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 Sierra Nevada. During therefore, is early Cretaceous. The principal gold belt of California 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 CRETACEOUS PERIOD. 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 Hum- 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 American River 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 soin 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 PERIOD. 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-JURATRIAS 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 tr achy tic tuffs. This contains fossils deposited the region underwent uplift and com­ NEOCENE PERIOD. 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-CARBONIFEROUS 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 put. 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 near the modern crest of the range, but the with lavas along lines which descend from north­ occur above sea-level is evidence that the rangeo east toward southwest, across the trend of the JURATRIAS PERIOD. 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 ondary, or those formed from the detritus pro­ were sufiicient to remove the silt while allowing BED-ROCK SERIES. intrusive granite; at Sailor Canyon, a tributary of American River; in Plurnas County at the duced by the erosion of the primary deposits. the coarser or heavier masses to sink to the PALEOZOIC ERA. 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 4: 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 Gteo. 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 Yose'mite 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 River, 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 Arnphibolite, amphiboliie-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-porphyrite. 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. Rhyolite. 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 Dlorite. 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 witfr 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 pel* 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 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, new channels and, rejuvenated, began the work but largely represent surface lavas which have G-ranodiorite. A granular intrusive rock hav­ pyroxene, and usually olivine. The silica 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 i 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 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 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.

FORMATION NAME CHARACTER OF ROCKS.

Soil and gravel. River and shore gravels Sand, gravel, atid conglomerate. River and shore gravels Gravel, sandstone,and conglomerate. Shale or clay rock. Sandstone. Coal stratum.

Clay and sand, with coal seams.

Sandstone and conglomerate.

Tawny sandstone and conglom­ erate. GREAT UNCONFORMITY

Monte de Oro. Black clay-slate, with interbedded Mariposa. greenstones and some conglom­ erate. Milton. Sailor Canyon.

UNCONFORMITY Intrusive granitic rocks

Argillite, limestone, quartzite, chert, and mica-schist, with inter- Robinson. bedded greenstones. Calaveras.

Intrusive granitic rocks. gr grd K A. >J-.J DESCRIPTION OF THE DOWNIEVILLE QUADRANGLE.

TOPOGEAPHY. porphyry, and granitic rocks. There are also maximum diameter, that resemble fossils, the in the North Yuba Canyon. The beds of the some amphibole-gabbro masses and dikes of origin of which is uncertain. Milton formation are less compressed and altered The Downieville quadrangle embraces a por­ augitic rocks of the diabase series. Considerable The impression of an ammonite on a piece of than the much older Calaveras black tion of the crest of the Sierra Nevada lying just masses of greenstones in the southwestern portion slate said to have come from the argillite lens at slates, quartzites, and magnesian lime- caiavera" south of the fortieth parallel. The district is of the quadrangle are interbedded with the schists the Phcenix mine would indicate a Juratrias age stone. In the Calaveras formation as a whole well wooded, and in large part cov­ of the Calaveras formation. These are presumed for this lens. Since the other similar lenses noted secondary schistosity is the most evident struc­ ered with brush. Those ridges that rise above to have been augitic tuffs of volcanic origin, on the map are in the same quartz-porphyry area ture, and the original bedding is with difficulty TOGO feet are, however, scantily clothed with formed at the time the inclosing sediments were as that of the Phoenix mine, it is supposed that discerned. At most places in the Milton forma­ vegetation, and on some slopes where glaciers deposited. These greenstones were originally all of them are of Juratrias age. tion the original bedding is evident, although have removed the superficial material, as at the similar to the Juratrias rocks described under the On the eastern spur of the Sierra Buttes is an systems of joint planes intersect the bedding Sierra Buttes and the basins of Bear and Long heading " Augite-porphyrite and diabase." Most argillite lens containing also coarser sediments, in planes and occasionally disguise the true dip. lakes, the rocks are almost entirely devoid of of the gold-quartz mines in this district occur in part of volcanic origin. In certain brecciated The Milton formation is later in age than the covering. The region is rugged in the extreme, the Calaveras formation. layers radiolarian remains were found, but these other rocks herein assigned to the Juratrias. It having many slopes with angles of 30° from the The rocks of the eastern portion of the large proved of no value for determining the age. The is, however, included in the Juratrias series, for horizon. The highest elevation is the main peak area of the Calaveras formation, forming the high vein of the Mountain mine is in this sedimentary the collective evidence is good that there are no of the Sierra Buttes, in the southeast section, ridge just west of Poplar Valley and ^agnesian lens, and the locality may be reached by the upper Cretaceous rocks represented in the Auriferous which has an altitude of 8615 feet. A view of the western slope of the Sierra Buttes of nsetone mine road, which ends at this point. The expo­ slate series of the Sierra Nevada. the peak is given on the page of illustrations. ridge, have been grouped together in sures along the road show remarkably well the The rock masses east and south of Mohawk Val­ The lowest part of the quadrangle is Indian Val- another publication* under the name magnesian relative age of the different rocks represented. ley, called Milton formation on the geologic map, limestone series. This series contains numerous Overlying the supposed Juratrias argillite lens may belong to another series. However, the tuffs lev,J " in the southwest corner,' with an ^Theu princi-. . elevation of about 2400 feet. The |?IJffiea= limestone masses which have in general a light- containing the radiolarian remains are beds of of the area by the Middle Fork of the Feather, Middle Fork of Feather Eiver, which tures' gray color, while the limestones of known Carbon­ fragmental quartz-porphyry, succeeded by heavy one-half mile upstream from the mouth of Willow crosses the central part of the district, is peculiar iferous age are usually of a darker, bluish-gray beds of augitic tuffs which represent the augitic Creek, are plainly bedded and appear to have in that .its valley is cut across the main Sierra color. Most of these limestone masses are noted greenstone series; and overlying the augitic tuffs been laid down in water. They strike north and Nevada range. The source of this river is in on the map. Analyses of two specimens show a are the red slates and tuffs of the Milton forma­ south and dip 68° to the east, and strongly Sierra Valley, which lies to the east of the higher magnesia content of 10 and 19.5 per cent tion, well exposed in the bed of the North Fork resemble the tuffs of the Milton formation. The crest of the range, and which is separated from respectively. of the North Yuba. outcrop forming a hilltop 2^- miles due east of the Great Basin only by the lower, eastern crest. In this series, in addition to the limestone, there Four miles west of north from the Mountain Denten's is composed in part of what is probably This second crest extends from Honey Lake are, in the neighborhood of Gold Valley, several mine argillite lens, to the east and northeast of wollastonite-schist. This same schist forms a por­ southeast across the Sierraville quadrangle, which deposits of magnetic iron. This belt of rocks, Deer Lake, is another mass of siliceous argillite tion of the supposed Juratrias mass just northeast lies immediately east of the Downieville. The from its relation to known Mesozoic rocks that interbedded with tuffs. The strata of this lens of the Sulphur Spring House. In the latter area main rivers in the Downieville quadrangle have lie east of it, is thought to be of Paleozoic age. dip to the east 35° at one point, but at other also are some tuffs and garnet rock. In all of a westerly course, but this is not true of their It may be older than the Carboniferous. points the dip is greater. On account of the com­ these small bodies the amount of igneous material tributaries, which flow in various directions. Robinson formation. The rocks of the Eobin­ paratively flat dip and of the erosion which the is large, and further investigation will be neces­ In the central and southeastern parts of the son formation consist of certain metamorphic tuffs country has undergone, the siliceous argillite sary before their age can be known. If the writer layers can not be traced continuously. The por­ is correct in regarding the district to Donbtful quadrangle are numerous lakes, which have been and non-volcanic sediments which form Litnology, ... , formed chiefly by glacial action. The group of several areas in the northeastern por- phyry-tuff is the most apparent part of the mass, the east of Mohawk Valley as a rela- «f M»tonces lakes on the east slope of the high ridge that joins tion of the Downieville quadrangle. In formation- and its prominence might lead the observer to tively sunken region lowered by fault- formatlon- the Sierra Buttes and Eureka Peak is particularly the largest area a number of fossils were found wrongly conclude that the rock should not be ing, these so-called Juratrias beds on the east of attractive. One of them is pictured on the page on the west side of Little Grizzly Creek. These represented as sedimentary, as it is only by a the valley may have been directly connected, of illustrations, fig. 3. A number of these lakes are of upper Carboniferous age, and consist in part careful search that the interbedded layers of before the faulting occurred, with the supposed serve as reservoirs for ditches which supply the of the same species as are found in the Eobinson black siliceous argillite can be found. Milton rocks extending from Howard Creek north neighboring gold mines with water. formation on the north side of Genesee Valley. It is probable that the Juratrias beds above to near Mohawk Valley. Fossils of upper Carboniferous age were also found described are nearly related to the Sailor Canyon GEOLOGY. IGNEOUS ROCKS. on the eastern slope of the high ridge east of Little formation of the southeastern part of the Colfax BED-ROCK SERIES. Grizzly Creek. district. Their exact age is not known, but all of Ampliibolite and metadiorite. Under this head­ The Bed-rock series consists of sedimentary The tuffs of the Eobinson formation contain the Juratrias beds of the Downieville quadrangle, ing are grouped much-altered igneous rocks con­ rocks which were turned into a nearly vertical large rectangular feldspars, which in some cases including the Milton formation, are presumed to sisting chiefly of green aluminous position during or before the post-Juratrias defor­ are orthoclase or microcline. The groundmass of be somewhat older than the Mariposa slates. amphibole, usually of the finely fibrous o/d?"erse es r * 4. T~ \ * origlns- mation, together with the associated igneous rocks. the tuffs is finely granular and contains much Milton formation. In the neighborhood of the uralite type, but in part a more The sedimentary rocks of this period represent secondary brownish-green mica, so that its original old stage station on the Middle Yuba known as coarsely fibrous recrystallized amphibole. They beds of clay, sand, and gravel which have been character is not evident. However, in some Milton are fine exposures of beds, may be called greenstones, but this name is also specimens it is plain that the groundmass was largely tuffs, all of which, being evenly tr1asrbedsaof given to other rocks occurring in the quadrangle. hardened and metamorphosed. These beds were O J ' I O ./ tuffs+,,ff,, andaI,H originally horizontal, but have since been folded originally glassy. Analyses made of three speci­ stratified, present the appearance of Some of the areas appear to have been originally and greatly compressed by forces acting chiefly mens of the tuff show that it has the composition having been laid down under water. augitic tuffs and surface lavas. Such is the from the NNE. and SSW. They have also been of a trachyte. These beds contain igneous fragmental (pyro- greenstone west of Laporte and its continuation subjected to extensive erosion, so that the upper At the west end of Grizzly Valley is a consid­ clastic) material of the augitic greenstone series, to the southeast, and most of the material in the parts of the folds have disappeared. Intercalated erable area of metamorphic tuff, and quartzite, fine-grained red slates, a little marble, southwest corner of the Downieville quadrangle. east of Mohawk Valley is another oc°cllirreunce and a variegated breccia or conglomerate. A It is possible that other areas of greenstone, form­ in these sediments are layers of metamorphic . ., T-, of the lavas and tuifs, showing that volcanic eruptions similar area. Both are presumed to be {JJJgjUJ, bed of the conglomerate occurs by the road at ing narrow streaks in the Calaveras slates for occurred while the sediments were being depos­ of the Eobinson formation, although Milton. It contains abundant small fragments of example, those northwest and south of the Eureka ited. Irregularly intruding the sedimentary rocks no fossils were obtained from them. chert and other rocks of various colors, and gravel mines were layers of volcanic material with their included volcanic layers are masses At the east edge of the map may be noted two appears to be the source of the variegated pebbles laid down in water along with the non-volcanic and dikes of various granular igneous rocks, such areas (symbol ms) colored as belonging to the subsequently referred to in a paragraph under sediments which enclose them. Again, there are as granite and gabbro. Eobinson formation, one lying east of Eed Clover " Auriferous river gravels." Marble was found at masses which are separated with difficulty from Valley and the other southeast of Grizzly Peak. two points only. In both places the change of the distinctly sedimentary material. Such is an AURIFEROUS SLATE SERIES. These rocks are thoroughly recrystallized and are the original limestone into marble is regarded as occurrence just west of Scales, where the indu Cala/veras formation. In the Downieville quad­ now practically gneisses. Another gneiss mass, due to the adjoining intrusive granite. This has rated rock contains much secondary hornblende; rangle there are extensive areas of sedimentary that may be a contact form of a sedimentary rock also caused the development of yellow and red­ but the original sedimentary nature of this rocks of the Auriferous slate series, all or tuff, occurs as a lens in the granite 4 miles dish garnets and wollastonite, both of which are material seems so certain that it is mapped as of which, excepting the Juratrias rocks Station* southeast of Milton. Although these areas are silicates rich in lime. These contact minerals part of the Calaveras formation. . . , , as Calaveras. of the Milton formation and the upper placed in the Carboniferous, they may be older, indicate that the granite is later in age than the On the west flank of Clermont Hill, to the east Carboniferous beds of the Eobinson formation, or if the recrystallization be the result of contact Milton formation. The hardened sandstone or of the serpentine area, is a lens of a coarse are placed in the Calaveras formation. This has metamorphism they may even be younger. quartzite of the Milton formation and the tuff amphibole-schist. Another area is crossed by the been done largely on general stratigraphic evi­ Juratrias beds. Associated with the so-called layers contain a great deal of brownish-green mica South Fork of Feather Eiver to the east of Little dence, for fossils have been found in them in this quartz-porphyries of the Sierra Buttes ridge are in minute foils, which developed in the rock and Grass Valley. Besides these two areas there is a quadrangle at only one point near the mouth of certain lenses composed largely of sili- Interbedded may likewise be ascribed to metamorphism. much larger mass of this peculiar amphibole-schist Onion Valley Creek. As may be seen on the ceous argillite. These lenses are in SJmitJf However, this body of tuffs and sediments as a between Gibsonville and Canyon Creek. On the map, the Calaveras formation forms the bed-rock part in the quartz-porphyry series and whole shows little evidence, either to the naked ridge north of Canyon Creek, at most points noted, of about one-half of the quadrangle. While in part lie on its lower (western) border and its eye or under the microscope, of having been the schist has a nearly east-west strike. The sedi­ divided into different portions by igneous masses, upper (eastern) border. One of these areas, greatly compressed. Not only has very little mentary schists on the ridge south of Canyon and covered up over many square miles by southwest of Gold Lake, is 2 miles in length, secondary schistosity been developed, but even Creek are likewise displaced in the same manner. Tertiary lavas, it nevertheless forms practically but usually the lenses are not so long. Certain under the microscope the grains usually show It is likely that the original rock of this coarsely one area. The formation consists in the main of thin layers, a fraction of an inch in thickness, little evidence of deformation. The dip of the fibrous amphibolite is not the same as the uralite- clay-slate, usually somewhat micaceous, found in some of the argillite, weather white, con­ beds in the neighborhood of Milton is quite uni­ schist and epidote-schist above described. andT mica-schists. , . andT quartzite. with. , USdlSoci-Lithologic trasting strongly with the nearly black color of formly to the east at from 40° to 50°, with occa­ Serpentine. It is known that serpentine, which numerous limestone lenses and smaller SSXnSi"8 the remainder of the rock. At other places there sional dips as low as 25°. At the contact with is a hydrous silicate of magnesia, is usually formed amounts of other schists. The older are oval bodies, about three-fourths of an inch in the mass of granite which lies to the east, how­ by the alteration of other minerals, Qr| inof igneous rocks that have been intruded in the ever, the dip is much steeper, and this is likewise chiefly olivine and pyroxene. It is thus serPentine- Calaveras formation consist of peridotite, quartz- * Journal of Geology, Vol. Ill, 1895, p. true of that portion of the Milton formation found a secondary rock, derived from other rocks by chemical changes, most commonly from peridotite Extending from a point on the east slope of area about Johns ville. It reappears about 1^ By far the most interesting gabbro area is that and pyroxenite. It has also been shown that the Grizzly Mountains north of the fortieth par­ miles north of that town and continues to Long lying in the Willow Creek drainage east of Pen­ many talc-schists and chlorite-schists were formed allel in a nearly south direction, passing just east Valley, north of which it is hidden under ande- man Peak. On the north it is in contact with an from basic igneous rocks, chiefly pyroxenites. of Tower Rock and thence across the crest of the sitic tuffs for a space of about 3 miles. Still area of quartz-mica-diorite. This gabbro is quite Such schists are often found associated with ser­ Grizzly Mountains and down the west slope to farther north the greenstone area broadens and variable in composition. While the average rock pentine, both being alteration products of the Long Valley Creek, is a long, narrow area of forms a part of the summit and much of the west­ is a normal gabbro, composed of monoclinic same rock mass. quartz-porphyry. This is believed to represent a ern slope of the Grizzly Mountains. If this belt pyroxene, brown amphibole, iron ore, and basic The largest area of serpentine is a huge dike- surface flow from an ancient volcano. A second­ of greenstone is continuous, as indicated, and all plagioclase, at certain points rhombic pyroxene like mass which continues with some interruptions ary schistose structure has been developed in parts are younger than the adjoining Calaveras largely replaces the monoclinic, and there are through the western portion of the Occurrences much of the area. The age of this lava is uncer­ formation, as is probable, the relations suggest developed porphyritic amphibole crystals more quadrangle south into the Coif ax dis- «fserPent|ne- tain, although immediately west of it is a long, the possibility of a synclinal structure for this than an inch long. An interesting orbicular trict. The mass crosses the North Yuba at Good- narrow strip of siliceous argillite similar to a portion of the district, as is more fully explained structure is also shown, in which there are con­ years Bar, Canyon Creek at Poker Flat, and Mount stratum which occurs north of the fortieth parallel in the paragraph on " Structural features." centric layers composed of olivine, hypersthene, Fillmore on its west slope. To the west of Onion associated with Silurian limestone. The age of The fact that the greenstones on the east slope iron oxide, and a little plagioclase, with lighter- Valley it broadens into an area 4 miles wide, and this siliceous argillite strip in the Downieville of the Sierra Buttes are superimposed on the colored layers of basic plagioclase and pyroxene extends across the canyon of the Feather River to district must, however, for the present remain an quartz-porphyry series indicates that they are in minor quantities. This rock may be called an Meadow Valley and beyond, in the Bidwell Bar open question, as it pinches out to the southeast younger than the quartz-porphyry. In the neigh­ orbicular norite or gabbro, and such a rock is quadrangle. From specimens^ collected at many of Tower Rock and its relation to the Silurian borhood of Bunker Hill dikes of greenstone cut known at only one other point Ronisas, Norway. points it is known that the mother rock of this siliceous argillite (Grizzly formation) has not been the quartz-porphyry, giving additional evidence G-ranodiorite (quartz-mica-diorite). Granodio- serpentine dike is a peridotite, or in places a ascertained. in the same direction. rite and quartz-diorite have in general the char­ pyroxenite. Remains of olivine and pyroxene An area of schistose porphyry similar to the The greenstone mass shown on the map to the acteristic appearance of granite, and are Qranitic rocks; defi­ are still to be seen in thin sections. One mile above extends from Eureka Peak to the ridge east of Little Grizzly Creek contains abundant commonly spoken of as such. The nition and west of St. Charles ranch the rocks of this mag- south of Sierra City. It forms the main mass of green crystals, which the microscope shows to be chief components are feldspar and occurrences. nesian belt are chiefly talc-schists and chlorite- the Sierra Buttes. A portion of the mass is dis­ uralite (secondary amphibole) having the exter­ quartz. The feldspar is chiefly soda-lime-feldspar schists. tinctly a breccia (see fig. 5), and it is supposed to nal form of augite, which the uralite has replaced. with a smaller amount of potash-feldspar. Usually There are several smaller dikes of serpentine, have been likewise an effusive body. This breccia Forming a dike that crosses Canyon Creek 1 mile biotite and hornblende are present. The feldspar usually but a few hundred feet in width, in the in places contains fragments of siliceous argillite upstream from Poker Flat is a green rock which varies from oligoclase to labradorite, with occa­ eastern part of the area of Calaveras Altered and layers and angular pieces of porphyrite, but the microscope shows to be a diabase with ophitic sional microcline or orthoclase. The rocks show formation that covers such a large por- %£££t£t3£~ most of the fragments in it show porphyritic structure that is to say, a rock composed of evidence, in their thoroughly crystalline texture, tion of the Downieville quadrangle. serpentine- quartz crystals. The line of demarcation between lime-soda feldspar in little laths with interstitial of having formed at some depth below the sur­ Two of these dikes have a length of about 4£ the quartz-porphyry and the augite-porphyrite pyroxene. face. Excepting an area just north of Scales, all miles. Associated with these dikes are small series, west and northwest of the Salmon lakes, The greenstone area shown on the east slope the rock masses properly referable to the quartz- masses of altered gabbro in which the diallage is is a somewhat arbitrary one. The contact of the of the Grizzly Mountains, which is crossed by diorite family are in the eastern part of the chiefly fresh, but is altered around the edges to quartz-porphyry with the Calaveras formation Little Grizzly Creek at the fortieth parallel, is in Downieville quadrangle, where the rock is abun­ fibrous amphibole. The feldspar is entirely lying to the west is usually sharp. Dikes of this part a typical diabase, but the mass as a whole is dant. It is quite probable that all of the areas of replaced by saussurite and epidote. porphyry are common in the Calaveras formation, a mixed one. The extremely steep and brushy this rock in this part of the district are actually The area 1J- miles southwest of Bunker Hill is particularly to the west of Sierra City in the can­ nature of the hillside and the altered character of connected and may be treated as one large mass, composed of serpentine and talc-schist. yon of the North Yuba. These dikes are supposed the material made it difficult to determine the although the connection between them can not be The area of magnesian rocks that surrounds the to have been intruded into the slates at the same original nature of all portions of the area. It is demonstrated on the surface. At two points south half of the Indian Valley granite area is time that the effusive quartz-porphyry just noted not unlikely that some of the material included quartz-pyroxene-diorite forms a facies or variety composed of serpentine, talc-schist, chlorite-schist, was thrown out from volcanoes, and furnish addi­ in this greenstone area is non-volcanic. of the quartz-mica-diorite. One of these occur­ and altered peridotite, in which the original pyr­ tional evidence that the porphyry is later in age Along Howard Creek, bordered on the west by rences is 6TL- miles southwest of Mount Ingalls on oxene and olivine may still be noted. than the Calaveras. Half a mile west of Milton the large area of augitic greenstone of Juratrias the west slope of the Grizzly Mountains, just There are a few streaks of serpentine along a quartz-porphyry dike about 40 feet in width age above noted, and on the east by a strip of below the andesite-tuff area that caps the ridge. Spring Garden Creek downstream from Pine contains abundant inclusions of black argillite, sediments presumed to belong to the Milton for­ The other locality is about Hay Press Valley, in Creek. One of these, 3^ miles northwest of into which it was intruded. Two and one-half mation, is a lens of porphyritic rock about 1^ the southwestern part of the quadrangle. Spring Garden ranch, forms a dike in the slates miles from the summit of Clermont Hill, on the miles in length. This lens is mapped as part of O-ranite and granulite. In the southwest corner extending south up the ridge slope. top of its high northwest spur, is a small mass of the greenstone series. The most noticeable fea­ of the Downieville quadrangle, forming a small Quartz-porphyry (rhyolite-porphyry). Rhy o- quartz-porphyry. There is another mass on ture of the rock is the large tabular feldspars, area having a maximum diameter of about 2 miles, litic rocks (lavas rich in alkali and Jamison Creek 4 miles northeast of Eureka Peak, often half an inch in length. The microscope is a coarse-grained granitic rock which approxi­ silica) T \ with ,! porphyritici *j.' crystals± i 01J» qNature f- rof . and a large area between Massick and Squirrel shows the groundmass to be microcrystalline, and mates a true granite in composition. In general quartz and feldspar in a fine-grained leou" ' creeks about 1^ miles northwest of Spring to be made up of grains and laths of feldspar, it is characterized by the abundance of biotite crystalline groundmass have usually can!crocks- Garden ranch. grains of iron ore, and grains of an augite-like and the lack of hornblende. The locality is a been called quartz-porphyry, although the name As may be seen by examining the geologic map, mineral. The small size of the augite-like grains very interesting one on account of the great abun­ rhyolite-porphyry is the more correct one. the main quartz-porphyry areas of the Downie­ makes their determination doubtful. The large dance and variety of dikes that cut the surround­ Most rocks called quartz-porphyry were erupted ville district form an interrupted belt extending tabular feldspars are labradorite. This Dlabase, ing rocks. Indian Valley, on the North Yuba, before the Tertiary period, and are often much across the quadrangle in a north-south direction, rock may be called a diabase-porphyry. p°rPhyry- has been carved out of this granite, which extends altered. Many of these rocks were originally a little east of the middle. It is likely that all On the east side of Mohawk Valley is a consider­ up the ridge to the south, forming the east part glassy, but have undergone a change called these rocks were erupted or intruded at about the able area of a fine-grained rock, in a portion of the eminence known as Indian Hill. The dike devitrification, whereby the various elements of same period. As was stated in describing the Jura- of which tabular plagioclase phenociysts are rocks will be described under the next heading. which the glass is composed crystallize out. The trias beds, in the quartz-porphyry there are lenses developed, although of smaller size than in the On the ridge 5 miles southeast of Downieville crystalline groundmass of such quartz-porphyries of siliceous argillite which may be of Juratrias Howard Creek rock. This rock may also be is a considerable mass of granitoid rock, which is thus a secondary and not an original one. The age, as an ammonite was found in one of them. regarded as a diabase-porphyry. Such rocks are extends south into the Colfax quadrangle. This rocks called quartz-porphyry in this text are to Augite-porphyrite and diabase (augitic green­ essentially identical with the labrador-porphyrites rock is characterized in general by an abundance some extent such devitrified rhyolites. That they stone series). Under this heading will be grouped of Rosenbusch. of quartz in large grains, sometimes half an inch in diameter, and by the lack of ferromagnesian sil­ were largely surface rocks seems plain, for, as a series of old lavas,' tuffs,' and dikes Greenstones_ Gabbro. As limited by the writer, the feldspar above noted, they appear to have been in part which in most cases show grains of of gabbro should be chiefly of the labradorite- icates. There are numerous quartz veins in this originally glassy, and they occur in part as breccias augite, and which in all cases can be anorthite series. In several of the areas Deflnition granitoid rock, and in Harris Meadows abundant formed by explosions from volcanoes. Typical shown to have been originally augitic rocks. called gabbro the feldspar is altered to green dikes were noted. quartz-porphyries contain more than 70 per cent Most of these rocks are probably of Juratrias age, saussurite, and is therefore not readily About 4 miles east of Downieville, in the can­ of silica, and more than 7 per cent of potassa and but as there is some doubt as to certain areas, no determinable. All of the rocks here called gab- yons of the North Yuba, is a small mass of gran­ soda. Those of the Downieville quadrangle have age is assigned to them on the geologic map. bros are medium- to coarse-grained, and, with one itoid rock similar to the Harris Meadows area on often less than 5 per cent of potassa and soda, The series as a whole shows much less evidence exception, of even texture. As noted under " Ser­ the ridge to the south. No certainly original and thus stand intermediate between typical of compression than do the greenstones included pentine," there are frequently gabbro masses in the ferromagnesian silicates were detected in the sec­ quartz-porphyries and quartz-porphyrites, which under amphibolite, although the two series repre­ serpentine dikes that cut the eastern part of the tion examined. The quartz has an. idiomorphic are altered quartz-bearing andesites. sent originally similar rocks and it is by no means larsreo area of the Calaveras formation. The mass tendency, and the feldspar is albite. The rock In almost any hand specimen of the porphyry unlikely that original augite may be found in that lies just east of the serpentine dike south of may be called a soda-granulite, or soda-aplite. there are numerous porphyritic quartz crystals to some of the amphibolite areas. Uralite, which is Goodyears Bar is composed of amphibole-gabbro, One and a half miles northeast of Rattlesnake be seen. In the larger part of the a secondary amphibole, and often epidote and some of which contains secondary quartz. To the Peak is a small area of a light-colored rock which quartz-porphyry a schistose structure SfqSart""8 chlorite are, however, present in variable quan­ east of this area, along the North Yuba and on the microscope shows to be a micropegmatite. It has11 been developed,j 'I J correspondingJ- in Porphyry. tity in nearly every hand specimen of the augite- the ridge north and south, is a peculiar garnetif- is classed with the granites, although the character strike and dip with the schistosity of the neigh­ porphyrite, but as a rule the larger augites are erous quartzite-like rock, greatly crushed and of the feldspar of the micropegmatite was not boring areas of sedimentary rocks and greenstones. still fresh. altered, which may be a product of silicification determined. Plagioclase is present, but is in part The prevailing color is very light gray, nearly On the east slope of the Sierra Buttes the of the amphibole-gabbro. This siliceous rock is, altered. white when massive. Much of the schistose augitic tuffs plainly overlie the quartz-porphyry however, placed in the Calaveras formation. Dike rocks. About Indian Valley there are quartz-porphyry, as at Bunker Hill, has a bluish that forms the summit of the buttes, and underlie On the east slope of Eureka Peak is an area of numerous white dikes cutting all the other rocks tinge. Where occurring in small amounts in the the tuffaceous sediments of the Milton formation. gabbro, and along its west contact with the quartz- of that neighborhood. The microscope shows other rocks the color and aspect of this rock are The Juratrias age of portions of this belt of green­ porphyry of Eureka Peak pieces of the gabbro these rocks to be porphyries having a less characteristic and the outcrops frequently stone can therefore be considered as probable. are enclosed in the porphyry, showing the gabbro finely granular groundmass of quartz" ofcicgneonces resemble the surrounding rocks. Dikes of this This greenstone area broadens in the lake region, to be the older rock. A smaller mass of a similar and feldspar, in which are embedded rock occur in the~ Calaveras formation at various having a width in the latitude of Gold Lake of gabbro lies about a mile north of the summit of larger crystals of feldspar, biotite, and points. At Wades Lake they follow the stratifi­ about 5 miles. Considerable portions of it seem the peak, and another little area about a mile hornblende, with occasional large grains cation, so far as noted, but southwest of the Sierra to represent massive lava flows, as at Mount southwest of Bunker Hill. A narrow gabbro of quartz. Such rocks may be called quartz-dio- Buttes and at other points are dikes which cut Elwell. To the north of the lake region the dike about 2 miles long cuts the Calaveras forma­ rite-porphyries. These dikes are particularly well th6 slates at angles to the bedding and cleavage. greenstone is buried under the large morainal tion a short distance east of Downieville. exposed in the rocky bed and banks of the North Yuba, east of the valley. At one point, about addition there was observed a fine white layer, a There are, however, two masses of white quartz 2 miles northwest of Bald Mountain, is a consid­ 1400 feet upstream from the mouth of Humbug few inches in thickness, composed almost entirely gravel at the north and west base of Blue Nose, erable body of white quartz gravel, which has Creek, cutting the black hardened slates, several of volcanic dust of rhyolitic origin. at the edge of the lava, about 500 feet been much exploitedJ- byJ means of LittleWIM Grass of these dikes, from 2 to 10 feet apart, were noted, Auriferous river gra/vels. The Sierra Buttes below the Bunker Hill tunnel, which shallow shafts. From its peculiar posi- VaIIey- all with a parallel strike of about 77° west of south, form the most prominent portion of the highest may be downthrown portions of the Hepsidam tion, lower than the South Fork of Feather River, or nearly east and west. The strike of the slates ridge in the Downieville quadrangle. river deposit. That extensive faulting has it has not been practicable to mine it profitably r -, , , Position of at this point is approximately north and south. I his ridge extends northwesterly to the occurred there can be no doubt, and that the on account of the excess of water. This channel Quartz-diorite-porphyry dikes cut the granite of Feather River, gradually decreasing in ^Neocenegan region about Blue Nose was formerly one of may continue under the Grass Valley Hill ridge Indian Valley and the serpentine area that partly altitude, but for 17 miles many points nvers' enormous volcanic activity, where the lavas issued, and thence northeasterly under the lava to Rich­ surrounds it, and are also found abundantly along on the summit are 7000 feet or more in elevation. is also apparent. Much of the lava here is mass­ mond Hill and Sawpit, northwest of Onion Valley. the south and southwest shore of Gold Lake, To the north of the Feather River the Grizzly ive andesite, in part dikes, occupying fissures in The white quartz gravel underlying Little Grass where they are irregular in outline and inclose Mountains form another ridge, with northwesterly the bed-rock. A little over a mile northeast of Valley is not all thoroughly rounded, and this is fragments of the neighboring greenstone. Similar trend. Both of these ridges are composed chiefly Mount Fillmore is some gravel under the andesite, also true of the gravel at Richmond Hill and dikes were noted in the greenstone-breccia south of pre-Cretaceous rocks. Together they appear and both are cut by basalt dikes. Laporte. of the Bear Lakes. There are also dikes about 2 to have formed a watershed between two distinct The Blue Lead gravel mine is on the ridge At the Richmond Hill hydraulic mine there is miles south of Limestone Point, and in the bed of river systems in Neocene time. To the east of north of Poorman Creek, 2^ miles northeast of no lava on the gravel, but the extension of the Kock Creek about one-fourth mile downstream the former divide extended a north-south river Pilot Peak. Here there is white quartz gravel same area to the east is covered by from Scales. The similarity in composition of system, remnants of which are still preserved at with a rubble of andesitic bowlders on top, like andesitic breccia, and on the east side oncionnlty of Valley. these dikes to granodiorite suggests a direct a number of points, while to the west the general the massive andesite of the area immediately west. of the breccia the Union Hill gravel genetic relation. course of the rivers seems to have been southwest­ The other fork of the Neocene river just mine is on the same channel. The latter mine Granite-porphyry dikes were found on the ridge erly. Faulting which occurred in late Neocene described is well exposed about Mount Pleasant. has also been worked by the hydraulic method. extending east from Indian Hill. These appear and Pleistocene time, and extensive erosion of the There is also between this point and Poverty Hill At Sawpit there is said to be white quartz gravel to be protrusions from an area of granite-porphyry river gravels, have in many cases made it impos­ a large amount of gravel made up chiefly of under the black basalt. If so the deposit is in the Colfax quadrangle. A dike of granite- sible to trace the former courses of the channels. siliceous pebbles of pre-Cretaceous rocks, forming undoubtedly part of the Richmond Hill-Union porphyry is shown on the geologic map about 2 The gravel deposits representing the little wooded ridges just south of the fragmental Hill channel. miles northwest of Goodyears Bar, and along this Neocene rivers to the west of the Neo­ lava of the ridge to the north. At the Iowa shaft Two and a half miles northwest of Onion Val­ dike is a quartz vein. Some granite-porphyry-like cene divide will be first described. north of Mount Pleasant this old channel has been ley, on the north edge of the andesitic breccia dikes were also noted in the bed of the east The white quartz gravel channel of Campton- mined. The next appearance of the gravel is on area, on the slope toward the Middle Fork of branch of Nelson Creek and in the bed of Onion ville and Depot Hill, which is east of Oak Valley the north side of the volcanic ridge Feather River, is an area of gravel on serpentine Valley Creek, in both cases cutting the rocks of in the Smartsville quadrangle, appears about one-half mile southwest of Port bed-rock. Immediately west is an area of the the Calaveras formation. first£ x in thexl DownievilleT\ ' -n districtj- x j. atj. Indian Hill. Wine. The claim is called the Bunker older basalt, which extends lower down on the Granulite or aplite dikes were noted in the Indian Hill, where it was formerly extensively Hill, and should not be confused with other mines slope than does the gravel. granodiorite east of Milton, by the main road south hydraulicked. It was there at one time covered of the same name in the Downieville quadrangle. There are some gravel deposits that have been

of Haskell Peak, and at other points. On the with andesitic breccia, a remnant of which is still Here a tunnel is being run in under the lava. mined by > tunnels on the east side of Wahoo, . west bank of Clermont Hill, cutting the serpen­ preserved on the summit of the hill. The next The Lucky Hill mine occupies some of the inter­ the high ridge of which Table Rock is district- tine, are dikes that may be called soda-granulites, trace of the channel is in the Bid well mediate ground of the lava-capped ridge between a part, about east of Port Wine. This is called Scales. being composed chiefly of quartz and of feldspar Bar district at Grizzly Hill, Brandy the Iowa shaft and the Bunker Hill claim. This the Wahoo district. rich in soda. Near Indian Valley, in addition to City, and Council Hill, and it reappears in the Lucky Hill property is as yet only partially A considerable river deposit has been exten­ the quartz-diorite-porphyry dikes heretofore noted, Downieville quadrangle south of Scales. From exploited, but such work as has been done indi­ sively mined on the ridge east of Canyon Creek. numerous dikes of diorite and diorite-porphyry there to Mount Pleasant and Poverty cates that there is a channel under the andesitic It is well exposed at Morristown and with primary hornblende needles were also found. Hill the gravel beds may be followed breccia which connects with the Iowa Hill gravel. Craigs Flat. At the latter place the SfffiSS" Poverty Hill. Flat. In rocks of the magnesian series just west of continuously. This channel and its two At Port Wine, where the deposit is next to be gravel shows evidence, by its very- the Indian Valley granite area a white dike with main branches are the only old river channels in seen, there were very abundant dikes of basalt uneven upper surface, of having been eroded porphyritic biotite crystals was observed. This the Downieville district that can be traced for cutting the gravel, and some of these may still be before being covered by the andesitic breccia. rock proved to be a pyroxene-syenite, the only any long distance. The channel appears to fork seen just south of the village and east of the road. The very large gravel mass at Eureka, 1^ miles dike of the kind noted in this portion of the north of Scales, one fork extending to Poverty From Port Wine the channel has been almost con­ southeast of Craigs Flat, is undoubtedly range. Hill and the other to Mount Pleasant and farther tinuously mined through Queen City, Grass Flat, a part L 01-C theJ.-U same river' deposit.J M. ItVI 0 north under the lava. The Poverty Hill branch Gardeners Point, Cedar Grove Ravine, St. Louis, may thence be traced to the ridge east of Eureka SUPERJACENT SERIES. appears on the north side of Slate Creek at Pine Grove, and Howland Flat. To the north­ Creek, where it has also been mined, and perhaps Barnards diggings, and then successively at thence across Goodyears Creek to the Monte The Superjacent series consists of late Creta­ west of the latter placej~ and to the west Howland , A ceous, Eocene, Neocene, and Pleistocene sediments Secret diggings, Laporte, Thistle shaft, Gibson­ of the road there are some fine expo- Flat- Cristo and Excelsior mines. lying unconformably on the Bed-rock series, ville, Whiskey diggings, and Hepsidam. sures of the gravel beds, showing, by their irregular This Neocene river may have flowed westerly together with igneous rocks of the same period. The white quartz gravel at Laporte may have upper surface, that they were considerably eroded to Craigs Flat and thence over to Scales, being During late Cretaceous, Eocene, and Neocene come in part from the quartz veins immediately before being covered by fragmental andesite. possibly the south fork of the same great river times, the Sierra Nevada was a mountain range adjacent, which may still be seen in the The Howland Flat channel disappears under that deposited the gravels at Laporte, Howland Laporte. and the Great Valley of California was under bed of the old river, now denuded of the lavas of the Mount Fillmore ridge at Potosi, Flat, and other points in the present drainage of water. During the same period, the rivers flow­ its former covering of gravel by hydraulic wash­ from which point it was followed through the Slate Creek. ing down the western slopes of the range depos­ ing. The gravel of the Laporte area is said to ridge by means of tunnels, and reappears on the On the summit of Craycrofts Ridge, east of ited the auriferous gravels. Volcanoes situated have been cut off under Bald Mountain by a wall south slope at Cold Canyon. There is said to Sailors Ravine, are three patches of gravel. Two mostly along the crest of the range poured out of gray lava (andesite). have been a rise in the channel up to of these, one 34 and the other 4 miles _ Cold Canyon. '2 ^ Craycrofts floods of lava, chiefly in Neocene times. During The channel between Laporte and Gibsonville the middle of the ridge, then a fall, northwesterly from Downieville, are Ridge- the Pleistocene also, portions of the Great Valley is covered with andesitic breccia. There is no until at Cold Canyon the elevation is about that composed largely of well-washed quartz gravel

were under water, but there were few volcanic doubt,' however,t ' of the existence tof the Poverty . Hill . at Howland Flat. This rise and fall was not, and are not capped with volcanic material. The eruptions. river deposit under this lava, as it has fork- however, gradual, but by steps, the channel being third occurrence is on the east edge of an area of .been penetrated at the Thistle shaft, about 4 suddenly cut off at several points by polished and andesitic breccia 5 miles northeasterly from NEOCENE PERIOD. miles northeast of Laporte, and the gravel is now striated walls, evidently fault surfaces. The Downieville. In 1893 this deposit was being Lake beds. Underlying the Pleistocene beds of being mined there. The Yankee Hill gravel near source of the channel east of Cold Canyon is mined by the Wide Awake Drift Mining Com­ Mohawk Valley are sediments which are chiefly Slate Creek by the road from Laporte to St. Louis unknown. Much and perhaps all of the gravels pany. white shales with some carbonaceous is perhaps a part of the Laporte channel, but representing it are now eroded. Nearly all of the river gravels that have thus layers. The beds are finely exposed SSJnn there is some doubt about it. From Gibsonville At Studhorse Canyon, below Cold Canyon, is a far been described are composed very largely of alongi the,1 JbeatnerT-I ,1 Kiver-r,' northi 01o the lake of the gravel may be followed continuously through mass of detached gravel, which is possibly a por­ white quartz and represent the earliest river sys­ Mohawk post-office, unconformably un­ Whiskey diggings to Hepsidam, where the tion of the same deposit. The dis- studhorse tem of which there are any traces in the district. derlying the later Pleistocene lake gravels, which channel disappears under the lava. From Laporte placed character of the gravel is well Canvon- Six and three-fourths miles north of Downie­ include blocks of the older shales. Similar Neocene to Gibsonville the average present grade of the seen at Briickerinann's tunnel, where the gravel ville, just north of Rattlesnake Creek, is a small beds may be seen at numerous points in Mohawk bed of the Neocene river is about 80 feet to the stratum stands in a highly inclined position. deposit of gravel known as Rattlesnake diggings. Valley, but the superficial exposures are in all mile; from Gibsonville to Whiskey diggings, Between the gravel and the bed-rock is a dike of The elevation of the locality is approximately cases too small to note on the geologic map. Beds about 250 feet to the mile; and from the latter fine-grained proxene-andesite. 5500 feet. Underlying the gravel is a bed of which are probably of the same epoch are exposed place to Hepsidam, a distance of over a mile, On the south slope of the ridge to the southeast rather fine-grained volcanic tuff containing many on the banks of the Feather River, about 1^ miles about 400 feet to the mile. The measurements, of the flat-topped hill known as Table Rock is a well-rounded particles. This forms part of the northeast of Wash post-office. Here very abun­ while not accurate, have a comparative gravel deposit called the California diggings. bed of Rattlesnake Creek for about one- _ ... Hepsidam. Rattlesnake dant fossil leaves were found, which show the value, and as the character of the gravel This, at the present time, is said to be separated half mile above the mouth of the run Creek< beds to be of Neocene age. The leaf beds have and of the old channel does not appear to have from the gravel at Howland Flat by a wall of that joins Rattlesnake Creek just west of the been disturbed and dip westerly at a gentle angle. changed in this distance, the rapid increase in lava, like that capping Table Rock, intruded after diggings. To the west, south, and east the bed­ At the same point are warm springs, which issue grade is in all probability to be ascribed to a sub­ the gravel deposit had formed. This lava is an rock (clay-slates, etc.) rise several hundred feet. along a line of faulting on which movements have sequent differential elevation toward the east. andesite of a type more recent than the andesitic To the north the volcanic material is continued taken place in recent years. A fuller account of The Hepsidam channel has been followed by long lavas which form the tuffs and breccias that cover up to the large area forming the high Rattlesnake this faulting and its relation to earthquakes may tunnels under the lava-capped ridge that extends the gravel beds. It is likely that at one time the Peak ridge. The gravel is subangular and evi­ be found in the Seventeenth Annual Eeport of southeasterly from Pilot Peak. California channel was connected with the How- dently was formed in a small watercourse. the United States Geological Survey, Part I, pp. On the other (eastern) side of this ridge the land Flat channel. On the south side of Canyon Whether the present position of the gravel and 592-594, and herein under "Faulting," p. 6. bed-rock at the present time sinks rapidly toward Creek, opposite the California diggings, vlcinity of tuff is due to displacement or other cause was not At Gray Eagle Creek, about 1J miles nearly the Feather River. The Bunker Hill tunnel, on is another deposit of white quartz Deadwood- ascertained. The gravel was washed by the south of Mohawk post-office, at an approximate the east side of the ridge, working westerly, is gravel, known as Deadwood diggings. This has hydraulic method. elevation of 4600 feet, is another exposure of the said to be on the same channel. From there east been followed under the lava in a southeasterly Two and a half miles west of the Richmond Neocene lake deposit, consisting of beds of clay no other gravel deposits are known to exist that direction to a mine known as Bunker Hill. Hill gravel area, at the south edge of the andesitic and sand with layers of carbonaceous shale. In can with any certainty be ascribed to this channel. Underlying the alluvium of Little Grass Valley, breccia and on the slope north of Onion Valley Downieville 3. accounting for the formation underground of such The basalt area of the point one-half mile south and Gold Lake. This morainal area merges on ridges, and may perhaps represent an inclosure fragmental material, a similar dike-like mass is of the summit of Haskell Peak presents the the east into the*terraces formed by a Pleistocene formed by the piling up of volcanic Qriz2| known in the Bidwell Bar quadrangle which con­ appearance of having flowed down over the rhyo­ lake that appears to have filled Mohawk Valley material about it. The clay and fine Valley- tains pieces of fossil wood, indicating a fissure lite, which had previously been partially eroded. at the time the glaciers were in .existence. Oppo­ sediment filling it are chiefly of early Pleistocene filled from the surface. The Poker Flat frag­ Other lavas. Besides the four main types of site Jamison, on the east side of Jamison Creek, age, with later bottom lands along Grizzly Creek mental dike may likewise be supposed to be a lava above described, there are certain olivine- there is an exposure of the moraine material at the and its branches. The deposits along the Feather fissure opened by an earthquake and filled in rich basalts and fine-grained hyper- point where the highest lake terrace begins. The River from Bells Bar up-t to the mouth Feather_ ,_ from above. sthene-andesites that occur as small basaitsand underlying coarse gravel and subangular material of Jackass Creek are chiefly coarse River- TI ',111 M fine-grained There is another mass of fragmental andesite in flows or as dikes,' intruded when molten andesites.hypersthene- are roughly stratified. In the bed of Jamison gravels, and materials of the same sort underlie the bed of Canyon Creek 1^ miles farther up the into cracks in other rocks. These Creek just below the bridge at Jamison a shaft Long Valley. On the west of Willow Creek, stream, at a point where a large branch enters it massive lavas often exhibit a columnar structure, was sunk some years ago to the depth of 270 feet, about 1 mile north of Nelson Point,' is Nelson_. , from the north. Here a dike of rhyolite forms due to contraction from cooling. all in gravel. This shaft is still to be seen. There a considerable area of early Pleistocene Point- part of the bed of the creek, and extends thence There are at several points small areas of mass­ must therefore be a great thickness of detrital gravel. The western border of this area has an up the south side, as previously described. The ive, dark-colored basalts, usually showing olivine material here. elevation perhaps 400 feet greater than that of andesitic mass is in contact with and in part with the aid of a hand lens. These are provision­ The sharpness of the line separating the bare the portion of the area bordering on Willow apparently overlies the rhyolite, of which it con­ ally grouped together, since they all appear to glaciated surfaces of the rocks that were covered Creek. Along most of the rivers are to be found tains fragments. The andesite-tuff forms the bed be later than the older basalt and differ from the with glacial ice and the morainal material is the usual bars, of which English Bar on the of the creek for about 300 feet, and thence extends doleritic basalt in structure and, to some extent, rather remarkable in this region. This may be Middle Fork of the Feather is a good example. north for about 600 feet, but does not extend up in mineral composition, although in some instances well seen in the canyon which contains the Sardine Along the North Yuba from Indian Valley to to the ridge top on either side, as at Poker Flat. the two types appear to grade into each other. Lakes, on the northeast slope of the Sierra Buttes. Sierra City are gravel banks or bars, The coarse andesite here described occurs as a Many of the specimens with a resinous appearance From the upper (west) end of the lower Sardine some 01o them,1 as muchi as 150«~ n zeeti> above-, North Yuba. massive lava on Blue Nose ridge; at Pilot Peak; resemble microscopically certain augite-andesites. Lakes (see fig. 3) to near the top of the buttes, the present river. In these gravels there are peb­ northeast of Onion Valley; on Mount On the geologic map these rocks are grouped all is bare, glaciated rock, with only occasional bles of Tertiary volcanic rocks mixed with those Fillmore; west of Gold Lake (where it SSSS^* with the doleritic basalts. erratics. The morainal material of the moraine of older rocks. Downieville rests on Pleistocene is columnar); on Fir Cap; and on Rat­ At many points in the large areas of andesitic south of this canyon begins at the west end of gravels, as does also Goodyears Bar. tlesnake Peak ridge. The massive andesite just material shown on the geologic maps are small the lake and extends diagonally up the ridge south There are also narrow areas of these ?nd*oood-le east of Bassetts, at the mouth of Howard Creek, bodies of massive andesite that usually has a of the lake, while to the east of that line the gravels extending for several miles and 2 miles north of Bassetts, where it is columnar, marked slaty fracture or laminated structure. ridge is composed entirely of glacial debris. along the branches of the North Fork of the is finer-grained than much of the coarser type, The same rock forms also some isolated areas, and There is practically one continuous sheet of North Yuba. but may be distinguished from the coarser por­ as it occurs in dikes in the fragmental andesite, moraine material from Gold Lake to the southeast GENERAL STRUCTURAL FEATURES. tions of the fine-grained hypersthene-andesite, as it is evidently later than that material, and is slope of the Sierra Buttes. All of the level land the pyroxene is chiefly augite. This fine-grained therefore considered separately, although on the east of Gold Lake, known as Church Meadows, As is shown on the structure-section sheet, the augite-andesite occurs also as a distinct dike geologic map it is not distinguished by color from appears to be underlain by moraine stuff. The slates and schists of pre-Cretaceous age in the cutting the clay-slates in the bed of Canyon Creek the coarser andesite just described. Macroscop- valleys of the streams to the west of the high Downieville area are highly compressed rocks 1 mile northeast of Poker Flat. This dike extends ically the rock is usually of light slate-gray color. ridge of the Sierra Buttes and Eureka Peak like­ with a steep dip, which is often vertical. As up the south slope, gradually broadening, and Under the microscope it is characterized by the wise formerly contained glaciers, the one in the stated under the heading " Augite -porphyrite forms a conical butte 1 mile south of east from universal presence of hypersthene in minute laths, canyon of the East Fork of the North Fork of and diabase," a synclinal structure is Poker Flat, where the lava in part lies in hori­ and in many cases the plagioclase occurs in the the Yuba originating north of Gold Valley. suggested by the succession and dip of structure. zontal columns, and up the north slope, widening same form. Often there is no hornblende present, Lake beds. The lake that filled Mohawk Val­ tne,-, °°, beds -, 01/ quartz-porpnyry, , and-, ^por- into an oval area 1^- miles north of Poker Flat. but about Poker Flat and Table Rock and at ley in Neocene time became much more extensive phyrite-tuffs and the rocks of the Milton forma­ At most points in this quadrangle the andesite- some other points there are needles of brown in early Pleistocene time. This is testified to by tion, all of which are supposed to be of Juratrias breccia exhibits little evidence of stratification, as hornblende, occasionally abundant. a series of terraces of gravel and sand, seen on the age. is shown in fig. 2 of the page of illustrations, ,but The fine-grained hypersthene-andesite forms west side of the valley. The highest of these The large area of the Calaveras formation lying occasionally the material is distinctly arranged in isolated areas at the following localities: on the have an elevation of about 5100 feet. Lower to the west of the crest of the high ridge extend­ layers, as on the east bank of the Middle Fork of high points 1-J- miles northeast and 2 miles north­ terraces are to be seen at numerous points in the ingo northwesterly«/ from the Sierra Calaveras_ , Feather River, northwest of Mohawk Valley. west of Goodyears Bar; about one-fourth mile valley. The lake at its highest level not only Buttes may be regarded as a mountain formation- These is a small patch of andesite-tuff on the east of Denten's, in Mohawk Valley, a dike-like filled Mohawk Valley but extended east into mass which was land at the time the volcanic south slope of the Sierra Buttes at an altitude of occurrence; and 1^ miles northwest of Saddle Humbug Valley, having a surface of about 35 tuffs and sediments of Juratrias age above noted about 7000 feet, and as the tuft' may be presumed Back. Dikes of hypersthene-andesite cut the square miles. The deposits at the highest stage were deposited in the eastern portion of the quad­ to rest on a bit of the old Neocene surface, there fragmental andesite dike that crosses Canyon were largelyo «/ rather fine material Character_. , of, rangle. These Paleozoic rocks are, as a rule, is here evidence that the Sierra Buttes formed a Creek at Poker Flat at several places southeast andesitic and morainal detritus which MoCwk1" more highly compressed and metamorphosed than mountain mass in Neocene time, the present alti­ of that village (see fig. 1). has since been much eroded, particu- Valley* the supposed Juratrias series. tude being 8615 feet, or 1615 feet higher than the larly on the east side of Mohawk Valley, where An extensive sheet of quartz-porphyry and tuff. PLEISTOCENE PERIOD. well-defined terraces are not to be found. The quartz-porphyry-breccia forms the crest and the Doleritic basalt. The doleritic basalt is macro- Grlacial moraines and drift. The ridge in the best-exposed terraces are crossed by the road from eastern slope of the Sierra Buttes. The original scopically a medium-grained, light-gray or pinkish northern part of the quadrangle known as the Mohawk post-ofiice to Johnsville. They are com­ dip of the breccia beds is nowhere apparent rock, often with large, scattered olivines. Grizzly Mountains supported a few small glaciers posed of loose sand and gravel distinctly stratified, except where interbedded with layers of siliceous Composition. Under the microscope it is seen to be on its eastern slope, but there appears to be no the bedding being approximately horizontal. Peb­ argillite. The secondary schistosity, as a rule, often nearly or quite holocrystalline, and it usu­ evidence of glacial action on the western and bles of andesite and rhyolite abound in the lake shows a nearly vertical dip in this series. Over­ ally contains much hypersthene. The largest southern slopes. On the steep east slope of the beds, and to the north and east of Mohawk Val­ lying this acid volcanic series are the tuffs and area of this rock is that forming the upper two- ridge, at Tower Peak, there are several crescentic ley thin patches of the lake deposit may be seen flows of augite-porphyrite, which still show thirds of Mount Ingalls, which appears to have masses of morainal material; and on the east side at many points resting on andesitic tuff. It is plainly the original bedding. Clearly overlying been a source of eruption. There is still on the of Little Grizzly Creek, opposite the mouth of the therefore certain that the lake attained its maxi­ these augite-porphyrite-tuffs are the red slates, summit of the mountain some red, scoriaceous stream which heads at Tower Rock, is a piled-up mum development after these andesitic eruptions. sandstone, and tuffs of the Milton formation. lava, which may have formed part of the crater. mass of quartz-porphyry and greenstone bowlders Moreover, the eruptions of the andesite appear to Where the original structure can be made out all It is thought that the eruptions of this late basalt that appears to be a terminal moraine, formed by have been the cause of the formation of the lake. of these Juratrias beds of the east slope of the took place in very early Pleistocene time, after the Tower Rock glacier. Judging from the present contours of the sur­ Sierra Buttes dip eastward at angles of 35° to 60°. the present drainage system had been initiated. Much the finest glacial district, however, is that face of the pre-Cretaceous formations (granite and In the northern portion of the quadrangle, in the It largely overlies andesitic breccia, and appears of the ridge joining the Sierra Buttes and Eureka the Auriferous slate series), the Middle Grizzly Mountains, we have a somewhat similar to have flowed down slopes that existed at the Peak. The east slope of the buttes (see _,,.. ., Fork of Feather River follows approxi- drainage series showing the same general relation but dip­ " \ Distribution i T -i -i i system. time of the basaltic eruptions. It occurs at num­ figs. 3 and 4), and the neve basins about of moraines- mately an older drainage system, which ping to the west. The rocks at the east base of erous points in the large volcanic area north of Gold, Bear, Wade, and the Jamison lakes, are well existed before the latest volcanic eruptions. These the Grizzly Mountains comprise the Robinson Mohawk Valley, as at Big Hill and Penman Peak. polished and scored. So also is the north slope eruptions, largely of fragmental andesite, filled up formation, known by fossils to be of Paleozoic age, There is a large flow just southeast of Mohawk of Eureka Peak, the moraines on the lower east the drainage that then existed, and the waters, with associated igneous masses. Forming the dammed back, formed the Pleistocene lake of crest of the Grizzly Mountains for several miles to Valley,* '. and another. at Haskell Peak.. Distribution_. . ^ and north slopes of which are very interesting, There is a considerable mass of it on offlows- showing by their forms especially well the course Mohawk and Humbug valleys. Since that time the south of the fortieth parallel is a long belt of the summit of the ridge 2J miles southeast of of the glaciers. Eureka Lake is a reservoir, the the river has cut through the barrier and drained quartz-porphyry which may be regarded as com­ Sierra City, and several areas on the ridge south construction of which was made easy by the the lake. For 3 miles north of the Mohawk lake posed of flows and tuffs deposited on the Pale­ of Downieville, one of which is known as Table manner in which the moraines surround it, form­ beds the Feather River of the present time flows ozoic rocks that lie to the east. Lying imme­ Mountain. It also caps the andesitic ridge west ing a natural basin, open only to the north. A through a canyon, the walls and bottom of which diately west, and perhaps stratigraphically over­ of the head of Woodruff Creek, and is found at medial moraine runs from the moraine on the east are composed entirely of andesitic material. lying the quartz-porphyry belt, is a strip of sili­ several points on the ridge between Fiddle and side down the slope in an easterly direction, form­ Alluvial deposits. The most recent Pleistocene ceous argillite which is indicated on the map as Canyon creeks. It forms the summit of Dead- ing the backbone of the side ridge north of Johns­ deposits forming the alluvial bottom lands occur being of Carboniferous age, but it may be of the wood Peak, southwest of Poker Flat, and the ville. In this moraine are abundant bowlders of along streams and form the lowest portions of same age as the lenses of siliceous argillite in the summit of Clermont Hill. There are several gabbro from the area on the east slope of Eureka most of the valleys. On the geologic map these Sierra Buttes ridge. To the west of this belt of masses on the north slope of Clermont Hill, an Peak. The terrace on which Johnsville is situ­ most recent deposits have not been separated siliceous argillite is a very large area of augite- area 4J miles south of Tower Rock on the west ated shows stratification in its upper layers. All from the earlier Pleistocene. American Valley porphyrite, which, so far as can be determined, is slope of the Grizzly Mountains, one noi*fch of Bells about Johnsville are enormous accumulations of appears to represent a basin formed in Alnerican continuous with the rocks of the same nature Bar, and one north of Long Valley. It is thus morainal material. Over an area of 15 square part by orographic causes, and to have Valley- lying to the east of the quartz-porphyry 'of the evident that this late doleritic basalt is scattered miles none of the underlying formations are been subsequently filled with gravel, sand, and Sierra Buttes. The general dip of these rocks in over nearly the entire Downieville quadrangle. exposed. On Frazier Creek, and at other points, other sediments from the large creeks debouching the Grizzly Mountains, at least in the southern It seems probable that the lava came out chiefly this moraine stuff has a depth of 700 feet. The into it from all sides. It is possible that this portion, is to the west. The rocks of the Milton at the points at which it is now found. Except bowlders are chiefly of quartz-porphyry and green­ valley at one time was the bed of a shallow lake, formation, so far as known, do not appear in this the scoriaceous material on the summit of Mount stone, from the extensive areas of these rocks in but no definite evidence of this has been obtained. portion of the area. There is here suggested, Ingalls, all the doleritic basalt is massive. the neve region of Eureka Peak, Mount Elwell, Grizzly Valley is nearly surrounded by volcanic then, an obscure synclinal structure the quartz- Downieville 5. accounting for the formation underground of such The basalt area of the point one-half mile south and Gold Lake. This morainal area merges on ridges, and may perhaps represent an inclosure fragmental material, a similar dike-like mass is of the summit of Haskell Peak presents the the east into the« terraces formed by a Pleistocene formed by the piling up of volcanic known in the Bidwell Bar quadrangle which con­ appearance of having flowed down over the rhyo­ lake that appears to have filled Mohawk Valley material about it. The clay and fine Valley- tains pieces of fossil wood, indicating a fissure lite, which had previously been partially eroded. at the time the glaciers were in .existence. Oppo­ sediment filling it are chiefly of early Pleistocene filled from the surface. The Poker Flat frag­ Other lavas. Besides the four main types of site Jamison, on the east side of Jamison Creek, age, with later bottom lands along Grizzly Creek mental dike may likewise be supposed to be a lava above described, there are certain olivine- there is an exposure of the moraine material at the and its branches. The deposits along the Feather fissure opened by an earthquake and filled in rich basalts and fine-grained hyper- point where the highest lake terrace begins. The River from Bells Bar up*- to the mouth Feather from above. sthene-andesites that occur as small ?asaitfarndh underlying coarse gravel and subangular material of Jackass Creek are chiefly coarse River- There is another mass of fragmental andesite in flowsn or as dikes,-..-. intruded. , , when, molten. fine-grainedhypersthene- ' andesites. are roughly stratified. In the bed of Jamison gravels, and materials of the same sort underlie the bed of Canyon Creek 1^- miles farther up the into cracks in other rocks. These Creek just below the bridge at Jamison a shaft Long Valley. On the west of Willow Creek, stream, at a point where a large branch enters it massive lavas often exhibit a columnar structure, was sunk some years ago to the depth of 270 feet, about 1 mile north of Nelson Point,' is .,Nelson , from the north. Here a dike of rhyolite forms due to contraction from cooling. all in gravel. This shaft is still to be seen. There a considerable area of early Pleistocene Point> part of the bed of the creek, and extends thence There are at several points small areas of mass­ must therefore be a great thickness of detrital gravel. The western border of this area has an up the south side, as previously described. The ive, dark-colored basalts, usually showing olivine material here. elevation perhaps 400 feet greater than that of andesitic mass is in contact with and in part with the aid of a hand lens. These are provision­ The sharpness of the line separating the bare the portion of the area bordering on Willow apparently overlies the rhyolite, of which it con­ ally grouped together, since they all appear to glaciated surfaces of the rocks that were covered Creek. Along most of the rivers are to be found tains fragments. The andesite-tuff forms the bed be later than the older basalt and differ from the with glacial ice and the morainal material is the usual bars, of which English Bar on the of the creek for about 300 feet, and thence extends doleritic basalt in structure and, to some extent, rather remarkable in this region. This may be Middle Fork of the Feather is a good example. north for about 600 feet, but does not extend up in mineral composition, although in some instances well seen in the canyon which contains the Sardine Along the North Yuba from Indian Valley to to the ridge top on either side, as at Poker Flat. the two types appear to grade into each other. Lakes, on the northeast slope of the Sierra Buttes. Sierra City are gravel banks or bars, North Yuba. The coarse andesite here described occurs as a Many of the specimens with a resinous appearance From the upper (west) end of the lower Sardine some of them as much as 150 feet above massive lava on Blue Nose ridge ; at Pilot Peak ; resemble microscopically certain augite-andesites. Lakes (see fig. 3) to near the top of the buttes, the present river. In these gravels there are peb­ northeast of Onion Valley ; on Mount On the geologic map these rocks are grouped all is bare, glaciated rock, with only occasional bles of Tertiary volcanic rocks mixed with those FiUmore; west of Gold Lake (where it with the doleritic basalts. erratics. The morainal material of the moraine of older rocks. Downieville rests on Pleistocene is columnar) ; on Fir Cap ; and on Rat­ At many points in the large areas of andesitic south of this canyon begins at the west end of gravels, as does also Goodyears Bar. tlesnake Peak ridge. The massive andesite just material shown on the geologic maps are small the lake and extends diagonally up the ridge south There are also narrow areas of these east of Bassetts, at the mouth of Howard Creek, bodies of massive andesite that usually has a of the lake, while to the east of that line the gravels extending for several miles and 2 miles north of Bassetts, where it is columnar, marked slaty fracture or laminated structure. ridge is composed entirely of glacial debris. along the branches of the North Fork of the is finer-grained than much of the coarser type, The same rock forms also some isolated areas, and There is practically one continuous sheet of North Yuba. but may be distinguished from the coarser por­ as it occurs in dikes in the fragmental andesite, moraine material from Gold Lake to the southeast GENERAL STRUCTURAL FEATURES. tions of the fine-grained hypersthene-andesite, as it is evidently later than that material, and is slope of the Sierra Buttes. All of the level land the pyroxene is chiefly augite. This fine-grained therefore considered separately, although on the east of Gold Lake, known as Church Meadows, As is shown on the structure-section sheet, the augite-andesite occurs also as a distinct dike geologic map it is not distinguished by color from appears to be underlain by moraine stuff. The slates and schists of pre-Cretaceous age in the cutting the clay-slates in the bed of Canyon Creek the coarser andesite just described. Macroscop- valleys of the streams to the west of the high Downieville area are highly compressed rocks 1 mile northeast of Poker Flat. This dike extends ically the rock is usually of light slate-gray color. ridge of the Sierra Buttes and Eureka Peak like­ with a steep dip, which is often vertical. As up the south slope, gradually broadening, and Under the microscope it is characterized by the wise formerly contained glaciers, the one in the stated under the heading " Augite - porphyrite forms a conical butte 1 mile south of east from universal presence of hypersthene in minute laths, canyon of the East Fork of the North Fork of and diabase," a synclinal structure is Poker Flat, where the lava in part lies in hori­ and in many cases the plagioclase occurs in the the Yuba originating north of Gold Valley. suggested by the succession and dip of ofulyncnnai ,-, &&, , / , -, -, ^ structure. zontal columns, and up the north slope, widening same form. Often there is no hornblende present, Lake beds. The lake that filled Mohawk Val­ the beds ol quartz-porphyry and por- into an oval area 1J- miles north of Poker Flat. but about Poker Flat and Table Rock and at ley in Neocene time became much more extensive phyrite-tuffs and the rocks of the Milton forma­ At most points in this quadrangle the andesite- some other points there are needles of brown in early Pleistocene time. This is testified to by tion, all of which are supposed to be of Juratrias breccia exhibits little evidence of stratification, as hornblende, occasionally abundant. a series of terraces of gravel and sand, seen on the age. is shown in fig. 2 of the page of illustrations, but The fine-grained hypersthene-andesite forms west side of the valley. The highest of these The large area of the Calaveras formation lying occasionally the material is distinctly arranged in isolated areas at the following localities: on the have an elevation of about 5100 feet. Lower to the west of the crest of the high ridge extend­ layers, as on the east bank of the Middle Fork of high points 1^ miles northeast and 2 miles north­ terraces are to be seen at numerous points in the ingo northwesterlyJ from the Sierra Calaveras_ , Feather River, northwest of Mohawk Valley. west of Goodyears Bar; about one-fourth mile valley. The lake at its highest level not only Buttes may be regarded as a mountain formation- These is a small patch of andesite-tuff on the east of Denten's, in Mohawk Valley, a dike-like filled Mohawk Valley but extended east into mass which was land at the time the volcanic south slope of the Sierra Buttes at an altitude of occurrence; and 1^ miles northwest of Saddle Humbug Valley, having a surface of about 35 tuffs and sediments of Juratrias age above noted about 7000 feet, and as the tuff may be presumed Back. Dikes of hypersthene-andesite cut the square miles. The deposits at the highest stage were deposited in the eastern portion of the quad­ to rest on a bit of the old Neocene surface, there fragmental andesite dike that crosses Canyon were largelyO «/ rather fine material Character_. , of. rangle. These Paleozoic rocks are, as a rule, is here evidence that the Sierra Buttes formed a Creek at Poker Flat at several places southeast andesitic and morainal detritus which MohIwklH more highly compressed and metamorphosed than mountain mass in Neocene time, the present alti­ of that village (see fig. 1). has since been much eroded, particu- Valley* the supposed Juratrias series. tude being 8615 feet, or 1615 feet higher than the larly on the east side of Mohawk Valley, where An extensive sheet of quartz-porphyry and tuff. PLEISTOCENE PERIOD. well-defined terraces are not to be found. The quartz-porphyry-breccia forms the crest and the Doleritic basalt. The doleritic basalt is macro- Glacial moraines and drift. The ridge in the best-exposed terraces are crossed by the road from eastern slope of the Sierra Buttes. The original scopically a medium-grained, light-gray or pinkish northern part of the quadrangle known as the Mohawk post-oifi.ce to Johnsville. They are com­ dip of the breccia beds is nowhere apparent rock, often with large, scattered olivines. Grizzly Mountains supported a few small glaciers posed of loose sand and gravel distinctly stratified, except where interbedded with layers of siliceous Composition. Under the microscope it is seen to be on its eastern slope, but there appears to be no the bedding being approximately horizontal. Peb­ argillite. The secondary schistosity, as a rule, often nearly or quite holocrystalline, and it usu­ evidence of glacial action on the western and bles of andesite and rhyolite abound in the lake shows a nearly vertical dip in this series. Over­ ally contains much hypersthene. The largest southern slopes. On the steep east slope of the beds, and to the north and east of Mohawk Val­ lying this acid volcanic series are the tuffs and area of this rock is that forming the upper two- ridge, at Tower Peak, there are several crescentic ley thin patches of the lake deposit may be seen flows of augite-porphyrite, which still show thirds of Mount Ingalls, which appears to have masses of morainal material; and on the east side at many points resting on andesitic tuff. It is plainly the original bedding. Clearly overlying been a source of eruption. There is still on the of Little Grizzly Creek, opposite the mouth of the therefore certain that the lake attained its maxi­ these augite-porphyrite-tuffs are the red slates, summit of the mountain some red, scoriaceous stream which heads at Tower Rock, is a piled-up mum development after these andesitic eruptions. sandstone, and tuffs of the Milton formation. lava, which may have formed part of the crater. mass of quartz-porphyry and greenstone bowlders Moreover, the eruptions of the andesite appear to Where the original structure can be made out all It is thought that the eruptions of this late basalt that appears to be a terminal moraine, formed by have been the cause of the formation of the lake. of these Juratrias beds of the east slope of the took place in very early Pleistocene time, after the Tower Rock glacier. Judging from the present contours of the sur­ Sierra Buttes dip eastward at angles of 35° to 60°. the present drainage system had been initiated. Much the finest glacial district, however, is that face of the pre-Cretaceous formations (granite and In the northern portion of the quadrangle, in the It largely overlies andesitic breccia, and appears of the ridge joining the Sierra Buttes and Eureka the Auriferous slate series), the Middle Grizzly Mountains, we have a somewhat similar to have flowed down slopes that existed at the Peak. The east slope of the buttes (see _...... Fork of Feather River follows approxi- series showing the same general relation but dip­ -t \ Distribution system. time of the basaltic eruptions. It occurs at num­ figs. 3 and 4), and the neve basins about of moraines- mately an older-i -i drainagei system, which1*1 ping to the west. The rocks at the east base of erous points in the large volcanic area north of Gold, Bear, Wade, and the Jamison lakes, are well existed before the latest volcanic eruptions. These the Grizzly Mountains comprise the Robinson Mohawk Valley, as at Big Hill and Penman Peak. polished and scored. So also is the north slope eruptions, largely of fragmental andesite, filled up formation, known by fossils to be of Paleozoic age, There is a large flow just southeast of Mohawk of Eureka Peak, the moraines on the lower east the drainage that then existed, and the waters, with associated igneous masses. Forming the crest of the Grizzly Mountains for several miles to Valley,J ' and another at Haskell Peak. Distribution...... and north slopes of which are very interesting, dammed back, formed the Pleistocene lake of There is a considerable mass of it on offlows- showing by their forms especially well the course Mohawk and Humbug valleys. Since that time the south of the fortieth parallel is a long belt of the summit of the ridge 2-J- miles southeast of of the glaciers. Eureka Lake is a reservoir, the the river has cut through the barrier and drained quartz-porphyry which may be regarded as com­ Sierra City, and several areas on the ridge south construction of which was made easy by the the lake. For 3 miles north of the Mohawk lake posed of flows and tuffs deposited on the Pale­ of Downieville, one of which is known as Table manner in which the moraines surround it, form­ beds the Feather River of the present time flows ozoic rocks that lie to the east. Lying imme­ Mountain. It also caps the andesitic ridge west ing a natural basin, open only to the north. A through a canyon, the walls and bottom of which diately west, and perhaps stratigraphically over­ of the head of Woodruff Creek, and is found at medial moraine runs from the moraine on the east are composed entirely of andesitic material. lying the quartz-porphyry belt, is a strip of sili­ several points on the ridge between Fiddle and side down the slope in an easterly direction, form­ Alluvial deposits. The most recent Pleistocene ceous argillite which is indicated on the map as Canyon creeks. It forms the summit of Dead- ing the backbone of the side ridge north of Johns­ deposits forming the alluvial bottom lands occur being of Carboniferous age, but it may be of the wood Peak, southwest of Poker Flat, and the ville. In this moraine are abundant bowlders of along streams and form the lowest portions of same age as the lenses of siliceous argillite in the summit of Clermont Hill. There are several gabbro from the area on the east slope of Eureka most of the valleys. On the geologic map these Sierra Buttes ridge. To the west of this belt of masses on the north slope of Clermont Hill, an Peak. The terrace on which Johnsville is situ­ most recent deposits have not been separated siliceous argillite is a very large area of augite- area 4J miles south of Tower Rock on the west ated shows stratification in its upper layers. All from the earlier Pleistocene. American Valley porphyrite, which, so far as can be determined, is slope of the Grizzly Mountains, one nonth of Bells about Johnsville are enormous accumulations of appears to represent a basin formed in American continuous with the rocks of the same nature Bar, and one north of Long Valley. It is thus morainal material. Over an area of 15 square part by orographic causes, and to have Valley- lying to the east of the quartz-porphyry 'of the evident that this late doleritic basalt is scattered miles none of the underlying formations are been subsequently filled with gravel, sand, and Sierra Buttes. The general dip of these rocks in over nearly the entire Downieville quadrangle. exposed. On Frazier Creek, and at other points, other sediments from the large creeks debouching the Grizzly Mountains, at least in the southern It seems probable that the lava came out chiefly this moraine stuff has a depth of 700 feet. The into it from all sides. It is possible that this portion, is to the west. The rocks of the Milton at the points at which it is now found. Except bowlders are chiefly of quartz-porphyry and green­ valley at one time was the bed of a shallow lake, formation, so far as known, do not appear in this the scoriaceous material on the summit of Mount stone, from the extensive areas of these rocks in but no definite evidence of this has been obtained. portion of the area. There is here suggested, Ingalls, all the doleritic basalt is massive. the neve region of Eureka Peak, Mount Elwell, Grizzly Valley is nearly surrounded by volcanic then, an obscure synclinal structure the quartz- Downieville 5. porphyry, augite-porphyrite, and Milton series likely that such a stream could have existed of the Sierra Buttes, is composed of a soft, brittle j on the ridge east of the Sierra Buttes, was also lying on the older rocks of the Sierra Bnttes ridge along the edge of a plateau having a steep escarp­ quartz, with porphyry walls, and is said to have 1 mined. At many points these Neocene river and dipping easterly; and the quartz- Qrjzz| ment. There can be no reasonable doubt that paid from $20 to $30 per ton. The quartz con- | ogravels are beingo worked at the J.present Neocene' porphyry, siliceous argillite, and augite- Mountains- since these gravels were deposited a profound dis­ tained very little sulphide, the ore being free j time by drifting. Thus the Laporte %£*£& by porphyrite lying on the older rocks of the Grizzly placement has occurred, in virtue of which that milling. The vein in general varies from 6 to 12 channel is mined at the Thistle shaft, dnfting- Mountains and dipping westerly. To the north portion of the former plateau lying to the east of feet in width. The Mountain mine, on the spur about 4 miles northeast of Laporte, where the of the fortieth parallel, however, there is some sili­ the present escarpment has dropped down 2000 south of the Sardine lakes, produces a high-grade gravel deposit is covered with lava, and at many ceous argillite in the Grizzly Mountains, associated or more feet and now lies in part buried beneath ore. Like the Young America mine, it is most points tunnels have been run in from the side of with Silurian limestone, forming the Grizzly for­ the sediment of the former Mohawk Lake and picturesquely located. The Phoenix mine is on the lava-covered ridges to strike the river gravels mation of Diller, and it is not impossible that the beneath Tertiary lavas. This downward displace­ the southeast slope of the Buttes. that lie beneath the lavas. The objections to belt of siliceous argillite noted above may be a ment must have been preceded by an elevation of There is considerable prospecting going on in hydraulic mining do not apply to any consider­ continuation of the siliceous argillite series of the the range. the ridge south of Sierra City and Downieville, able extent to the Downieville district, for the Grizzly formation. While there is thus a reason­ Mohawk Lake, just referred to, occupied the where quartz veins are numerous, but no mines of distance from the gravel deposits to the Sacra­ able doubt as to the quartz-porphyry of the Grizzly present Mohawk Valley in Upper Miocene or Pli­ importance are being worked there at present. mento Valley, following the course of the drain­ Mountains being of the same age as that of the ocene time, although the lake at this The veins occur both in the slates and in the age, is so great that it is difficult to believe that Sierra Buttes, there is little doubt that the augite- period i may noti ihave ibeen a deepill body vaiieyMohawk the granite area of Harris Meadows. At Gold Val­ any considerable amount of the detritus can reach o , mi -i T 7 ,1 « result of a porphyrite of both regions is of the same age, as 01 water. I he basin occupied L by«/ this displace-merit. ley two quartz veins have been developed, the the Sacramento River, where alone it can do any it forms one large area. lake may be regarded as the result of Gold Valley Mining Company's vein and the vein harm. In a general way it may be said that the strike the displacement above referred to, in which case of the St. Johns mine; both strike northeast and The Pleistocene gravels forming bars usually of the older rocks of the Downieville district is to the faulting probably took place or was initiated dip southeast. The veins are about 5 feet wide. of some elevation above the present rivers have the northwest and southeast,' or north Strike_ ., of. in early Miocene time. The lake beds lie at nearly In the Gold Valley mine sulphides were so abun­ in nearly all cases been rich in gold. These are and south. Local displacements, how- older rocks' all points approximately horizontal. Keference is dant that a chlorination plant was necessary. referred to under the head of alluvial deposits. ever, are not uncommon, as is the case with the made here particularly to the older Tertiary beds, Two mines have been developed on the west They may be well seen in the canyon of the North mass of sediments of the Calaveras formation form­ composed chiefly of fine white sediments. It is of side of the North Fork of the North Yuba, to the Yuba and its branches, as well as along the ing the ridge south of Canyon Creek, of which course equally true that the Pleistocene lake beds, north of Downievile. The Good Hope, about 500 Middle Feather. At Downieville remnants of Deadwood Peak is the culminating point. These which cover up and largely conceal the older beds, feet above the stream, is a vein between slate and gravel benches at a considerable elevation above slates in general have an east-west strike, parallel are also approximately horizontal. These more serpentine. The strike of the ledge is northwest the present river are still preserved. These to the contact of this area of sediments with the extensive Pleistocene beds were deposited by a and southeast. The Gold Bluff, farther up the gravels were formerly mined with good returns. amphibolite-schist just north. The schistose body of water of some depth, due to the damming ravine, is still being worked. The strike of the Some years ago a shaft was sunk in the middle structure developed in this amphibolite-schist, as back of the water by andesitic breccia, as has been ledge is said to be north and south and the dip of the American Valley, but little if any gold was seen along Canyon Creek and to the east of St. described under the heading " Lake beds." Slight 45° to the west. taken out. The gravel in the present stream beds Louis, trends likewise approximately east and displacements have, however, occurred in com­ Quartz veins are abundant in the slates along has also been mined at many places, the streams west. There is no apparent explanation of the paratively recent times in the lake beds, indicat­ Willow Creek east of Clermont Hill. being temporarily diverted. displacement of this body of Paleozoic sediment ing a zone of weakness, presumably established at Gold in quartz-porphyry (rhyolite-porphyry). At some points also the soil and subangular unless we regard the amphibolite-schist as an an early date, as above indicated. The lake beds One mile east of Onion Valley is a dike of rhyo- loose material of subaerial origin have been suf­ altered form of an intrusive mass, very probably are disturbed in the immediate vicinity of this lite-porphyry in the Auriferous slate Free oldat ficiently rich to repay mining. The Gold in soil. pyroxenite. This amphibolite-schist is a coarsely line of faulting, but are horizontal a few feet to series, at the head of Poorman Creek, poorman Hayes mine on the ridge 3 miles east fibrous variety, containing little else than amphi- the east and west of the fault line, so far as in a decomposed portion of which free of Gold Lake is in material of this nature. bole, and quite different in composition and struc­ determined. gold occurs, associated with little veins of quartz. In 1890 morainal stuff was being mined on ture from the uralite-epidote rocks which are the Faults with a small throw in Pleistocene sedi­ As the gulch was very rich below this dike it is Poorman Creek, northeast of Pilot Peak, by the result of metamorphism of augitic tuffs. ments were noted on the east side of the Middle possible that much of the gold in it came from the hydraulic method. The gold was mostly angular There is no evidence in the Downieville quad- Feather Eiver opposite Wash post- Recentfaults dike. At the time of the writer's visit the gravel and perhaps came chiefly from the white dike of .rangle that any of the granitic masses are older office, and on the south bank of the ^a«ie?le and morainal debris about the gulch were being rhyolite-porphyry previously noted. On the slope than the pre-Cretaceous rocks by which they are river about 1 mile upstream from Rlver' mined by the hydraulic method. This gold west of Mohawk Valley, 2 miles Qold|n surrounded. As a rule there is a distinct zone of Wash post-office. About a mile farther east, deposit is of unusual interest, as being in a dike south of Wash post-office, is Bennett's moraines- greater metamorphism bounding the granitic where most of the fossil leaves were collected of late Cretaceous or Tertiary age. The rock may hydraulic gold gravel mine. The bed-rock is masses. that are referred to under the heading "Lake be termed a rhyolite-porphyry. It is composed of granite. On the north side of the ravine the Faulting. That faulting has occurred at many beds," a fissure was formed in the tuff and breccia crystals of brown mica, sanidine, plagioclase, and material lying on the granite is seen to be made points in the Downieville quadrangle is shown in beds at the time of an earthquake, about 1876. quartz, in a microgranular groundmass of appar­ up of clay considerably compacted, with numer­ a general way by the topography, and in a more In 1889 warm, moist air still issued from this ently both quartz and feldspar. ous pebbles and bowlders, which are generally definite way by the manner in which the old river fissure, which when first formed is said to have Gold-bearing gravels. The gravels that were well rounded. On some of them scratches were channels are suddenly cut off under the lavas by been 2 feet wide. A little east of the Earthquake,,..,, deposited by the Tertiary rivers in the Downie­ evident. The upper material was composed of walls of other rocks which often appear smooth fissure is a warm sulphur spring. Near flssure- ville district have been described under the head­ sand and gravel, with many fragments of granite, and striated, indicating movement. The continu­ this is another spring, which soon after the earth­ ing "Auriferous river gravels." In a general and of pre-Cretaceous and Tertiary volcanic rocks. ation of the gravel channel has then to be found quake is said to have been so warm that the way it may be stated that all of these gravels are Traces of a rough stratification were noted. The by sinking or upraising, according as the displaced hands could not be held in the water. As further auriferous and in places exceedingly rich. Since material is supposed to be part of the higher mass was lowered or raised. Such faults were evidence of a disturbance at this point, it may be the distribution of these Tertiary gravels has terraces of the Pleistocene Mohawk Lake, and to found in the Hepsidam channel, in the Potosi- said that the leaf-bearing lake beds underneath already been noted in detail, only a brief notice have been furnished by the glaciers on the slope Cold Canyon channel, and at other points.* the tuff dip from 5° to 30° to the west. of them will be here given. above, and therefore to be partly morainal. To one standing on the summit of the high, Farther south, just east of the springs at the Those gravels that have produced the most in Iron ore. There is a mass of magnetite by the wide plateau west of Mohawk Valley and looking Sulphur Spring House, the lake beds are likewise the Downieville district lie in the present drain­ creek north of Gold Valley, the surface of which eastward, the region to the east of the valley pre­ flexed, at one point dipping 22° to the southwest. age of Slate Creek. They have been has been smoothed by glacial action. On this sents all the appearance of being a downthrown The so-called sulphur springs had in 1890 a mined by the hydraulic method very surface are numerous figures drawn in former area. The slope to the west of the valley is an temperature of 75° F., as tested by an immersed extensively at Whiskey diggings, Gib- creekadrain= times by Indians. This mass is noted on the .-,-, -r o. -.. . T age mined by exceedingly steep one, rising at some points 2500 thermometer. sonville, Laporte, secret diggings, and " " " economic sheet, as is also a smaller mass just feet in If miles. This slope, moreover, is com­ Artesian wells along the west side of Sierra Barnards, on the north side of Slate southeast of the Spencer Lakes, and another just posed wholly of pre-Cretaceous rocks, although Valley are said in some cases to strike hot water, Creek; and on the south side at Howland Flat, west of Wades Lake. these are hidden at some points by morainal which may be regarded as evidence of the exist­ Pine Grove, St. Louis, Cedar Grove Eavine, Grass Chrome iron. In the gravel along Goodyear debris. To the east of the valley Tertiary vol­ ence of a fissure. The fault shown on the map Flat, Queen City, Port Wine, and intermediate Creek waterworn pebbles of chrome iron have canic material covers large areas and forms the along Spring Garden Creek and near Cromberg points. The channel on the south side of Slate been found. These doubtless came from the summits known as Jackson Mountain, Penman on the Middle Feather may be correlated with Creek is last seen at the Bunker Hill drift mine, neighboring serpentine area. Similar pebbles Peak, and Pilot Hill. Where pre-Cretaceous the zone of faulting above indicated. about half a mile southwest of Port Wine. It is from the gravel at Howland Flat indicate bodies presumed here to go under the lava ridge, and to in place not far away, as the pebbles are too rocks occur theyJ are at low elevations,' Plateau , A at no point within 3 miles of the valley Mohawk ECONOMIC GEOLOGY. be the same as has been worked at the Iowa shaft heavy for distant transportation undei ordinary having a greater altitude than 5300 Valley> Gold-quarts veins. No portion of the Sierra north of Mount Pleasant. At Mount Pleasant circumstances. feet, or 800 feet above the lower part of Mohawk Nevada is more prolific in quartz veins than the the Neocene rivers of the Slate Creek drainage Limestone and marlle. The very abundant Valley. The best evidence that a great displace­ Downieville quadrangle. Large and continuous join and continue toward the south as one river. limestone masses that form a zone in the Paleozoic ment has occurred here in Tertiary time consists veins are, however, not the rule, and the mines as The hydraulic mines at Scales, Union Hill, Coun­ slates just west of the quartz-porphyry belt of of the presence of river gravels on the high pla­ a whole have not proved nearly so rich as in other cil Hill, Brandy City, and Indian Hill are on this Eureka Peak and Sierra Buttes are chiefly of a teau to the west. At one point, about 2^- miles districts. The most notable mines are those on main channel. The Neocene river gravels were dirty light-gray color. A similar mass is found northwest of Haskell Peak, there is a heavy mass the eastern slope of Eureka Peak, known as the also mined by the hydraulic method at on the main southeast spur of Clermont Hill, and of well-rounded gravel at the very edge of the Plumas Eureka, and the Sierra Buttes mine several points north and west of Onion deposits of is known as Limestone Point. Analyses of two escarpment, and a considerable portion of this northwest of Sierra City. These have been ValleyJ and north of Blue Nose Moun- Ilicked.1.y1drau= samples of these limestones are given in the para­ mass has by gravity traveled down the slope, so worked with much profit for many years, but are tain, and also at California diggings graph on the Calaveras formation, and show that that the apparent thickness of the deposit is about now nearly exhausted. The vein matter in the and at Deadwood, near Poker Flat. At Morris- they are highly magnesian. On Little Long Val­ 500 feet. To the northeast and west of Haskell Plumas Eureka consists of a firm, white quartz, town, Craigs Flat, Eureka, Monte Cristo, and other ley Creek is a mass of crystalline limestone or Peak, and also close to the edge of the escarp­ carrying a large percentage of pyrite, with galena points the deposits of another river were hydrau- marble. Some of this has been burned for lime. ment, are smaller masses of similar gravel, all of and some zinc-blende, the gold being about evenly licked. A river entirely distinct from those above There is also a long reef of limestone that crosses them capped with rhyolite and all at an elevation distributed through the quartz and the sulphides. noted existed in Neocene time in the eastern part the ridge north of Onion Valley Creek at Last of about 7000 feet. It is evident that we have To the south of the Plumas Eureka the Little of the district. The gravels of this river have Chance, and small lenses at other points, as noted here remnants of an old river deposit formed by Jamison quartz vein is said to promise well. been mined at the Cascade gravel mine on the on the map. a stream flowing at a moderate grade. It is not The Sierra Buttes vein is a large one, in places east slope of the Grizzly Mountains, and at a H. W. TUENEE, 40 feet in width, dipping to the east from 40° to point farther south in the drainage of Little Long *See Whitney, Auriferous Gravels, 1880; and the author's 50°. Valley Creek. In the southeast portion of the Geologist. paper in the Seventeenth Annual Report of the United States Geological Survey, Part I, 1896. The Young America vein, on the eastern slope district the Neocene river deposit at Chips Hill, June, 1896.