Rock- Basin a large dike-like mass of granite breaks quadrangle. mechanics of the intrusion, the slates in this bound Valley. A much contorted, calcareous across the bay and connects the granite masses on Granodiorite. The granodiorite is the prevail­ vicinity being especially torn up by deeply incised quartzite was noted to the northeast of Mount the north and the south. South and southwest ing rock, occupying a broad belt extending across bays of granitic rock. Horizontal and inclined Tallac. Outcrops of quartzite also occur east of of the sharp, isolated peak, which has an eleva­ the whole quadrangle from north to south. It is joints also traverse the schists, separating them Fallen Leaf Lake. tion of 9500 feet, the granite assumes a finer- of an easily crumbling nature, and falls readily into rhomboidal fragments. The contact of the Four miles southeast of Silver Lake there is an grained habit and weathers to a bright-red color. to pieces under the destructive forces of erosion schists with the granitic rock is usually best ill-defined area of white quartzitic rock mixed in Here its outcrops are strikingly rounded. Four and weathering. The grayish-white outcrops are defined where the contact line runs parallel to the a very irregular manner with granitic dikes and miles west of Morattini's ranch a narrow belt of of rounded form. This form and the brilliant schistosity; wherever it cuts across the strike a masses. This area has doubtfully been referred singularly mixed rocks, dioritic and granitic, light color are especially marked in the glaciated stronger metamorphism accompanied by a feather- to the Juratrias series. extends across the granite. It begins on the east region. The granodiorite is a medium-grained ing-out of the schists and by an injection of gran­ by an extremely sharply defined triangular area to coarse-grained rock, the average diameter of IGNEOUS ROCKS. itic magma is often noted. of dark hornblendic diorite, bordering with dis­ the grains being 2 to 3 mm. The grayish quartz Juratrias. A few isolated areas of schists, Details of character and distribution. tinct contacts against both the granite and the and white feldspar grains are of about equal size; quartzites, and highly altered tuffs are scattered Granite. A normal biotite-granite, or granitite, granodiorite. Westward this soon becomes very the quartz is decidedly less prominent than in the on both sides of the crest in the northern part of as the variety is also called, occupies several ill-defined and mixed with light-colored granitic granite, and the feldspar does not reach the the Pyramid Peak quadrangle. They are in most large areas at Loon Lake, and others along the rocks. Breccias may often be noted, the acid dimensions attained in the latter rock. Black cases clearly of sedimentary origin and have been Pyramid Peak Range, at Echo Lake, at Mokel- rock always forming the cement. mica and hornblende are usually present in about extensively metamorphosed by the granitic rocks umne Peak, and about the headwaters of the The granite of Echo Lake, forming the sharp equal quantities. The foils of the former reach along the contacts. There is no reason for con­ Cosumnes River. Its outcrops are generally dis­ peaks on the north and south sides, is similar to 2 or 3 mm. in diameter, while the hornblende is sidering them as older than the main sedimentary tinguished by a light yellowish or reddish color, that of Pyramid Peak. roughly prismatic, the crystals sometimes reaching series on the western slope; in fact, they are due to the sequioxide of iron contained in the The area of Mokelumne Hill is composed prin­ 1 cm. in length. A slight porphyritic aspect may sometimes less disturbed and less metamorphosed orthoclase, and, especially in the glaciated portions, cipally of a medium-grained, light-colored rock occasionally be attained by the rock in conse­ than the former. One of the principal reasons for they contrast strongly with the grayish-white containing small scattered foils of biotite and quence of this crystallographic development of referring them to the Juratrias a reference croppings of the granodiorite. Usually coarse­ weathering reddish. In places, however, as on the hornblende. Titanite is nearly always pres­ which is made with doubt, however is their grained, it consists of quartz, orthoclase, albite, the summit of the peak, the rock is of coarser ent in small, isolated, brownish grains. A typical position in the continuation of strata known to be and biotite. It is harder and of a firmer texture grain. The microscope shows it to be composed rock from the northwestern shore of Silver of that age in the Truckee quadrangle, adjoining than the latter rock, and its masses form the chiefly of orthoclase, microcline, and quartz. The Lake contained 67.5 per cent silica, 3.6 per cent northward; another is that the principal mass, highest and roughest ridges in the region. For rock is not uniform, and on the northern slope of lime, 3.66 per cent potassa, and 3.47 per cent near Mount Tallac, is intimately connected with the same reason bowlders of granite are much the peak there are many scattered streaks of dark soda. large masses of dark-green porphyrite and por- more abundant than those of granodiorite. While hornblendic rock, sometimes schistose in struc­ The appearance and composition of the rock phyrite-tuff, which is characteristic of the Jura­ it varies somewhat in appearance and constitu­ ture. The contacts are generally indistinct are very constant over large areas, with only trias at Sailor Canyon (Truckee quadrangle) and tion, yet it is a typical granite. The contacts except on the northwestern side, the passage small variations in the quantity of hornblende northward. The color of the outcrops of these with the granodiorite are sometimes sharp, but from granite to granodiorite taking place within and black mica. In a few places the quantity of schist areas is usually reddish-brown, contrasting more commonly much pegmatite, diorite, and short distances. hornblende diminishes, and the rock then assumes strongly with the light-gray granodiorite. granite-porphyry occur on the contacts, making The large mass of granite in the southwestern a habit more similar to that of granite, as at Buck The two small areas, at the northern boundary them indistinct. In other places transition forms part borders on the north against the diorites of Island Lake, between Rubicon Peak and Rubicon consist of quartzite and black slates, the latter I may be observed, hornblende and plagioclase the South Fork of the American, on the west Point, and in the area east of Fallen Leaf Lake. Microscopical and chemical investigation shows very coarse, containing large crystals of horn­ minor intrusions of acid and basic magmas, and the granitic rocks fractured the sediments along the rock at these points to be a granodiorite, blende ; nearer to the schist contact, biotite- that there are diorites, pegmatites, and aplites of extremely irregular lines; sedimentary masses though rather rich in orthoclase. gabbro appears. A small mass of coarse granite the age of the granodiorite and of the granite, were torn away and now lie in the granitic bodies Diorite. A number of smaller areas of diorite reaches the road one-half mile east of Moores, the granite being the older rock. Only on this as detached fragments. The granitic rocks are occur connected both with the granite and with and penetrates and brecciates the surrounding supposition can the contradictory testimony be entirely unaffected by the prevailing schistosity. the granodiorite. The typical diorite is a dark- diorite. On the grade leading up to the Ice House, explained. The diorites of the canyon of the All the rocks are more or less traversed by joints, green, medium-grained to coarse-grained rock, light, medium-grained diorite or granodiorite first South Fork of the American and the smaller but it is along a narrow zone following the summit composed chiefly of dark-green hornblende in appears beyond the granite, while nearer to the areas along the western boundary would then region that the jointing becomes especially promi­ prisms or irregular grains, a little black mica, and schist contact gabbro was noticed. belong to the period of granitic intrusions, while nent. From Tells Peak to the vicinity of Round white plagioclase. Normal quartz-diorites are not From Moores up to Slippery Ford mica-diorites those of Round Top and the Pyramid Peak Top the jointing or sheeting is strongly developed, abundant, as the rock usually takes up consider­ prevail, frequently changing in grain, and in many Range would belong to the granodiorite. and it is excellently exposed in the glaciated able orthoclase at the same time as quartz. Many places running over into nearly normal granodio­ Augite-porphyrite. Most conspicuous among canyons of Summit Creek and Silver Fork. There rocks along the South Fork of the American from rite. The best exposures seen along the ditch the mountains rising south of Lake Tahoe is is one set of joints extending ENE.-WSW. Slippery Ford down are, however, undoubtedly are as follows: From south of Moores to Plum Mount Tallac, with its dark, rocky escarpment which appears to be vertical. Joints belong­ quartz-diorites. A normal diorite from Pyramid Creek there is a dark, fine-grained diorite contain­ facing east, its broad shoulder sloping more ing to this system were also noted in the grano­ Peak contained 51.5 per cent silica, 10.2 per cent ing dikes of granite. At the point one-half mile gently southward, and its precipices crowning diorite along the upper parts of Camp Creek and lime, 1.08 per cent potassa, and 2.9 per cent soda. north of Ditch Camp 7 there is again the same the glacial cirques toward the north. It is built the North Fork of the Cosumnes. There are two The diorite at many places contains pyroxene, diorite with dikes of granite. On the point up of a dark, massive rock which frequently has other sets, both of which have a direction of forming transitions to gabbro, and it is almost before Wolf Creek is reached the rock is a nearly been called basalt. The normal rock is dark- NNE.-SSE., but dip in opposite directions at impossible, in fact, strictly to separate these two normal granodiorite, changing within a short dis­ green in color, and often shows well-defined angles varying from 15° to 40°. While these rocks. Gradual transitions from granodiorite to tance to coarse, dark diorite, which continues to crystals of augite mostly converted into second­ systems have not been studied in detail, slight diorite are extremely common, but the latter rock within 1000 feet of Wolf Creek; then follow ary hornblende. The feldspars are rarely promi­ faulting has been noted in places along the joints. is quantitatively entirely subordinate. Normal lighter, medium-grained quartz-diorites. One mile nent; the groundmass is dark and fine-grained; Only at one place are there indications of syenite has not been recognized; rocks intermedi­ beyond Wolf Creek dikes of granite again begin in places it is somewhat amygdaloid. It ranges extensive faulting. The abrupt slope from Rubi­ ate in composition between a syenite and a to appear. Fine exposures of granite, slightly in structure from a typical diabase-porphyrite to con Peak to Echo Lake has been interpreted as diorite occur to some extent, but can scarcely be porphyritic, appear in Alder Creek. Beyond an augite-porphyrite, according to the variations an old fault scarp, erosion not being adequate separated from the diorites. Smaller areas of Alder Creek coarse, dark diorite appears. Beyond of structure in the groundmass. Schistose struc­ to account for it. The reasons for this interpreta­ diorite sometimes occur, both in granodiorite, as the narrow wedge of highly altered gneissoid ture sometimes appears; at the summit of Mount tion can not be fully set forth except by describ­ at Rubicon Point and west of Rubicon Peak, and schist nearly normal granodiorite appears, but Tallac the rock is converted to a dark-brown or ing areas beyond the limits of the Pyramid Peak in granite, as at the head of the Middle Fork of this soon becomes darker and passes over into the spotted dark-green and dark-brown, roughly quadrangle. Suffice it to say that the evidence is the Cosumnes. They also occur at the contacts gabbro areas below Slippery Ford. schistose rock. A specimen taken 200 feet below clear that Lake Tahoe already existed in Neocene of granodiorite and schist, as at Ellicott's bridge Two miles below Slippery Ford, on the main the summit, on the trail, proved to be a uralite- time and that it is probable that this faulting and near Westmoreland. The diorite occurring road, the granodiorite begins to grow darker, and porphyrite, which along certain lines has been took place toward the close of the Cretaceous in the Pyramid Peak Range has already been a coarse diorite appears. Extending half a mile subjected to strong crushing, producing schistose period. Along the western shore of the lake the mentioned. farther down are diorites of varying grain, the structure. Specimens from the top show porphy­ slope continues steep below the water-line to a The large area between Devils Basin and Echo lighter varieties at many places having been ritic augite and plagioclase in a greatly altered depth of about 1000 feet. The fault is probably Lake lies between granite and granodiorite, with injected into the darker; then several dikes of groundmass filled with brown mica and some located at some distance from the shore, and the schist and augite-porphyrite joining on the north. pegmatitic granite appear, beyond which diorites tourmaline. Other specimens from the top are throw gradually lessens toward Echo Lake. The larger part consists of a dark, medium- of changing habit are again found. These short tourmaline-biotite-schists, carrying also some horn­ South of Echo Lake the canyon of the Upper grained, dioritic rock with frequent transitions to notes will illustrate the extremely variable char­ blende and feldspar; there can scarcely be any Truckee has obviously been excavated by erosion. quartz-diorite and granodiorite, especially on the acter of this area. doubt that this rock represents merely an There are no evidences of Neocene or post- western side. At the western end of Echo Lake Gabbro. Although there are many occurrences extremely altered form of the porphyrite. On Neocene faults. the rock is impregnated with pyrite, the decom­ of gabbro among the western diorites, only a few the road one-half mile east of Glen Alpine SUPERJACENT SERIES. Springs, also, the porphyrite is changed to a dark- position of which gives the outcrops a reddish areas are occupied by normal gabbro which can NEOCENE. color. One mile southwest of Angora Peak there be mapped as such. Of this nature are the irreg­ green schist, often filled with pyrite. Secondary is considerable variation in grain and basicity; ular masses in granodiorite or diorite near the brown mica is very frequently present, developing An account of the river gravels, lava flows, and former valleys in their relations to one streaks of gneissoid rock also appear. Dark, Kirkwood and Woods settlements and northwest in all the constituents of the rock. The occur­ another and to modern topographic features. coarse-grained diorites continue southeast of Echo of Round Top, very dark green, coarse-grained, rence of breccia along the western contact has Lake to the eastern boundary of the quadrangle. heavy rocks. In the Kirkwood area a uralitic rock already been mentioned. The northern contact Auriferous gravels. The Neocene gravels The diorite and quartz-diorite of Round Top occurs, evidently derived from gabbro or pyroxen­ is generally sharp, and along the excellent out­ which lay upon the old bed-rock surface of the show very gradual transitions toward the sur­ ite, while other specimens show the composition crops in the glacial cirques there is no indication range, such as it was before being flooded by lava rounding granodiorite and toward the enclosed and structure of an augite-diorite. Specimens that the porphyrite forms a flow on top of the and trenched by the modern canyons, are very patches of black gabbro and pyroxenite. from the Woods mass show a biotite-gabbro, granitic rocks. At the southern contact with the sparingly represented in this quadrangle, if not The diorite area contained in the granite about closely related to a diabase, while the area near diorite, where Glen Alpine Springs bears N. 15° entirely missing. The rivers which accumulated Pi Pi Valley and Dogtown is of greatly variable Round Top contains an entirely normal gabbro. W., many veins are found filled with quartz, such large masses of gravel lower down on the habit. Pyroxene occurs at many places. At All transitions between granodiorite, diorite, and epidote, and tourmaline, the latter being in the slope of theSierra Nevada deposited scarcely any Lanes Tunnel the rock is an augite-biotite-diorite, gabbro may be seen in this vicinity. center of the vein, the epidote next, and the debris near their headwaters. Only along the while one-quarter of a mile below Dogtown an The irregular areas north and south of the quartz next the walls. The projecting point of western boundary of the quadrangle is this for­ olivine-hornblende-gabbro is found. A uralite- river a few miles west of Slippery Ford generally porphyrite 1 mile west of Glen Alpine Springs mation found, and generally in such very small gabbro occurs at the head of Steeley Fork. consist of dark-green, coarse rock composed of contains very much epidote and is partly schis­ exposures that it can not be indicated on the map. The area near the eastern boundary on Camp dark-green pyroxene and large grains of reddish- tose. Certain of the phenomena of metamor- It occurs in intimate connection with rhyolitic Creek and the North Fork of the Cosumnes gray, basic plagioclase. Microscopic evidence phism in this rock, notably the development tuffs, and, as is natural, only in the bottom of the varies from a biotite-diorite to an olivine-uralite- shows that they are entirely normal uralite- of biotite and tourmaline, may with great prob­ old depressions. gabbro. The contacts of these areas toward the gabbros. A specimen from the area 5J miles ability be attributed to the influence of the The three small areas indicated as rhyolite on granite are usually indistinct, and many cases of west of Slippery Ford shows the composition of granitic magma. There is no doubt of the inti­ the hill northwest of Bullion Bend are occupied by apparent transitions occur. East of Dogtown a very basic olivine-gabbro, almost to be classed mate connection of the augite-porphyrite with gravels of the rhyolitic period reaching a thick­ pegmatitic dikes are injected in the diorite. as a peridotite. The contacts of the gabbro with the adjoining sedimentary series. Whether it ness of 40 feet. The gravel is well-rounded river- At the contact of granite and granodiorite on the granite are fairly sharp and well defined, but should be considered as an intrusion or a flow or wash, and contains pebbles of rhyolite, quartz, Camp Creek south of Morrison the two rocks toward the diorite abundant transitions may be succession of flows is a question which is still slate, and granitic rocks. A few feet of gravel lies are separated by a zone one-fourth of a mile wide noted. unsettled, but it is probably of intrusive origin. in the deeper, post-rhyolitic, lava-capped channel a containing diorite mixed with pegmatitic dikes. The succession of the granitic rocks. There is Similar porphyrite masses occur connected with little farther east. Eight feet of granitic gravel is An almost bewildering complexity is found in hardly any doubt that all of the granitic rocks Juratrias strata at Sailor Canyon (Truckee quad­ exposed below the rhyolite at Ditch Camp 7. A the diorite area following the canyon of the South are later than the altered sedimentary rocks and rangle) and at other places farther north. few feet of often imperfectly washed gravel, com­ the augite-porphyrite, but it must be confessed posed mostly of quartz and metamorphic schist, Fork of the American River from Bullion Bend SEQUENCE OF EOCKS AND STEUCTUEAL FEATUEES. up to Slippery Ford. The area is enclosed by that in spite of good exposures the evidence as to lies at different places under the lava along the An account of the relative ages of the rock-masses granite and schists on the north and south sides, the relative age of the granite, granodiorite, Neocene depression extending down to Fort and is bordered by granodiorite on the east diorite, and gabbro is not decisive and in some and the effects they have suffered. Grizzly by Dogtown, Mayer, and Barneys. One and west. Good exposures are seen along the respects is even contradictory. There is some In the sequence of rocks deduced from the mile west of Dogtown the channel is cut by two road, as well as along the Placerville ditch on evidence, based on the general form of the Pyra­ study of this district the Paleozoic slates and creeks and offers the best exposures. The follow­ the southern side of the canyon. Normal grano­ mid Peak granite area and the manner in which schists of the Calaveras formation are determined ing section is exposed: On the granitic bed-rock diorite appears at the boundary, continuing west­ it includes the slate fragments, as well as on the to be the oldest. The schistose structure was rests 10 feet of gravel and sand; above follows 3 ward into the granodiorite area of Soldier Creek occurrence in it of dikes of a rock allied to grano­ produced before the granitic eruptions, which feet of white rhyolitic tuff, again covered by 15 (Placerville quadrangle). This changes gradually diorite, tending to show that the granite was affected the adjoining schists only by making feet of coarse and fine sand. The whole is capped to the complex diorite area of Bullion Bend. On intruded earlier than the granodiorite. On the them more crystalline. Next in age follow the by andesitic breccia. Near Fort Grizzly there the northern side of Bullion Bend the rock is other hand, it is unquestionably true that the probably Juratrias areas along the crest, and must be accumulations of similar or greater thick­ largely gabbroitic, typical gabbro, sometimes granite of the southwestern corner sends out their accompanying tuffs and porphyrite masses. ness, but the bottom of the channel is not well with approximation to diabasic structure, occur­ numerous dikes in the diorite of the South Fork This series is similarly affected by contact met- exposed. ring at many places. of the American River; this diorite again shows amorphism, but shows much less evidence of Basalt. In two places evidences appear of an At Moores station there are very fine exposures numerous local transitions to apparently normal schistose structure. Still later occurred the intru­ old basaltic eruption antedating the andesite and of granite dikes injecting and brecciating the granodiorite; so that, if it be conceded that this sions of the rocks of the granitic series; these are probably also the rhyolite. The first of these is fine-grained, dark niica-diorite. Westward along diorite area is of appromimately 'the same age as almost certainly all of very late Jurassic or early 2 miles south of Kirkwood's ranch, where a small the. road, the rock changes one-half mile from the main granodiorite mass, it follows that the Cretaceous age. It can not be said that the suc­ exposure of a black normal basalt occurs below Moores to coarse and dark olivine-gabbro, and granite is later than the granodiorite. The prob­ cession of the different granitic rocks has been the rhyolite. The second is near the southern the dikes of granite cease to appear; three- ability is that the intrusion both of the granite 'definitely ascertained, but the granite is probably boundary, southeast of Hams station, where sev­ quarters of a mile from Moores the rock becomes and of the granodiorite was accompanied by older than the granodiorite. By their intrusion eral smaller areas of a similar basalt cap the Pyramid Peak 3. ridges. While the exposures do not seem decisive, roughly rounded gray or brown andesite bowlders valleys with steep slopes, not quite so abrupt, Neocene drainage line, and that the Tallac system it is probable that this basalt also is older than are abundantly strewn over the surface. In the however, as the modern canyons. of peaks rose to more pronounced level summits the andesite. eastern, glaciated part the exposures are very A well-defined depression is indicated by the to the east of the Rubicon. Indeed, evidence JRhyolite. The occurrence of rhyolite is con­ much better; in numberless places the beauti­ contact lines in the region lying between the from adjoining regions shows that the whole fined almost entirely to the southern part of the fully bedded structure is brought out, resulting Rubicon and the Little South Fork. The deepest depression of Lake Tahoe Avas already outlined quadrangle and only a relatively small number from the superimposing of numerous flows of part of this channel is now from 600 to 700 feet in Neocene times. of square miles are occupied by this rock. A slightly differing structure. These long slopes above the bottom of the present canyon, and it It has already been noted that the canyon of flow of rhyolite once filled the bottom of the of a somber, dark-gray or reddish-gray color, continued westward to Ellicott's, joining the the present South Fork is suddenly cut off at Neocene valley of the South Fork of the Ameri­ covered by scanty herbage or scattered trees, Neocene Middle Fork in the northeastern corner Johnsons Pass, near Audrain Lake, by the deep can and the lower Mokelumne and its Dogtown alternate with precipitous walls strongly resem­ of the Placerville quadrangle. Immediately south and narrow trench of the Upper Truckee, and tributary, but is now partly eroded, partly cov­ bling fortifications with scarps, parapets, and of this channel the bed-rock rises abruptly, 1000 that the river has no normal headwaters. The ered by andesite. The places of eruption of this buttresses. In places where erosion has carried feet in a mile, to the flat-topped ridges of Robbs andesite occurring in it proves that the Canyon acid lava were located without doubt in the east­ its work still further, as in the vicinity of Thim­ Peak, which is a conspicuous feature in the land­ of the Upper South Fork existed practically in ern portion of the area here mapped. The prin­ ble Peak, peaks and pinnacles of the most fantas­ scape even at the present time. This high pla­ its present form before the andesitic eruptions. cipal flow can be traced to the high volcanic com­ tic form result. Among other fine exposures teau, now deeply dissected by the forks of Silver From this peculiar windgap of Johnsons Pass plex about 4 miles south of Echo. Flowing down may be noted Castle Point, on the Amador grade, Creek, was undoubtedly more extensive in Neo­ (elevation 7400 feet) another entirely similar a steep tributary, it found the main Neocene river and Old Round Top, north of Twin Lakes. cene times, though already at that time it was windgap, 1500 feet deep, lying to the southeast, near the present bend of Silver Fork, and fol­ The andesitic flows consist almost entirely of considerably eroded. Silver Creek probably may be perceived across the Little Truckee Can­ lowed it down by Morrisons and Plum Creek. tuffs and tuffaceous breccias in an indefinite drained into the South Fork of the American yon. This is Luthers Pass (elevation 7700 feet), A small flow of this lava followed the Dogtown number of sheets, differing in hardness as well as below the andesite table of Peavine Ridge. As leading over into Hope Valley (Markleeville tributary by Pi Pi Valley and Sopiago Creek to in size and frequency of the andesite bowlders, indicated by the andesite areas, Silver Creek has quadrangle), and the watershed of the Carson the larger areas at Fort Grizzly. Whence this the latter ranging up to several feet in diameter. deepened its channel by only a few hundred feet River. No other explanation of these facts flow and that of the Dogtown tributary came is They all consist of angular andesite fragments at most at the junction of the forks, and it is evi­ appears possible than that the South Fork of the uncertain; it appears as if there might have been bound in a cement of finer andesitic detritus dent that the granodiorite in its upper drainage American formerly rose in Hope Valley, that the a local eruption in this vicinity, for between Fort very little non-andesitic material is present, formed an extensive plateau, somewhat lower than has captured the headwaters, and Grizzly and Silver Lake no trace of the rock is though occasional granitic bowlders may occur. that of Robbs Peak on account of the softer char­ that the Upper Truckee has cut the canyon in found. The andesite is a dark, rough, and porous rock acter of the rock. two. As it is known that Hope Valley was a Before the andesitic eruption the surface of the containing porphyritic crystals of plagioclase and The river-channel corresponding to the South few hundred feet lower than Luthers Pass at the rhyolite suffered considerable erosion, so that its almost invariably pyroxene, principally augite, Fork of the American left this quadrangle at the time of the andesite flows, it may be concluded thickness differs much in different places. The but also hypersthene; hornblende is less abundant, hydraulic mines 1 mile northwest of Bullion that these events happened long before the begin­ heaviest masses are not found near the place of but also common; the groundmass varies from Bend, to continue down the slope by Pacific ning of the volcanic eruptions. eruption but near the western boundary of the microcrystalline to glassy. Flows of massive House (on the south side of the present river) The other branch of the Neocene South Fork quadrangle. A maximum thickness of 400 feet andesite occur rarely, but near the volcanic and Placerville. Near this point, where it is 600 headed, as noted above, near Round Top, and its is found on Plum Creek, while ordinarily it does centers the tuffs and breccias often contain necks feet above the present river bed, it branches, and headwaters were of a normal character. Near not exceed 300 feet, and is often much less. A of massive hornblende-andesite, as on Old Round the two forks can be traced upward as follows: the western boundary of the quadrangle it was tendency to form steep bluffs distinguishes the Top, north of Twin Lakes. The largest volcanic The first, crossing the present river again, is separated from the tributaries of the Mokelumne rock in many places. It is commonly massive, neck is represented by Round Top Peak, made found near Ditch Camp 7 and on Plum Creek by a comparatively low divide, but its canyon tuffs occurring only near the western boundary. up chiefly of hornblende-andesite; and a flow of above this; then it appears again at Morrison's deepened rapidly eastward and is well exposed The normal rhyolite is a white, gray, or pink, a similar gray hornblende-andesite is noted to the ranch, on Alder Creek; it then makes a more by Alder Creek, cutting across the channel, at fine-grained rock, somewhat porous and easily east of Thimble Peak. Andesites containing northerly swing, and is found on both sides of Morrison. A narrow ridge 1500 feet high sepa­ dressed with the hammer. It carries small sani- mica are rare, though found on Round Top and Hells Delight Valley, and crosses Silver Fork rated this branch from the northerly fork, just dine and quartz crystals in a fine-grained, streaky at the southern edge of the complex 4 miles south at the great bend of that stream; thence it crosses described. Southward the slopes also rose rapidly groundmass (lithoidal rhyolite). Very rarely a of Echo. The marked stratified structure and again and enters under the eastern andesite areas, 1500 feet to the rolling high granitic plateau little brown mica appears. A somewhat peculiar the similarity of the breccias over the whole one branch heading at Thimble Peak and another culminating at Leek Spring Hill. East of this variety occurring near the bend of Silver Fork slope of the Sierra are evidences that the rocks north of Twin Lakes. A smaller tributary is the topography gradually grew" more rugged, and weathers in gray, rounded outcrops and contains were spread over the area as thin successive mud noted descending from the high complex north­ the character of broad shoulders of granodiorite very abundant crystals of quartz, feldspar, and flows. All of the flows slope westward at an east of the bend of Silver Fork. The second fork, separated by deep canyons is very clearly indi­ mica. A tendency to a tuffaceous and brecciated angle of from 1 to 3 degrees. Andesitic sandstones a very important one, joining the first near cated by the contact lines. From the western texture may often be noted. A small mass of bearing more evidences of having been deposited Bullion Bend, followed very nearly the present boundary of the quadrangle to Morrison the Neo­ white rhyolitic tuff is found in a gulch in the in a volcanic lake occur at Round Lake. While river course up to Audrain Lake. cene channel has an approximate grade of 130 andesite north of Twin Lakes. no well-defined craters are now visible, it is prob­ Along this latter channel the andesite, at sev­ feet to the mile, while from Morrison to the bend Andesite. The andesitic flows were the latest able that all of the volcanic material was erupted eral places between Bullion Bend and Slippery of Silver Fork the grade may be in the vicinity of the Neocene series of eruptions and cover large from rounded orifices or vents under the lava Ford, descends suddenly on the northern canyon of 110 feet; between the latter place and Twin areas in the southern part and in the northwest­ masses north and south of Twin Lakes, the vents slope, as near Moores and west of Brownells, Lakes the grade is nearly 200 feet to the mile. ern corner of the quadrangle, while the northeast­ finally being stopped up by necks of massive plainly indicating the existence of a deep and The watershed of the Neocene Mokelumne ern part is remarkably free from them. In andesite. The breccia flows in the northwestern narrow channel. The andesite is in place, its posi­ River in this region coincides, roughly speaking, general, the andesitic rocks now form the tops of corner originated in the volcanoes of Mount tion being due neither to faults nor to landslides. with its present drainage, but also takes in the the ridges, but the contact line with the underlying Mildred, shown on the Truckee sheet. The lowest elevations reached are 600 feet above headwaters of the present Cosumnes. The Neo­ granitic or schistose series is far from being as The Neocene topography. As there are, within the present stream channel, the bed-rock rising cene channel of the Mokelumne is exposed near regular and even as it often is at lower elevations this region, no evidences of Neocene or post- 1000 feet within a mile of the lowest depression. Fort Grizzly, from which place it continues south­ on the slope of the Sierra; indeed, proofs are Neocene faulting, nor evidences which would lead One and a half miles southwest of Slippery Ford west below the andesite ridge into the Jackson everywhere abundant that the surface upon to the belief that any strongly marked deforma­ post-office a small andesite mass is found, 600 feet quadrangle. It can be traced upward, crossing which the andesitic lavas flowed out was an tions of the surface have occurred, it follows that above the river, and others occur at low eleva­ Tiger Creek at Tarrs Saw Mill and Panther irregular one, possessing considerable relief. The a study of the numerous contact lines of the tions east of this. The old river channel here Creek near Dutch Henry. It probably crossed present canyons are, however, cut considerably Neocene eruptives with the underlying bed-rock probably makes a bend, following Silver Fork a the southern boundary near Westmoreland, and below the Neocene surface, and during this proc­ series may give a correct idea of the detailed few miles up, and then, crossing under the is again found in the Big Trees quadrangle south ess a great part, perhaps half, of the original topography of the surface on which these flows andesite area, again appears in the canyon of the of the present river. volume of the lava flows has been removed. It were spread. Over a large portion of the region modern river, 500 feet above its bed. The lowest South of this channel line the andesite contact is evident that the flows once covered continu­ it would indeed be feasible to reconstruct the andesite on the trail from Slippery Ford up Silver rises several hundred feet, but the great Mokel­ ously nearly the whole southern half of this quad­ Neocene surface and indicate the relief by con­ Fork has been located, by repeated aneroid meas­ umne Canyon has eroded the larger part of the rangle, and that only a few higher bedrock points tour lines. urements based upon Slippery Ford, at an eleva­ Neocene valley slope. Northeast of Dutch near Round Top, Mokelumne Hill, and possibly The different degrees of resistance to disinte­ tion of 5000 feet, and this figure agrees well Henry the Neocene surface rose 1700 feet in 2 Leek Spring Hill projected above the volcanic gration offered by rocks influence the Neocene as with the probable grade of the old river. West miles, to the level of the plateau of Leek Spring plateau. On the other hand, it is also evident well as the Recent topography, the highest points of Georgetown Junction the old canyon was deep Hill. The modern canyon of the Mokelumne is that the larger part of the northern, higher half in both consisting of granite or schists. Along and abrupt, the slopes rising rapidly 1500 feet in this vicinity no less than 1200 feet below the has never been submerged in a similar manner. the main Neocene valleys of the South Fork of to the rolling high plateau. East of this point Neocene river. The andesite areas north of In the deeper part of the old channels the ande­ the American and the Mokelumne there is evi­ isolated andesite areas occur southwest of Echo, Mokelumne Peak show the existence of a deep site often rests on rhyolite, but over the larger dence of the existence of two channels, the later 700 feet above the modern river, and between and narrow channel, a tributary to the main part of the area it lies directly on granitic or one being eroded between the rhyolitic and the Echo and Phillips, 500 feet above the same. Neocene river, Mokelumne Peak rising 2000 feet schistose rocks. These appear, in the few cases andesitic flows. This inter-volcanic erosion pro­ These facts are extremely interesting, showing above it in a distance of 1 mile. where good exposures are seen, to be soft and duced an irregular surface of the rhyolite, and in that the modern canyon is, all along the river, An important tributary, which will be referred crumbling, but no evidence of any notable accu­ many places the new channel cut through the only about 500 feet below the Neocene trench. to as Dogtown Creek, joined the Mokelumne at mulations of debris has ever been found except rhyolite and trenched the bed-rock surface below Near Phillips the latter was 2000 feet deep and Fort Grizzly and extended northward to Camp in some of the channels, as stated above. The that rock. This is shown near the bend of Silver 3 miles wide; north and south extended the Creek. With its several branches it occupies thickness of the flows is considerable. In the Fork, northwest of Bullion Bend, near Morgan, Neocene high plateau. The andesite areas the rather wide Neocene valley lying between northwestern corner it reaches 1000 feet; in the and on Sopiago Creek, while along Plum Creek between Wilson and Georgetown Junction indi­ the Leek Spring Hill Plateau and another high southwestern part it ranges from a few hundred it is evident that the rhyolite flows, which here cate clearly that this plateau rose gradually plateau in the adjoining Placerville quadrangle of up to 1000 feet along the deeper drainage are very deep, had not been cut through. In toward the Pyramid Peak Range. Though which Baltic Peak is the remnant, rising to an channels. The greatest thickness is found on the no case does the later channel lie more than along this range the volcanic rocks are absent, it elevation of 5100 feet. The channel has an aver­ northeast side of Silver Lake, where it reaches 100 feet below the old one, and the general char­ is certain that this lofty barrier of hard rock, age grade of 100 feet to the mile and was about 2000 feet. acter of the surface was not affected by this ero­ now dissected into peaks and pinnacles, in 13 miles long, the present elevation along its The andesitic flows form, in the western part of sion. Neocene times formed a continuous level-topped deepest part ranging from 5100 feet on the north the quadrangle, well-defined, flat-topped ridges, In general the Neocene topography consisted ridge, corresponding to that of Robbs Peak, and to 3800 feet at its junction with the anain river. well covered by vegetation and affording few good of broad plateaus and high, level-crested ridges ; rising 3000 feet above the Neocene channels. It East of it the slope rises gradually to the Leek exposures. The soil is dark-red, and angular or the rivers flowed between these in sharply defined is also probable that the Upper Rubicon is a Spring Hill Plateau. Evidence collected from other parts of the more imposing sights than the ice-swept rock- comparatively small terminal moraines now lie. There remains to be mentioned certain glacial range has shown that the Neocene channels with deserts of the upper Rubicon or the Devils Basin. A short distance above Echo there is a knob 700 deposits along the canyon of the Rubicon which a direction NNW.-SSE. have the lowest grades, While a striation of the rounded outcrops is fre­ feet high, rounded especially on its eastern side tend to show that the great glacier, fed by its while the maximum and abnormal grades are quently observed, it is often absent where it by ice action, to the south of which rises the many tributaries, extended much farther west­ exhibited by the channels running ENE.-WSW.; would be expected. In explanation of this the nearly perpendicular cliff of Lovers Leap, 1000 ward than had at first been supposed. Among and it has been held that this is strong evidence observation may be recorded that the polished feet high, which has great similarity to the cliffs these is the scattered but shallow drift on the in favor of a westward tilting of the range as a and striated surfaces have a marked tendency to of the Yosemite. The vertical joints traversing andesite table in the extreme northwestern cor­ whole. scale oft' in thin flakes, only a fraction of an inch the granodiorite have evidently facilitated the ner, extending over into the adjoining Colfax, While this kind of evidence is much more thick. This was especially noted in the glacial forming of this scarp, and glacial sapping at its Placerville and Truckee quadrangles. Further, a meager in this quadrangle than farther west, cirques on the west side of the Pyramid Peak foot has also been an important factor. The well-defined, though short, moraine lies on the where Neocene auriferous stream channels abound, Range. The areas designated moraines on the moraines of the Devils Basin glacier lie chiefly on andesitic ridge west of Uncle Toms Cabin; and yet some facts indicate that similar relations map include only the heavier deposits of the the eastern side of the great bend the ice stream scattered, often large, granite bowlders occur obtain here. Being nearer to the headwaters, it terminal, lateral, and ground moraines; scattered was forced to make where it reached the main along the crest of the ridge from Uncle Toms is natural that steeper grades in general should thin drift is not indicated. The moraines are river. High up on the trail leading to Ralstons Cabin up to an elevation within 300 feet of the be expected. composed of rough and angular, not waterworn, Peak a magnificent view of this glacier path and summit of Robbs Peak on the northern side. The fact that the relatively small Neocene bowlders of all sizes, admixed with sand and its moraines is obtained. Above rises a vast Scattered drift of granodiorite also occurs gener­ channel extending from north to south and join­ finer detritus. The topographic form of the extent of clean-swept and polished, white grano­ ally over the andesite table between the Rubicon ing the old Mokelumne channel at Fort Grizzly valleys changes as soon as the lower limit of diorite, with the towering Pyramid Peak in the and the Little South Fork of the Rubicon. These has a fall of only about 100 feet to the mile, glaciation is reached. Below, they are narrow background; below recedes the narrowing canyon facts admit of scarcely any other explanation than while the principal Neocene river channels, and V-shaped; above, broader and U-shaped, often of the river, with the moraines, which appear that the whole basin of the Rubicon River in this running in general from east to west, have an also characterized by stretches occupied by small insignificant in comparison with the denuded vicinity was at one time filled with ice. If it average grade of 150 feet to the mile, certainly meadows separated by rocky .portions with steep surface. On the western side the upper limit of was, the tongue of the glacier in the canyon points in the same direction as the evidence from grade. The long lateral moraines at lower eleva­ the ice sheet is marked only by a narrow but must have projected into the adjoining Placerville the lower slopes. tions often form conspicuous topographic features. sharply drawn lateral moraine, sloping downward quadrangle, reaching a least elevation of about The region of the high peaks of the Pyramid at the rate of about 800 feet to the mile. On 3000 feet. This seems, however, difficult to PLEISTOCENE. Peak and Tallac ranges is characterized by fre­ the east a crescent-shaped and double lateral believe, for the canyon of the Rubicon does not An account of the former distribution of glaciers quent glacial cirques, separated by sharp ridges moraine is thrown across the river like a dam. It in the Pyramid Peak quadrangle present such and the features of the scenery which have (aretes) leading up to rocky, jagged points. is 1^ miles long, three-fourths of a mile wide, decided evidence of glaciation as would be resulted. Lakes of glacial origin, in basins either formed and 600 feet high at most. Of the two parallel expected if the whole deep valley had been filled JSarlier epoch of erosion. The post-andesitic by morainal dams or scooped out of the rock, are walls, indicating two stages of glaciation, the with ice. More detailed examination may be period, which here is taken to coincide with the common in the once ice-bound region. Many outer is the larger. Minor glaciers, one coming needed to settle this point. An earlier and more Pleistocene, comprises two divisions. During the grassy flats or meadows represent dried-up or down the creek south of Pyramid Peak and extensive period of glaciation can scarcely be earlier time intense erosion excavated the canyons drained lakes. others occupying Strawberry Creek and Sayles assumed, for elsewhere within the quadrangle which now score the slope. There is no evidence The principal glaciers which projected from Canyon, also reached the river. Behind the there are no such occurrences as those stated of glaciation during this time. the main ice and neve fields were as follows: The crescent moraine the river was dammed up to a above, outside of the clearly marked glacial limit. Basalt. In the northwestern part of the Pyra­ largest glacier, that which once filled the Rubicon depth of at least 200 feet, sandy and gravelly North of the boundary, in the Truckee quad­ mid Peak quadrangle are several smaller basaltic Valley, probably terminated in the Truckee deposits now marking the extent of the flooded rangle, the glaciation of the Rubicon Valley is flows, generally capping the ridges, which con­ quadrangle, adjoining northward. The Gerle area, but in the course of time the morainal clearly and indubitably indicated. tinue northward beyond the boundaries. Their Creek branch, filling the valley of the same name barrier was trenched by a narrow canyon, and Glaciers of the eastern slope. The peculiar occurrence indicates several independent vents of to the north of the boundary of the Pyramid Peak meadows now extend over the site of the lake. and narrow drainage basin of .the Upper Truckee ejection, and the adjacent topography shows that quadrangle, deposited its terminal moraines a Above these meadows, in the vicinity of River, which with its deep trench separates the considerable erosion has taken place since their short distance below this boundary line. The Phillip's ranch, there is much scattered drift, but watershed of the American from that of the Car­ eruption. On the other hand, the basalt is dis­ Little Rubicon branch filled the well-polished no well-defined moraines are found until the can­ son River, bears ample evidence of once having tinctly later than the andesite, as is directly rock-basin of Loon Lake, and left its well-defined yon which heads 1 mile north of Bryan's ranch is contained a glacier, the neve fields of which must proved by the occurrence of a well-defined dike terminal 2 miles below Forni. An excellently reached. This tributary contained a well-defined have connected with those of the adjoining in andesite on the point between the Rubicon and defined morainal wall indicates its lateral extent glacier, which threw its western lateral moraine glaciers on the west. The upper valley is, as usual, Little South Fork. The same is proved by the on the basalt table northeast of Filipini's ranch, across the main river, damming it again and swept bare. Where the road leaves Lake Valley small isolated basalt flow between Strawberry and it connects across a gap with the less well causing swamps and meadows about Audrains to wind up to the summit of Luthers Pass, toward Creek and Sayles Canyon, which decidedly indi­ defined lateral of the large glacier filling the Lake. Immediately east of this lake is Johnsons Hope Valley, a considerable amount of lateral cates that the surface on which the andesite North Fork of Silver Creek. In these, as well as Pass, at the suddenly ending canyon of the South and ground moraine is found. The final terminal flowed out had been eroded several hundred feet numerous other instances, it is very plain that Fork. The Echo Lake glacier pushed part of its moraines begin about 1 mile above Myers. There before the basaltic eruption took place. Abun­ when the glacier made a bend most of the load lateral moraine over into the valley of the main are at least six of them within a distance of 3 dant morainal matter frequently covers these was deposited on the outside of the curve. The river, while the glacier itself escaped toward Lake miles, indicating stages in the retreat of the areas, so that the age is certainly pre-Glacial. well-marked lateral moraines of the latter glacier Valley. The glacier once filling the canyon glacier, and they are well marked by low, crescent- The rock may therefore be considered of early cap the ridges southwest of Bassi's ranch, the heading 1 mile northeast of Bryan's a-anch like­ shaped ridges, best visible from a high point on Pleistocene age, the andesitic eruptions being southeasterly one being especially well preserved. wise threw part of its moraine over toward the adjoining ridges. Below Myers no morainal supposed to close the Neocene period. The basalt Its terminal moraine is not clearly marked, but Audrain Lake, while the main ice stream found matter is found. The glacier filling the basin frequently forms tables bordered by steep bluffs is probably represented by the morainal masses its way into Lake Valley. whose lower part is now occupied by Echo Lake with imperfect columnar structure. The rock is in Union Valley at an elevation of 5000 feet, South of the American River the tongues of seems to have flowed out through a gap about black, fine-grained, sometimes scoriaceous, usually thus reaching an unusually low elevation. The the ice sheet divided themselves between Silver one-half mile northwest of the present outlet, the showing abundant brown or yellow olivine crys­ rocks in the low gap just east of the large basalt Fork and the Mokelumne tributaries. The Silver knobs rising above the lake and the slope toward tals, and is in all respects a normal basalt. It area of Union Valley bear every evidence of hav­ Fork glacier, fed by the neve fields of the cirques Lake Valley being worn smooth. It flowed down contains but little glass. ing been worn by ice. of Round Top, Thimble Peak, and the vicinity of the steep rocky wall, a veritable cascade of ice, Epoch of glaciation. The later part of the The extensive moraines of the Middle and Silver Lake, was of imposing extent, and its ice- and its terminal semi-circular moraines, joining Pleistocene period was characterized by exten­ South forks of Silver Creek join in the vicinity swept, broad valleys rival those of the Rubicon those of the Lake Valley glacier, lie around the sive glaciation, the traces of which are plainly of Wilson's and Windmuller's ranches. The system. The deep flows of andesitic breccia of little lake at the foot of the scarp. marked everywhere near the crest of the range. accumulations may in places reach a depth of this region do not present the desolation of the While the glaciers draining toward Lake Tahoe Finally the glaciers receded, giving to the region 100 feet. Wilson Valley was evidently once a glaciated granitic area, for, owing to the crumbling were comparatively short, their moraines are its present aspect. Even the smallest traces of lake or swamp, retained by the terminal moraine nature of the rock, it soon produces a fair soil, unusually well marked and of large size. The actual glaciers have now disappeared, though below. On the ridges on both sides of the South which supports scattered timber and in summer Fallen Leaf glacier, occupying the basin of the patches of snow remain during the summer above Fork of Silver Creek there are no glacial traces is covered with patches of grass and flowering same name, swept everything bare above the an elevation of 8000 feet. The largest snow in the shape of scattered drift; it would be easy alpine herbs. The well-defined lateral and head of the lake, the only well-indicated lateral fields lie on the northeast side of the Pyramid to discover any granitic bowlders on the flat lava terminal moraines, which are singularly small in moraine of the upper drainage being found north­ Peak Range. tables on the south side had the glaciation once extent, lie near the junction of the main branches east of Half Moon Lake. At the head of Fallen The area once completely covered by ice, neve, extended farther west than is indicated by the of Silver Fork, no indications of glaciation being Leaf Lake the glaciated surface reaches at least and snow comprises nearly one-half of the 930 end of the moraines on the map. A smaller found below an elevation of 5500 feet. The 1000 feet above the valley, but Angora Point square miles of the quadrangle. Its western glacier extended down the southwesterly slope of Bear River glacier left its lateral moraines piled (elevation 8625 feet) shows by its rocky crags limit is clearly indicated by the moraines stretch­ Pyramid Peak for a few miles, as far down as up against the breccia flows high up on both that it must have been above the ice line. On ing from the head of Gerle Creek down to Bear Forni's meadow. There are no indications that sides of the canyon, and a terminal moraine lies both sides of Fallen Leaf Lake the lateral and Mokelumne rivers. East of this irregular any of the glaciers from the western side of the a mile below the dam. The glacier extended moraines are very large and typical. The eastern line everything was ice-covered, except the peaks, Pyramid Peak Range extended down into the down at least to an elevation of 5500 feet. The is 3 miles long, from one-half to 1 mile wide, and above an elevation of about 8500 feet. The lower canyon of the American River. bare, desolate canyon of Cold Creek also contained 900 feet high, gradually sloping at its northern part of Lake Valley and Mokelumne Canyon Owing to the peculiar character of the drainage an ice stream, which left a lateral moraine 3 miles end. Being on the outer side of the bend the were also free from ice. Tongues of ice extended of the South Fork of the American River, it con­ north of Mokelumne Peak and other well-defined glacier was forced to make, it is natural that this from this grand mer de glace down the valleys tained no large glacier, being in this respect morainal walls near the mouth of the canyon; moraine should have received the largest amount and canyons, carrying with them and depositing unlike the other forks of the same river. No the glacier may have descended into the Mokel­ of debris. The moraines are composed of granitic as morainal heaps and walls the enormous masses decided evidence of former glaciation has been umne Canyon. Summit Creek was doubtless and porphyritic rubble. The westerly lateral of loose debris swept away from higher eleva­ discovered below the mouth of Strawberry Creek. also occupied by a glacier, but it probably did moraines are beautifully indicated; there are at tions. While smaller debris heaps may be found Devils Basin, a broad depression east of Pyramid not extend down to the junction with the main least three of them, forming sharp parallel ridges, at higher elevations, the great moraines lie at Peak, dotted with little lakes filling rock-basins, river. The glacier which once filled the great sometimes splitting in two at the lower end. elevations ranging from 5000 to 7000 feet. Above is really a continuation of the Rubicon Valley, Mokelumne Canyon to the east of the boundary The terminal moraines have dammed Fallen Leaf this extend vast stretches of dazzling white from which it is separated only by a low divide. line of the quadrangle may have extended a short Lake, the surface of which lies less than 100 feet granitic rock-surface, worn bare and rounded and The large ice sheet once filling it extended down distance into the quadrangle, but this is doubt­ above Lake Tahoe. There are at least three, and frequently scratched and striated by rocks held to the South Fork of the American, reaching, ful. No moraines are found in these precipitous probably four, low terminal moraines, from 15 firmly by the moving ice sheet. There are few however, no farther down than Echo, where its canyons. to 45 feet high, forming crescent-shaped walls Pyramid Peak 5. surrounding the lower end of the lake. These and that the lake must have been filled with ice­ scale. Four miles west of Mayer, on the same granodiorite is characterized by an almost com­ moraines generally split in two near the western bergs and icefloes. ridge, is another and smaller channel, on which plete absence of quartz veins, while they occur, end; between them lie little flats covered with Alluvium. Alluvium is present only in very some work has been done at the head of the rich though not abundantly, in the Calaveras forma­ pebbles and sand. subordinate quantities. Small gravel bars occur Russian Ravine and at Acksley's claim, one-half tion and in granite and diorite. The glaciers once occupying Cascade Lake and at intervals along the larger rivers and creeks. mile northeast of Lanes Tunnel. The gravel is In the area north of the South Fork a few Emerald Bay are similar to the Fallen Leaf Within the glaciated area are many smaller shallow and covered by a white tuff. Quartz irregular, nearly barren quartz veins are found in glacier, but smaller. Cascade Lake lies somewhat meadows, produced by the draining of glacial pebbles are common here, as in the other channels. the Calaveras formation. The quartz occurring over 100 feet above Lake Tahoe. Its upper, lakes. The largest alluvial areas lie at the south­ Lane's tunnel was driven 900 feet under the lava in the granodiorite is generally of a pegmatitic clean-swept and polished drainage basin is nearly ern end of Lake Tahoe, and consist of grassy and in order to find this channel, but is said to have origin. circular in shape and contains several small lakes. swampy meadows only a few feet above the level been located at too high an elevation. South of A very well defined quartz vein occurs a mile On both sides of Cascade Lake lie lateral of the lake. Mayer, across the Middle Fork of the Cosumnes, north of Round Top in diorite. It is from 1 to moraines up to 2 miles long and 500 feet high, a little hydraulic work has been done on the 3 feet thick, and has been prospected by means and a well-defined terminal moraine dams its ECONOMIC GEOLOGY. same channel. Southwest of this, small hydraulic of several small holes and tunnels. Assay values outlet. This glacier evidently did not reach the A statement of the occurrences of gold and oilier cuts indicate where the higher or rhyolitic chan­ up to $100 per ton are said to have been obtained present shore of Lake Tahoe. The ridge between mineral resources. nel crosses Sopiago Creek. A lower post-rhyolitic from this quartz, which has a favorable appear­ Emerald Bay and Cascade Lake forms a medial channel crosses the same creek at Barneys. At ance and carries a little iron and copper pyrite, as moraine common to both glaciers. Emerald Bay AURIFEROUS GRAVELS. Fort Grizzly this important tributary joined the well as some galena. A short distance eastward connects with Lake Tahoe by a narrow inlet. Auriferous gravels of Pleistocene or earlier old Mokelumne River. A great deal of placer the mining districts of Alpine County begin. A The upper glacial basin and the lateral moraines, periods are very sparingly present in this quad­ mining has been done in this locality, and a little small quartz vein carrying arsenopyrite was 600 feet high, are as well defined as those pre­ rangle. is still being done. The rhyolite here attains a noticed on the north side of Mokelumne Peak. viously described. No terminal moraine is vis­ Neocene gravels. The more important Neocene maximum thickness of 300 feet, and at Fort Several quartz veins occur in the granite south ible, but the debris dropped at the end of the drainage channels and their deposits are described Grizzly goes down to the level of the creek, so of Slippery Ford on the South Fork, one vein glacier now forms a shallow bar across the inlet above. In the southwestern part of the quad­ that it is not probable that the very bottom of carrying some copper being found on the south­ to the bay. rangle the Neocene gravels contain gold, and have the Mokelumne channel is exposed. There is a ern side of the first high point between the During a part, at least, of the glacial period been worked in many places. The main channel considerable quantity of partly washed quartz river and Silver Fork. Similar nearly barren the surface of Lake Tahoe stood several hundred of the Neocene South Fork, coming down from and metamorphic pebbles, but the actual thick­ veins occur near Ditch Camp 7. On the North feet higher than at the present time, so that the vicinity of Round Top, passed by the sites of ness of gravel below the rhyolite is probably not Fork of the Cosumnes River, from 1 to 2 miles many of the glaciers must have projected into the Morrison, Ditch Camp 7, and Bullion Bend. The great. above the mouth of Van Horn Creek, there are, lake. But there is no evidence that the glaciers first gravels are met with at Ditch Camp 7, where Pleistocene gravels. The gravels in the whole in granite, several quartz veins and superficial once filled-the whole of the basin of Lake Tahoe. a small patch, about 8 feet thick, has been northeastern part of the region are practically gravels derived from them, which have been Lake-beds. The lower part of Lake Valley is hydraulicked with satisfactory results. On the barren, though in some streams occasional colors prospected to a slight extent. Quartz prospects filled with sand and fine, well-washed gravel, hill northwest of Bullion Bend, 600 feet above may be found. In the southwestern part the are also found on the North Fork of the forming terraces rising gradually a few hundred the present river, are several small patches of gravels in some places are rich enough to be Cosumnes, north of Ham's road station. feet above the river, which in places has produced gravel, with a maximum depth of 40 feet, con­ worked, though poor compared to the deposits IRON. shallow, alluvial flats of recent date. On the taining rhyolite pebbles. They are marked " Nr " farther down on the slope of the Sierra. The point of the ridge southeast of Myers waterworn on the map. These have been washed with good workable deposits, as a rule, begin to appear Abundant loose fragments of magnetite are pebbles are found at an elevation of 6800 feet, success, and some gravel still remains. A deeper along the eastern edge of the Calaveras formation, found in a gulch emptying into the North Fork but in large quantities only from 6700 feet post-rhyolitic channel, though less rich, has been though some are found on the adjoining granite of the Cosumnes 2 miles above Van Horn Creek. down. West of Myers the terrace rises to 6700 worked under the lava 1 mile eastward. and diorite. Some placer gold is found on Little feet; north of the same place pebbles are found The rest of the Neocene gravels in the quad­ Silver Creek and on Silver Creek, in the Cala­ BUILDING STONES. on the southern end of the granitic hill up to 6600 rangle are found along the tributary joining the veras formation; a little occurs also on the South The granodiorite of many parts of the glaci­ feet, while on the northern point they reach to old Mokelumne River at Fort Grizzly and extend­ Fork of the American River, at the western ated country would furnish good building stone, 6500 feet. On the northern end of the moraines ing, branching with a general north-south direc­ boundary of the quadrangle, and the different were there any demand for such material. The of Fallen Leaf Lake pebbles are found as high tion, up toward the old divide north of Camp branches of the Cosumnes have been worked with rhyolite also furnishes an excellent stone which as 6800 feet, though these may well have been Creek, the Cosumnes not being represented in some profit at various places along the western may be easily dressed. the Neocene drainage. Beginning on the north, border. One of the richest gulch deposits was derived from small local streams of the glacier. SOILS. On the western side of Fallen Leaf Lake a fairly the shallow gravels under the lava have been found at Russian Ravine, a small tributary from well defined pebble beach occurs at 6500 feet. worked at and west of Van Horn Creek, a tribu­ the north to the Middle Fork of the Cosumnes, 3 The distribution of the deep soils has already Taken in conjunction with observations from tary to the North Fork of the Cosumnes; near miles west of Morgans, from which it is said that been alluded to under the head of Vegetation other parts of the lake, these facts clearly indicate the head of Steeley Fork, where some hydraulic gold to the value of $50,000 was extracted. The and culture. As practically no agriculture is that during the Glacial period Lake Tahoe stood work has been done; and 1 mile east of Dogtown, various tributaries to the Mokelumne River along carried on, the subject has no particular interest. at a higher level than now, and that some time it where the channel crosses the two branches of the southern margin of the quadrangle are prac­ Deep, dark-red, clayey soils cover the lava-capped stood continuously at about an elevation of 6500 the North Fork of the Middle Fork. Here a few tically barren. Some fine gold occurs in the ridges, especially along the western margin of feet, or 275 feet higher than at present. The feet of gravel resting on granitic bed-rock is cov­ gravels of Silver Fork, north of Hells Delight, the quadrangle, and are evidently very fertile. higher benches near Myers may in part have ered by rjjyolitic tuffs, and considerable work but scarcely in workable quantities. Some rich At some places, such as Slippery Ford on the been formed by the ancient flood plain at the has been done by means of sluicing and hydraul- placer ground is said to have been found long South Fork, the granodiorite also produces a good mouth of the Upper Truckee. There are also icking, the banks reaching a height of about 30 ago in a gulch about 1 mile southwest of Mokel­ soil. Clover, alfalfa, vegetables, and apples are indications that before the Glacial period the feet at Candell's and Estee's claims. The ground umne Peak. easily raised below an elevation of 4500 feet, level was still higher. Since that period it has is said to have been very rich in places. The wherever water for irrigating purposes is avail­ been steadily receding, and terraces are formed channel continues southward to Mayer, and is GOLD-QUARTZ VEINS. able. in places from 20 to 50 feet above the lake level. there indicated again by rhyolitic tuffs covering No quartz veins are at present worked in the WALDEMAR LINDGREN, It is thus evident that in Glacial times the ice gravels of slight thickness. At this point the Pyramid Peak quadrangle, though at a few places Geologist. streams must have projected far into the lake, gravels are very profitably washed on a small some prospecting is carried on. On the whole the December, 1896.