DESCRIPTION OF THE MOTHER LODE DISTRICT.

INTRODUCTION. The strip of country included within the area ness in the rocks across which they flow, although Alluvial or bottom land is found only in a few mapped lies along the lower portion of this unfin­ the Stanislaus west of Tuttletown is deflected relatively small patches. Grain aiid hay are

In the' preparation of the text of the Mother ished and inclined t:peneplain , Jr about one- Relation_ , .. of. southward for some distance in the soft Mariposa grown, however, on the gentler slopes. Lode Special folio an attempt has been made to third of the distance from the edge of sierra2**0 slates before entering the deep gorge through The red soils resulting from the decay keep in mind the needs of mining men. Purposeof the Great Valley of California to the which it traverses the resistant volcanic masses of the old volcanic rocks of the Bed-rock tjomplex

The chief interest of the district centers the folio- crest of the range. It is a portion of what is of the Bear Mountains. These rivers Consequent are well adapted for growing grapes and most of in the gold veins of the Mother Lode rather than commonly known as the " foothills " of the Sierra are consequent streams, i. e.,~ their main streams' the common fruits, wherever irrigation can be in any of the more general problems of geology or Nevada, this somewhat vague term being applied direction is a consequence of the general south­ resorted to during the summer. The lack of con­ petrology. The miner wishes to know what the to the hilly country bordering the alluvial floor of westerly inclination given the western slope of venient railway transportation is, for the greater so-called " Mother Lode" really is, how it was the Great Valley and rarely attaining an elevation the sierra at the close of the Neocene. Many part of the district, an obstacle to supplying other formed, in what rocks the veins occur, how these of more than 2000 feet. Owing to the general smaller streams, although not heading so far back than a local demand for agricultural products. rocks originated, and how they came to occupy evenness of this slope no definite line can be drawn in the range, exhibit a similar consequent direc­ their present position, whether a vein in one kind between the " foothills" and the " high sierra." tion of flow. Such for example are Dry, Amador, GENERAL GEOLOGY. of rock is likely to prove richer than in another, The Mother Lode district is hilly throughout, Sutter, Jackson, Willow, and San Domingo creeks. why some veins are rich and others are poor, and in places mountainous, the present Between these streams and those of subsequent For a general account of the geological history . Relief. and to have many other queries of like import topography being the result of the character, such as Moccasin Creek, are countless of the whole Gold Belt, of which the present area answered. Some of these questions are still unan­ energetic Pleistocene (Sierran) and recent erosion small tributaries, too short or too irregular to be is but a part} the reader is referred to the Descrip­ swerable, but wherever actual knowledge has of the uplifted, rolling, Neocene surface. In the safely classified with either of the types mentioned. tion of the Gold Belt prefixed to the regular Gold been gained the aim has been to impart it in the northwestern part of the area the hills are of The Mokelumne, Stanislaus, Tuolumne, and Mer­ Belt folios. Such portions of that history as clearest way and in the simplest terms consistent moderate height. A few conspicuous eminences ced rivers are fed during the summer by melting apply to the limited area forming the subject of with accuracy. only, such as Jackson Butte and Golden Gate snow and by springs far up in the crest region of this folio will be, however, briefly summarized The present folio is intended, first of all, for use Hill, rise above 2000 feet, and the slopes, except the range, and carry considerable volumes of water after the rocks themselves have been described. in the field by those interested in this district. close to the larger streams, are comparatively even in dry seasons. The larger creeks also flow, It is sufficient for the present to keep in mind Consequently the different rock formations are gentle. Toward the south the relief is more pro­ as a rule, all the year round, although the flow is the fact that the rocks of the Sierra described with rather more local detail than nounced. The great area of meta-andesitic brec­ very small in summers of exceptional drought. Nevada fall naturally into two great g££pssofnct would be warranted by their general geological cias and tuffs (" diabase and porphyrite " of earlier Many of the smaller streams, particularly such groups: (1) An ancient complex of interest alone. As far as is practicable a dis­ folios, "greenstone " of the miners), extending from as derive no part of their water from, the forest steep-dipping schistose rocks invaded by intrusive tinction has been made between facts of general the Tuolumne River down to the Merced River zone of the higher slope, become entirely dry in igneous masses the Bed-rock complex (Bed-rock importance and those of local interest, the latter near Horseshoe Bend, has a particularly bold and summer. series of earlier folios), and (2) a very much being printed in smaller type. The principal aim rugged topography. The principal summits in The region is one of moderate rainfall and is, younger group of flat-lying rocks, the Superjacent has been to provide a working geological guide this vicinity range from 2500 to 3000 feet in alti­ as a whole, below the altitude at which any series, resting upon the upturned and eroded for use on the ground. tude, the ravines are deep and narrow, and the important part of the precipitation falls edges of the older complex. To the older group Climate. The map forming part of this folio was origi­ slopes steep. as snow. The normal annual precipita­ belong the slates, schists, and "granites"; to the nally intended to accompany a report on the gold As a whole, the Mother Lode district exhibits tion is 43 inches at the Kennedy mine, near Jack­ younger, the auriferous gravels and Neocene veins of the district, by Dr. George P. Becker. greater geological complexity than any other son, 33.42 inches at Jackson, 32.11 inches at lavas. The two groups are separated by a very This report has been unavoidably delayed. The equal portion of the western slope of Geolo |cal Mokelumne Hill, and 32.04 inches at Drytown. conspicuous unconformity. reader is referred to it for a fuller description and the Sierra Nevada, Rocks of various comPlexity- The rains usually begin in October as showers, THE BED-ROCK COMPLEX. a more complete theoretical discussion of the gold kinds occur, often in small masses and in narrow separated by days of delightfully clear weather veins than is possible in a folio. belts or lenses. This disposition of material has free from the dust and heat of summer. The The Bed-rock complex consists of both sedi­ The field work upon which this folio is based in a measure controlled the erosive processes grass which springs up shortly after these first mentary and igneous rocks. The sedimentary was finished in 1898, and consequently mining which have deeply etched and. obscured the origi­ showers flourishes through winter and spring, but rocks were originally beds of mud, sand, and developments of later date than this are not avail­ nal Neocene surface upon which they began the is generally brown and dry before the end of May. gravel. They represent the mechanical waste able for description or deduction. work of the present cycle. A somewhat intricate The summers are,dry and hot. The temperature of ancient land surfaces which were long ago geological structure, involving rocks which vary frequently rises above 110° during the daytime in deformed and have been carved by erosion into GEOGRAPHY. in resistance to decay and erosion, exposes many July and August, but the nights are usually com­ new forms or buried under later sediments and points of attack to disintegrating and eroding fortably cool. The normal mean annual tempera­ perhaps partly fused by invading, deep-seated The district treated in the present folio com­ agents and is readily carved into a topography ture at the town of Jackson is 55°. The normal igneous intrusions. The sediments derived from prises a comparatively narrow belt of country characterized by considerable detail and variety annual range of the mean daily temperature is this old land mass remain,' but the( land Sediments,_ extending in a nearly northwest-southeast direc­ of form. The soft and thinly fissile rocks, such as from 41.6° in January to 72° in July. Rain sel­ mass itself has disappeared during the anStru- tion along the western foothill region Posltion of the clay slates of the Mariposa formation or some dom falls between May and October, although slow progress of geological change. slves' of the Sierra Nevada. The length of di°Stric°" ° varieties of the amphibolite-schists, are worn into rarely one of the thunder showers which are not Carried down by the streams, this material was this area is very nearly 70 miles, and its width is ravines or small valleys, while the more massive infrequent higher up the sierra slope becomes spread by waves and currents over the sea bottom 6^- miles. It lies mainly within the Jackson and and resistant rock masses, such as the meta- sufficiently general to embrace the Mother Lode in nearly horizontal layers. There were periods Sonora quadrangles, but includes also the extreme andesite breccias or the siliceous schists and district. of volcanic activity during the deposition of these southwest corner of the Big Trees and the extreme quartzites of the Calaveras formation, are but In the character of its vegetation the Mother early sediments. Igneous material, thrown out northeast corner of the Oakdale quadrangles. It slowly reduced in altitude, and persist above the Lode district stands midway between the rolling, either as loose fragments or as volcanic mud, was contains the principal mining portions of Amador, general level as ridges, peaks, and knobs. nearly treeless, grassy hills, which char­ also spread over the sea bottom, accompanied prob­ Vegetation. Calaveras, and Tuolumne counties, and a smaller In spite of the linear arrangement of the closely acterize the transition ground between ably by flows of molten lava. These beds and part of Mariposa County. The boundaries of the compressed belts of schistose and slaty rocks in the floor of the Great Valley and the sierra slope, the associated volcanic rocks have been folded map are merely arbitrary lines, so chosen as to the Mother Lode district the drainage and the real " timber belt" or forest zone of the and compressed chiefly in a northeast-southwest Drainage. permit the presentation, in convenient form, of the is rather wanting in regularity, and sierra. The most characteristic trees are the direction, and have been irregularly intruded by topography and geology of the region immediately shows noticeable lack of the parallelism which scrawny digger pine (Pinus sabiniana) and a masses of igneous rocks, such as granite, diorite, adjacent to the auriferous quartz veins of the might be expected from the structure of the rather stunted variety of white oak (Quercus and gabbro. The sediments have been hardened Mother Lode system. The general trend of this underlying rocks. The occurrence of a belt of douglasii). These trees grow in thin groves or and change^, by the long-continued action of system is northwesterly and southeasterly, corre­ clay slate, compressed between harder rocks, usu­ scattered over the grassy slopes, the oaks being underground solutions, the pressure and move­ sponding to the trend of the more important ally determines a small stream or ravine, such as particularly abundant on areas underlain by the ment of folding, and the heat of igneous intrusions structural features of this portion of the range, Murphy Gulch south of Jackson, Black Gulch and older volcanic breccias and tuffs (meta-andesites). until they often bear no resemblance to their and accordingly the district mapped constitutes a Rich Gulch opening on the Mokelumne River, Some yellow pines (Pinus ponderosa) occur on original condition. The Bed-rock complex is long, narrow rectangle which the meridians of and several others. But the only cases in which the higher ridges along the eastern edge of the therefore made, up of both sedimentary and longitude and parallels of latitude cross at angles streams of any length or importance Subse uent area, being an outlying fringe of the great forests igneous rocks. It is an intricate assemblage of of nearly 45'°. have excavated courses along such belts strealns- of the timber belt, which consists largely of this many different kinds of rocks, differing consider­ Looked at in a broad and general way, without of weak rocks are afforded by the South Fork of pine. Many of the hills are thickly clothed with ably in age. It contains probably more than one regard for the moment to the topographical forms the Calaveras River and by Woods and Moccasin the so-called grease wood (Adenostoma fascicula- sedimentary series, as that term is commonly used in geology. For these reasons it seems best to which have resulted from the very > General creeks, which empty into the Tuolumne River tuni), an evergreen shrub whose dark-green color active post-Neocene erosion, the middle otfnrna'T Jacksonville. Such streams, classed by is a pleasant contrast in summer with the brown speak of it as a complex. western slope of the Sierra Nevada is topography- physiographers as subsequent streams, originated of the parched grass, and with the pale gray-green SEDIMENTARY BOOKS. a peneplain, a word in common geological use to at a later date than the larger streams next to be foliage of the; digger pine and stunted oaks. This indicate a surface which erosion has reduced described. shrub requires but little moisture and flourishes Calaveras formation. The rocks making up almost to a plain. Its surface is of prevailingly The prevailing course of the regional drainage, well on the poor soil derived from the clay slates. this formation are the oldest in the area. They low and gentle relief and was carved in pre- and that pursued by all of the larger streams, or Various other shrubs, such as several species of comprise conglomerates, sandstones, Neocene time upon the Juratrias and older rocks rivers, as they are generally called, is southwest­ wild lilac (Ceanothus), the Christmas berry^ or slates, limestones, cherts, quartzites, which constitute the Bed-rock complex ("Bed­ erly. These main streams, such as the Mokelumne, toy on (Heteromeles arbutifolia), and manzanita and mica-schists, with smaller amounts of other rock series " of the earlier Gold Belt folios). Upon Stanislaus, Tuolumne, and Merced rivers, have (ArctostapJiylos) grow thickly over many of the schistose rocks. These various rocks are not dis­ this worn-down surface were spread the nearly their sources far to the eastward in the high sierra slopes. tributed uniformly in the different Calaveras horizontal rocks of the Superjacent series. Such and on their way to the Great Valley flow across Mining, is naturally the principal industry of areas, but, as will be more fully shown later, cer­ remnants of these rocks as exist in the region of the strike of the steep-dip^ping schistose rocks of the Mother Lode district, such agriculture as tain areas or belts are to some extent characterized the Mother Lode consist chiefly of gravels and the Bed-rock complex As a rule, their courses is practised being chiefly directed toward supply­ by rocks of certain kinds. The occurrence of volcanic conglomerates, breccias, and tuffs. are but slightly influenced by differences of hard- ing the local demands of a mining population. limestone, usually in small lenticular masses, is a characteristic feature of the Calaveras formation, still other cases, facies (varieties) may occur town, and one a little north of east from Jamestown, are pebbles of this conglomerate consist largely of chert, quartz, and as the only fossils thus far found in the series which, while not typical arnphibolite-schists, yet merely portions of a single very irregular area, as may.be seen and slate, with apparently a littlejmaterial of igneous origin. by placing together the geological maps of the Sonora and Just southeast of the ferry a tongue of the Calaveras con­ occur in these limestone lenses, the latter acquire show by the development of green hornblende Big Trees quadrangles. The other large mass of limestone, glomerate extends into the Tuttletown area^of amphibolite- special significance. The fossils are neither and other minerals that they originally contained northeast of Algerine, curves around a large intrusive granitic schist. This conglomerate is accompanied by slaty mica- mass, and, as shown in the Sonora folio, divides into several schists and is itself much more altered than that exposed abundant nor varied, but indicate that the lime­ a certain proportion of igneous detritus in addi­ branches toward the southeast. The limestone of these east­ along the river. It has become a mica-schist in which, how­ stones are of Carboniferous age. Accordingly, tion to the ordinary sands and clays of nontuffa- erly areas is generally recrystallized and may properly be ever, the original pebbles still show on weathered surfaces. that portion of the Calaveras formation in which ceous sediments. As a general rule such rocks termed marble. No fossils have been found in these masses The same belt of Calaveras rocks is also well exposed along within the limits of the area mapped, but they are in the the open bottom of Mormon Creek, where the- rocks consist the limestone lenses occur must also be Carbonif­ have been mapped with the amphibolite-schist line of strike of a similar group of limestone areas in the of slates, usually somewhat micaceous, squeezed conglomer­ erous. It is possible that the Calaveras formation, series. Yosemite quadrangle in which foraminifera (Fusulina) are ates, and cherts, with limestone lenses. The latter contain known to occur. round crinoid stems, indicating that they are of Paleozoic age. as mapped in the Gold Belt folios, may also In common with all the sedimentary rocks of The Calaveras rocks comprising the broad belt west of Dry- Near French Flat, hard, "blocky," dark slates make up the include some still older rocks, but such doubtful the Bed-rock complex, the Calaveras rocks have town differ, considerably from those so characteristic of the mass of the belt. Small lenses of limestone are exceedingly portions lie almost wholly to the eastward of the been closely folded and strongly com- Foldin and area just considered. They are more hetero­ abundant northwest of French Plat, and the formation con. geneous and less altered. Clay slates and con- The western tains also some bands of conglomerate, and streaks of basic area now under discussion and need not be con­ pressed, and the resulting isoclinal folds comPression- glomerates predominate, while small lenses of veras rocks, tuif and talc-schist, usually not large enough to map. This sidered here. have been deeply truncated by erosion. As a limestone are often very numerous. The lime­ belt of Calaveras rocks is of unusual interest from stone is apparently not limited to any single geological hori­ the fact that it swings directly across the line of Caiaveras* At many points in the Calaveras formation result of this folding and compression, a more or zon, but its small masses occur irregularly distributed through the Mother Lode, between Tuttletown and Raw- there are conglomerates containing pebbles of less 'perfect schistose or slaty structure is every­ the belt. No fossils were detected in these limestone lenses hide, and from lying to the west of the main veins near within the area mapped, but Mr. H. "W. Turner has reported veins, comes to. he,. on their,, . eastern,, or ,hanging . Rawhide. vein quartz, chert, slate, mica-schists, Source of where characteristic of the rocks of this formation, sediments. fossils from several points in this same Calaveras belt, within wall, side. This feature will be again referred to in the dis­ sandstone, and grit, indicating that the In many cases the planes of schistosity do not the Jackson quadrangle. These fossils were referred by Mr. cussion of the Mother Lode veins. Northeast of Rawhide, and oldest sedimentary series in the district was depart sensibly from the bedding planes of the C. D. Walcott to the Carboniferous, and are as northwest of Table Mountain, this area of Calaveras is again follows: Fusulina cylindrica, Zaphrentis (?), Fossils in directly connected with the main eastern area of these rocks. derived from still more ancient and in part meta­ original sediments. Frequently, however, the bed­ Lithostrotion (?), and crihoid stems. There are formation. It is difficult to account for the irregularity of the contact morphosed sediments. Practically nothing is ding planes have been obliterated and schistosity also some beds of light-colored chert in this area, between the dark, slaty schists of the Calaveras and the known of the extent and form of the rock masses is the only structure recognizable. Cases in which particularly in Grapevine Gulch, but they constitute only a amphibolite-schist on both sides of Table Mountain northeast small fraction of the whole. The slates of this western belt of Rawhide without supposing that the original folding of whose disintegration furnished these materials. schistosity cuts across the bedding are exceedingly of the Calaveras formation frequently resemble very closely the rocks in this region was of a decidedly complex character. To the eastward, the granitic rocks of the high uncommon. Such structure would occur in the the clay slates of the Mariposa beds, but they lack as a rule the very homogeneous character and even cleavage of the In the vicinity of Jamestown, particularly to sierra have invaded the Calaveras rocks and oblit­ crests of anticlines, but erosion has long since latter rocks. Compared with the slaty schists of the eastern the south and west, the ordinary Calaveras slates erated the basement upon which the latter were carried away these portions of the folds. Folds area of Calaveras, they are much less altered. They have grade frequently into amphibolite- laid down, and from which their materials may are never of infinite length, but every anticline not undergone recrystallization, and such metamorphism as they have been subjected to has been chiefly of a mechanical schists in such a manner that the abso- lntodlniphib= have been in part derived. The eastern limit of must ultimately die out at the ends, and in a sys­ nature and is recorded in the evidences of compression and lutely accurate separation of the two the Calaveras formation is everywhere, as far as tem of folds will be succeeded by a syncline in the development of cleavage. rocks upon the map is impossible. The amphibo­ known, an eruptive contact of intrusive rocks. the line of its prolongation. Wherever an anti­ A conglomerate of somewhat peculiar character alternates with the beds of slate and almost or quite equals the latter in lite-schist in this case was originally a somewhat It is possible that the conglomerates may indicate cline pitches downward and passes into a syncline, amount. Typical exposures of this conglomerate basic tuff or volcanic ash, which was deposited one or more undiscovered unconformities within the bedding planes will cross at varying angles occur on Sutter Creek, about ^ miles below the town of the same name. The beds vary in coarse­ at the same time that the clayey sediments from the assemblage of rocks which has been called the the planes of schistosity developed by the forces ness from fine grits to moderately coarse conglomerates. which the slates were formed were being laid Calaveras formation. producing the general folding. Consequently Pebbles of quartz and chert predominate, but slate, fine mica- down upon the sea bottom. In such a case not Light-colored cherts are of frequent occurrence cleavage and schistosity across the bed­ schist, grits, and a fine-grained rusty sandstone were also noted. The pressure involved in folding these beds into their only would there sometimes be an actual mingling in some portions of the Calaveras formation and ding planes will be developed. Such 5eLavag" to present nearly vertical position has squeezed the pebbles, f j. i i j. beddi°g- of materials, but there would also be a certain not infrequently form lenticular beds cases ot transverse cleavage or schist­ many of which were originally flat or schistose, and has ren­ » , T Cherts. dered the conglomerate roughly schistose, or flaky. At this amount of overlapping of the layers of clay and several feet in thickness. In some osity have been but rarely observed in the Cala­ point the conglomerate contains little or no volcanic material. tuif, due to changes in supply and shifting of cur­ cases these cherts are closely associated with the veras rocks. Their absence, however, is taken But at very many places, particularly in the western portion rents. With the subsequent folding, compression, limestones and may be due to siliceous replace­ rather as an indication of the intense pressure of the area, conglomerates occur in which pebbles and frag­ ments of meta-andesite and related volcanic rocks occur so and truncation of the beds, there has resulted the ment of calcareous material. But many of the concerned in the compression of the folds and in abundantly as to render it extremely difficult to discriminate intricate relationship exposed on the surface larger and more persistent beds have a different the production of schistosity, than as pointing consistently between the non-volcanic conglomerate of the Calaveras formation and the meta-andesitic tuffs and brec­ to-day. origin. They were originally highly siliceous to the original absence of pitching and complex cias All gradations may indeed be found, ranging from a Mariposa formation. The rocks making up conglomerates in which the pebbles have been so folds. Exposures of Calaveras rocks show planes conglomerate, such as that described on Sutter Creek, through the Mariposa formation consist chiefly of very flattened and welded by pressure as to make an of cleavage or schistosity having a prevailing various tuffaceous facies to a more or less angular volcanic breccia. Such a transition can be well studied 4 miles south­ cleavable and homogeneous clay slates, Characterof almost homogeneous rock. The selective action of northwest-southeast strike and a dip of 70°-80° west of the town of Sutter Creek, near the edge of the area with locally varying amounts of sand- rocks- the weather on exposed surfaces reveals, however, NE. Locally, however, both direction of strike mapped, and also south and west of the hill called Sugarloaf This intimate relation between the sedimentary Calaveras stone and conglomerate. Slates, sandstones, and the true structure. Such squeezed conglomerates and amount and direction of dip may vary widely, formation and the ancient clastic andesites (or meta-andesites) conglomerates sometimes alternate in well-defined can be well studied 2J miles northwest of San even when such irregularities are not traceable to will be reverted to when the latter rocks are described in their beds, but very frequently the last two form irreg­ Andreas, north of the North Fork of Calaveras the disturbing influence of igneous intrusions. turn. Dikes of light-colored porphyry are numerous, ular lenses in the slates and have but little linear River. Such variations in the dip and strike of the especially along the eastern border of the belt. Dikes in m? ;; , ' , i -, Calaveras persistency. In the vast majority of cases the Although made up chiefly of rocks that were cleavage planes indicate that the rocks undergo­ The larger of these have been mapped and will formation, be described with the igneous rocks. original bedding planes and the planes of cleavage originally siliceous and argillaceous sediments, the ing deformation did not behave as a homogeneous A much smaller but interesting strip of Calaveras rocks are parallel, so that the strike and dip of the slaty Calaveras formation, as outlined upon material. Lack of homogeneity caused local reso­ begins at a point about 1^ miles east of Amador and, passing cleavage corresponds also to the strike and dip of the, map, contains. some material -I 01P Tuffs- lution of the main stresses into minor stresses j ust east of Jackson, extends down to within less than a mile of the Mokelunine River. This belt consists of the slaty the beds. Speaking broadly, the rocks of the igneous origin which it was not practicable to acting along lines making considerable angles schists (or micaceous slates) characteristic of so large a portion Mariposa formation constitute a remarkably per­ map separately. Thus, lenticular bodies of old with those of the major forces. of the Calaveras formation, together with cherty beds and at least one rather thin lens of limestone No fossils were found sistent narrow belt that, with the exception of a andesitic or basaltic tuft's are not infrequently In describing the rocks of the Calaveras formation in in this strip, and its assignment to the Calaveras formation sweep to the south westward, southwest of Fourth intercalated in the clay slates or siliceous schists. is made purely on lithological grounds. The presence of lime­ greater detail it will be convenient to consider first the large Crossing, which carries it for a distance outside Such tufts are invariably altered. In some cases and very irregular area of these rocks which lies stone, the frequent occurrence of cherty beds, and the mica­ chiefly to the eastward of the Mother Lode veins Tne eastern ceous character of the slates are features not found in any of the limits of the area mapped, traverses the dis­ they have been indurated so as to form hard, ring­ T -i -i ^ -i * ^ area °f Cala- and occupies many hundred square miles of the veras rocks, the areas of known Mariposa rocks. Just southeast of Jack­ trict from end to end. Looked at more closely, ing rocks of fine grain and massive aspect. But higher mountain slopes, beyond the northeast son, however, it is difficult to draw the line sharply between they are more often schistose, and have frequently boundary of the area mapped. Attention will afterwards be the Calaveras and Mariposa slates. It is certain that the this belt is seen to be sometimes very irregular. directed to the smaller, not necessarily entirely detached areas, unaltered clay slates of the Mariposa formation are wholly cut It branches or divides into two or more belts and been transformed into amphibolite-schist by which are more closely associated with the Mother Lode vein off, just west of Scottsville, by micaceous slates, cherts, and recrystallization of the original minerals. Such system or lie to the west of that zone. limestone of thoroughly Calaveras aspect, and having a line at one point, possibly at two, its continuity is The rocks composing this large eastern mass of the Cala­ of strike different from that of the belt of Mariposa slates interrupted. As in the case of the Calaveras for­ altered igneous material has always a greenish veras formation are fairly uniform in character, and consist underlying the town of Jackson. mation, northwest-southeast strikes and steep tint, due to green hornblende, chlorite, and other for the most part of .dark, fine-grained, slaty mica-schists such The relation just shown between the Calaveras greenish minerals, and weathers to a rusty soil. as, in common with the clay slates of the Mariposa formation, northeast dips greatly predominate; but both dip are usually termed "black slates" by the miners. These and Mariposa slates carries with it a and strike are subject to wide local variation. In certain portions of the area the argillaceous rocks, however, seldom exhibit the very even and character­ strong suggestion of unconformity ityCbetweTn The Mariposa rocks as a whole contain very few slaty and schistose rocks of the Calaveras forma­ istic cleavage of the unaltered clay slates, and close inspection -T i P TTTI -i Calaveras of a fresh specimen generally reveals minute mica scales lying between the two iormations. Wnile formations.and Mariposa organic remains. Sufiicient fossils have been tion pass so insensibly into amphibolite- Gradation in a finely crystalline groundmass. They are, indeed, very between tuffs not in itself conclusive, this case is in found, however, at various points to schists derived from ancient tuffs that and sedi­ fine-grained crystalline schists, consisting of ments. line with several other occurrences which indicate establish the Jurassic age of the main no sharp boundary can be established abundant quartz, biotite, and black dust-like fch*sts.lotite= particles (carbon ?), which latter give to these the same unconformity. belt shown on the maps (see Jackson and Sonora between the two rocks, even with the best of schists their dark color. Typical schists of the character folios). In the case of smaller isolated belts or exposures. The intimacy of the relation between described may be seen along the main road between Priest The several tongues of Calaveras schists lying southeast and Big Oak Flat and at many other points. Some thinly of Middle Bar do not call for detailed description. Some detached lenses, the lithological character is as a the two classes of rocks may be revealed in some­ fissile slates do occur, particularly in the western portion of difficulty was experienced in drawing the boundary between rule the chief factor which determines whether what different ways. A belt of siliceous or argil­ the mass, but even these slates are frequently micaceous and the Calaveras and Mariposa formations southward from Mid­ laceous schists and slates, when followed along plainly represent much more altered sediments than do the dle Bar. This, however, is not at all uncommon whenever such small areas shall be mapped as Mariposa or ordinary Mariposa slates. Not infrequently the siliceous bio- the two formations are directly in contact. It was encountered the strike, may be found passing irregularly into Calaveras. Thus it is always possible, though titic schists are intimately associated with streaks and lenses in delineating the tongue of Calaveras cherts, siliceous con­ scarcely probable, that the future discovery of a belt of amphibolite-schist. Good exposures of amphibolitic schist, which weather to a rusty red soil. glomerates, and micaceous slates which, passing 1^ miles west across such a transition zone will sometimes show Such bodies of schist- may in some cases represent altered of San Andreas, extends northward across the Calaveras River fossil remains within any small area of slates may dikes, but have probably in most instances been derived from toward Golden Gate Hill. It was also met with on Cherokee change the designation given that area upon this an intimate interlamination of clay slates or basic tuffs, deposited contemporaneously with the argilla­ Creek, 4 miles west of Angels Camp.- ' phyllites with slaty amphibolite-schist, together ceous and siliceous sediments. These lenses, when of suffi­ An important area of Calaveras rocks begins about 4 miles map. cient size, are shown upon the map, but many of them are northwest of Angels Camp and extends across the Stanislaus with considerable slaty material that is neither too small, or too poorly defined, to make it practicable to River, past French Plat, Rawhide, Jamestown, and Quartz Near the town of Plymouth the belt of Mariposa rocks has typical green amphibolite-schist nor an ordinary indicate them on a map of this scale. Such amphibolites Mountain to a point a little beyond Sullivan Creek. West of an average width of about a mile, and consists almost wholly occur just north of Jackson Butte, east of Dogtown, about Angels Camp this area consists of clay slates, micaceous slates, of homogeneous clay slates with very uniform strike and dip. slate derived from residual non-volcanic sediments. the reservoir 3 miles northeast of Angels Camp, north of slaty mica-schists, squeezed conglomerates, and sandstones, Some conglomerate occurs near the western edge of the belt In other instances bands of amphibolite-schist, Carson Plat, and at many other localities. with some chert and limestone. There are also occasional and continues down past New Chicago and New Philadelphia too narrow to map individually, alternate with The limestone bodies in this main area of Calaveras rocks thin lenses of altered basic tuffs, or amphibolite- almost to Amador. The conglomerate occurs usually in two are noteworthy both on account of their extent and because schist. At Robinsons Perry, on the Mokelumne tuf?Sjnscaia- or three beds interbedded with clay slates and schistose sand­ bands of slate or siliceous schist and it becomes of their frequently exceedingly irregular out- ,. River, the micaceous slates and schistose con- veras forma- stones. The pebbles consist, of quartz, chert, slate, and schist, necessary to generalize and map the area either as lines.T BothT> 4.1- theseJ.-U facts* j. are, however,-C onlyi par­" Limestone. glomerates are connected with the main mass of and as a general rule are not noticeably distorted by pres­ tially illustrated within the limits of the present map. The Calaveras rocks to the eastward. For at least half a mile sure. Between the conglomerate and the meta-andesite area Calaveras or as amphibolite-schist, according as three patches of limestone shown near the edge of the map down the river from Robinson Perry the Calaveras is repre­ to the west there is usually a narrow strip of clay slate. one or the other rock appears to predominate. In (section 3), one at Natural Bridge, one northeast of Tuttle- sented by a squeezed, flaky, schistose conglomerate. The Near Amador the.main belt of slates contracts to a width of 3 about one-eighth of a mile. For a distance of a mile, how­ original structure and mineralogical constitution tions of the minerals already enumerated. The hornblende coarsely crystalline schistose rock consisting of ever, northeast of the town, the Mariposa clay slates occur in of eruptive rocks, but also those which have been is sometimes porphyritically developed, as in an epidote-rich quartz, calcite, and muscovite, with abundant close association with the tuffaceous and more coarsely clastic facies forming a narrow band just west of the Mokelumne rocks of the meta-andesitic volcanic series. From Bunker directly derived from igneous masses through Hill meta-diorite area, and well exposed in the canyon of the small cubes of pyrite. It occurs as a lenticular Hill to a point just east of the town of Sutter Creek the expo­ various processes of change (inetamorphism). Mokelumne, about a mile above the bridge at Middle Bar. mass inclosed within the Calaveras slaty schists sures show that the basic tuffs and clayey sediments from East of Golden Gate Hill there is a break of about 2 miles in which the Mariposa slates were formed were deposited prac­ Between these two classes of rock no perfectly the amphibolite-schist belt. At San Andreas, however, the about 4 miles a little north of east from San tically contemporaneously, and under similar conditions. sharp line can be drawn in the Mother Lode green schists have again attained a width of over a mile. Andreas. On account of its striking difference Sharp contacts sometimes occur, but often slate and tuff region. In this district it is exceedingly uncom­ Between San Andreas and Angels Camp the amphibolite-schists from the ordinary schists of the Calaveras forma­ . grade into each other, or alternate in such thin beds that it is occupy an extensive and very irregular area. A broad tongue impossible to separately delineate them, on a map of this scale. mon to find in any portion of the Bed-rock com­ of them extends to the eastward up San Antonio and Indian tion and its probable derivation from a granitic Where gradation Occurs the fact is indicated by omitting the plex an igneous rock mass which microscopic creeks, and another large mass extends into the main area of dike, the mass has been indicated on the geologi­ line separating the colors of the two rocks upon the map. Calaveras rocks northeast of Angels Camp. The rock compos­ Where the alternating streaks are too small to map the struc­ study does not show to have undergone consider­ ing these tongues is, as a rule, re crystallized and thoroughly cal map. ture is somewhat generalized. able alteration. The ancient volcanic tuffs (nieta- schistose. The strike of the green schists is generally con­ Meta-andesite. The name meta-andesite is Just southeast of the town of Jackson, the main belt of andesites) have been rendered roughly _ formable with that of the enclosing Calaveras rocks. The applied to andesites which have undergone dis­ Mariposa slates is cut off, as already described, by supposedly / o J Deep-seated schistosity near contacts is always approximately parallel to Calaveras rocks. A quarter of a mile southwest of the town, schistose, and contain secondary amphi- alteration- the latter. These relations, taken in connection with the tinct alterations, but which still retain Definition. however, is a very narrow strip of Mariposa slate which bole, chlorite, epidote, and calcite. Even the varying strikes and dips of this portion of the area, indicate in large part their original structure is noteworthy in containing the important Kennedy and originally complex folding. Southeast of Lower Calaveritas Argonaut mines. There is at present no means of telling freshest-looking of the granular intrusive rocks and northeast of the narrow strip of Calaveras rocks which and mineralogical composition. In the earlier whether this narrow strip really connects to the northward are generally full of secondary minerals and fre­ extends southeastward toward Bunker Hill the fine-grained Gold Belt folios such rocks have been termed with the broader band of slates lying east of it, as it passes quently pass into schistose peripheral modifica­ amphibolite-schists are associated with numerous narrow porpliyrites (a name in common use by German beneath andesitic breccia east of Martell. They probably do streaks of black slate. A portion of these schists appears, more­ unite however. Toward the south this narrow ribbon of tions. The feldspars are commonly altered to over, to have been derived from material consisting only in petrographers for andesites of pre-Tertiary age), slate continues without break to the Mokelumne River. aggregates of epidote (or zoisite), white mica, and part of igneous detritus. This portion of the area, like that diabase-porphyrites and diabase-tuffs. The meta- Before reaching the river, however, it joins the spur-like pro­ already referred to near Jamestown, probably preserves the jection of slates extending up Black Gulch and so merges into more obscure secondary products. Since Tertiary record of a time when comparatively fine volcanic tuffs and andesites of the Mother Lode district, as will be the wider slate band of Rich Gulch, in which is located the time the streams have in some places cut down argillaceous mud were mingled together on the sea bottom, or shown, are chiefly fragmental or clastic rocks. Gwin mine. Thus the Kennedy, Argonaut, Alma, Anita, into these older igneous rocks to a depth of over deposited thereon in overlapping layers. They are old volcanic tuffs and breccias. The Amador No. 2, Mammoth, and Gwin mines, all occur in the As already stated, the rocks comprising the large eastern same remarkably narrow and continuous strip of black slate. 1000 feet without exposing unaltered rock, area of the Calaveras formation are as a whole more altered original character of an ancient altered tuff is not Miners are inclined to draw from such a fact deductions not whereas the Tertiary lavas are, as a rule, fresh than those of the western area. A similar statement may be always exactly determinable, so that it is not to warranted by a comprehensive study of the Mother Lode dis­ made with reference to the amphibolite-schists As a rule, trict. At present attention is directed simply to the dis­ and unchanged. Thus this general alteration of the thoroughly reerystallized and typically schistose facies be understood that every variety or facies of tribution of the slate, the discussion of its bearing upon min­ the older igneous rocks is not a mere superficial occur in the middle or on the eastern side of the irregular igneous rock within the areas mapped as meta- ing problems being reserved. phenomenon, but is profound in its character, and amphibolite belt. Along the western edge, however, south­ andesite was originally an andesite. It is very The main belt of Mariposa slates, after being cut off west of west of San Andreas and west and south of Fourth Crossing Scottsville for an interval of about a quarter of a mile, reap­ is a result of the stresses to which the region has the amphibolite-schists pass into less altered facies which still probable that basaltic tuffs were laid down in pears and crosses the Mokelumne at Middle Bar with a width been subjected, and of the passage of solutions retain more or less of the original igneous minerals and struc­ subordinate amount with those of 'andesitic char­ of half a mile. It passes beneath the andesites of Golden Gate ture. Such facies not uncommonly show porphyritic horn­ Hill and finally wedges out about a mile south of San Andreas. through the rocks similar in character to those blendes and might be taken for some of the porphyritic acter. Basaltic flows and sheets appear also to Clay slates predominate in the greater portion of this area, which deposited the Mother Lode veins. Such meta-andesites presently to be described. The microscope have played a part, although a small one, in but southwest of San Andreas the slates are largely replaced widespread alteration of the country rock is not shows, however, that the groundinass of the rock has under­ building up these great igneous accumulations. by conglomerate. This conglomerate varies from a grit or gone recrystallization and has the character of an amphibol­ very coarse sandstone to a conglomerate with pebbles 4 or 5 peculiar to the Mother Lode district. It is com­ ite-schist. None of the material, however, so far as is known, inches in diameter. The latter consist of quartz, chert, schist, mon in many mining districts, and is a natural Typical fine-grained amphibolite-schist underlies and sur­ contained olivine, and the series as a whole pre­ slate, and probably a little volcanic material, and usually accompaniment of the conditions which produce rounds the town of Angels Camp, and extends down to Carson sents andesitic rather than basaltic characters. show the result of intense pressure Near the contact with Hill and the Stanislaus River. On the south side of Carson .the Calaveras on the east is a breccia composed chiefly of large and productive veins. Hill and in Indian Grulch the amphibolite-schists are bleached These older volcanic rocks must at one time have fragments of chert and quartz and apparently derived from Amphibolite. Rocks embraced under this head and light-colored in superficial exposures and separable with been very similar to the andesitic tuffs the adjoining chert beds of the Calaveras. A similar con­ some difficulty from the Calaveras rocks lying to the west of Clastic origin. glomerate occurs 1£ miles southwest of San Andreas in the occur in both massive and schistose forms. They them. Tuttletown lies in an area of the green schists, which, and breccias of Neocene age which cap belt of Mariposa rocks which is shown just on the edge of the while evidently a portion of the main belt of these rocks, has have been derived from igneous° rocks,' Component_ so many of the present ridges of the Sierra map. of basic or intermediate chemical com- minerals- yet been completely isolated by the complex folding. A Nevada. But they were probably on the whole Southwest of Angels Camp this western belt of Mariposa rather coarsely crystalline fibrous facies showing abundant rocks again swings within the area mapped, with a width of position, by processes of metamorphism, through remnants of augite crystals is common in this area. The large more basic, and among themselves more variable, 1J miles. It occupies the southwestern border of the dis­ which the augite and plagioclase of the original mass of amphibolite-schist folded in with the Calaveras rocks than the Tertiary lavas. trict as a broad band which extends across the Stanislaus northeast of Jamestown has been derived from igneous rocks » and beneath the lava of Table Mountain. Slates, sandstones, rock have been obliterated, and green amphibole, of more than one kind. Much of it, particulai'ly just east and The meta-andesites occupy extensive areas, not and conglomerates, with lenses of basic tuff, are the prevail­ biotite, chlorite, white rnica (sericite), feldspar, southeast of Jamestown, is fine-grained, with slaty cleavage, only within the district, but also just outside its ing rocks of this area, the slates largely dominating all other epidote, zoisite, calcite, quartz, and other minerals and was probably originally a fine tuff. About 1J miles north­ southwest border. They occur in broad rocks in amount. east of Jamestown, on the road to Columbia much of the Occurrence. have been formed by secondary crystallization. schist has been derived from a volcanic breccia. Although bands parallel with the general trend of On Woods Creek the belt of slate contracts The characteristic mineral of such metamorphosed the original minerals have given place to new mineralogical the sedimentary series and in smaller lenticular somewhat in width, and toward the southeast combinations, yet the grosser structure of the originally frag- rocks is a bright-green amphibole and the univer­ mental rock still remains. In the central part of the area a masses in the Calaveras and Mariposa sediments. is well denned and much simpler in sal greenish tint of the amphibolites is, in almost finely fissile schist/occurs which is composed chiefly of quartz As a general rule, bands of meta-andesite are outline than farther north. The bed all cases, due to the presence of abundant slender and a pale-green sericite. This schist may be well seen on the less persistent, and more variable in width, than road between Jamestown and Sonora, about a mile southwest of Woods Creek and the ravines open­ prisms of this mineral. of the latter town. It was evidently derived from a much less the belts of clay slates with which they are asso­ ing into it afford the best exposures in the whole The typical schistose amphibolite, or amphibo- basic rock than most of the amphibolitic series, and is not, ciated. The clastic origin of the great bulk of area for studying the structure and character of strictly speaking, an amphibolite-schist. On the eastern edge lite-schist, (the so-called "gray-slate" of the of this area, south of Sonora, the amphibolite-schists comprise the meta-andesites is evident merely from a the Mariposa beds in detail. As usual, slates miners)> ' shows,' as a rule,' no trace. of^ Typical_ , , unusually hard and rather massive rocks, which appear to be careful study of the shape of the areas, and predominate, but there are numerous beds of the minerals or structure characteristic facies- altered diorites. They are completely recrystallized and when of their relation to the belts of clay slate and examined under the microscope differ from the more common sandstone and conglomerate. The sandstone is of the igneous rock from which the schist was facies of the schists in the larger size of the green hornblende conglomerate. often somewhat schistose, the schistosity being derived. Occasionally, however, remnants of the crystals. There is the usual granular mosaic of quartz, feld­ Some shade of grayish-green is characteristic of usually parallel to the bedding planes and to the spar, and caleite. Epidote is frequently very abundant, and original structure remain, and not infrequently in some portions of the mass a paragonite-like mica and small all the meta-andesites. The green color, however, cleavage of the slates. The beds of conglomerate areas of amphibolite grade into masses of com­ garnets are plentiful. The maximum width of this mass of is not original, but is due to the altera­ Green color. are not always persistent, but often pass irregu­ paratively little altered igneous rock. interesting schists where crossed by Sullivan Creek is about tion of the rock, and to the develop­ half a mile. larly into sandstone and slate. On Sullivan The amphibolites were, in part, derived from Between Jamestown and Stent fine-grained, slaty amphib ment of such secondary green minerals as chlorite Creek, for' five-eighths of a mile from its mouth, olite-schists are intricately dovetailed or interleaved with the massive igneousO rocks. But by«/ far the Ogreater and uralitic amphibole. These rocks were once there is exposed an excellent section of the Mari­ Calaveras slates. These amphibolites were at one time igne­ proportion of them, and especially the ous tuffs deposited over the sea bottom synchronously with gray, like the Neocene andesites. A very common posa slates, with but little sandstone or conglom­ thoroughly schistose or slaty facies the Calaveras muds and afterwards folded up together with facies of the meta-andesite is a breccia composed erate. Along much of this section it is apparent (varieties), were derived from tuffs and the latter. of fragments up to 3 or 4 inches in diameter Southward from a line connecting Stent and Algerine fine­ that the principal slaty cleavage makes a consider- volcanic breccias originally similar to the Neocene grained facies of imperfect schistosity are closely associated embedded in a finer matrix. The fragments £,ble angle with the planes of original bedding. andesitic tuffs and breccias which cap many of with the more common amphibolite-schists. These have in usually show small porphyritic crystals of augite, This is a very uncommon occurrence in these some cases been derived from tuffs of various kinds, from the present ridges. volcanic breccias, and perhaps in some cases from massive or more rarely, of hornblende. As a rule the rocks, as far as known. Practically all of the amphibolite-schist of the lavas. Thus considerable variety in texture and color may be breccia has become so thoroughly indu­ Induration. The Mariposa rocks along Moccasin Creek call district is comprised in a very irregular and fre­ observed in the amphibolites west of the Algerine meta-dio­ rated that its clastic structure is appar­ rite area; but they manifestly are all part of the main for no particular description. They resemble in quently interrupted belt lying north­ ent only on weathered surfaces or under the Occurrence. amphibolite-schist belt, and grade into more nearly typical general character those composing part of the east of the main strip of Mariposa rocks. facies toward Jacksonville. microscope. All gradations can be found, from same belt on Woods Creek. Four miles north­ Sometimes the amphibolite-schists separate the On Rattlesnake and Jackass creeks, south of Priest, the fine­ the coarse breccias, down to the finest tuffs, and grained altered peripheral portions of the meta-diorite can west of Coulterville the belt is cut in Intrusive, . . Mariposa from the Calaveras areas, but in many not be sharply separated in the field from the amphibolites to in nearly every case the induration has been such two by an intrusive mass of serpentine serPentine- places they are so irregularly involved with the the west, which are here much restricted in width, owing to as to convert a once loose mass of fragments into a mile in width. West and south of Coulterville, the intrusion of the diorite. Calaveras schists as to indicate that the two rocks Southeastward from Jackass Creek the belt of amphibolite a hard, tough rock. A rude schistosity is of very the Mariposa rocks consist chiefly of clay slate, have been folded and compressed together and broadens toward Coulterville. Although consisting in the frequent occurrence, particularly in the finer tuffs, with uniform dip and strike, constituting a belt that they are essentially of the same age. The main of recrystallized, more or less schistose amphibolite, this and is probably in most cases coincident with, portion of the area contains other fine-grained green rocks about a mile in width. Conglomerates and amphibolite-schists are readily distinguished from which are very much altered and uralitized basic tuffs or and partly determined by a rough original sandstones, however, are not present. A coarse adjacent Calaveras schists or slates of sedimentary eruptives. The recrystallization in such rocks has not been bedding. The surface exposures of such schistose conglomerate with pebbles 5 to 6 inches in diam­ origin by their invariable greenish tint when fresh so complete as in the typical amphibolite-schists. Although facies show the usual steep northeast­ composed almost wholly of secondary minerals, they are Outcrops. eter occurs on the ridge west of Whites Gulch, or only slightly weathered, and by the red color turbid and obscurely crystallized when examined under the erly dip common in the sedimentary while along the Merced River is much hard, fine­ of the soil resulting from their decay. The Cala­ microscope. The typical amphiboiites, on the other hand, series, and project from the soil in long, parallel, have been clarified by thorough recrystallization, and their grained, thin-bedded sandstone. This main bed veras rocks, containing as a rule but little iron, component minerals are fresh and clear. comb-like outcrops the so-called " gravestone of Mariposa rocks extends far beyond the limits generally give rise to a gray soil. slates." The meta-andesitic breccias and tuffs are of the area mapped, and in Hell Hollow, just The strip of amphibolite-schist which enters the area Southeast of Coulterville the amphibolite- more resistant as a whole than the neighboring south of the Merced River, were found the fossils mapped northeast of Plymouth and extends southeastward to schists pass, without definite contact, Mariposa or Calaveras sediments, and form some of which first determined its age as Jurassic. a point about 2| miles south of Mokelumne Hill the boldest mountain masses of the foothill region. is composed almost wholly of the greenish, thor- Description into the meta-andesite area of Buck- T i i x i j> * j. ^ ± °f areas in horn Flat. This area is to be regarded They decompose to a red soil which supports a IGNEOUS BOOKS. oughly schistose rock, of somewhat fibrous tex- detail. ture, which represents the most widespread and as a residual mass which has escaped metamor­ growth of oaks and grass, and is much more fer­ characteristic facies of the amphibolite-schists. From place Under this heading are included not only such to place the rock varies in degree of schistosity, in coarseness phism into amphibolite-schist. tile than the gray soil derived from the clay slates members of the Bed-rock complex as preserve the or fineness of grain, in hardness, and in the relative propor- Quartz-muscovite-schist. This is a rather of the Mariposa or Calaveras formations. Mother Lode. One of the largest areas of meta-andesite occurring within andesite or meta-diabase, occur in the neighbor­ usually medium-grained granular rocks, commonly ined in thin sections under the microscope the the limits of the area mapped is the broad belt lying west of hood of Quartz and Stent. These, however, call termed " granite " by the miners. They occur in rock usually shows abundant green hornblende Jackson, Sutter Creek, Amador, and Plymouth, and separating the main eastern belt of Mariposa J-^^llJ11 for no special description. They are altered areas of variable size and irregular form and are in rather irregular areas, and usually some biotite. slates from the western belt of Calaveras rocks, detail. greenish rocks, easily distinguished from the ser­ in all cases intrusive into, and therefore younger Occasionally the outlines of the original feldspar This area (the same one that farther south includes the elevations of Mount Joaquin and Bear Mountain) pentine and Calaveras rocks with which they are than the other rocks of the Bed-rock complex areas are preserved, but more often the horn­ is made up chiefly of volcanic breccia of the kind described, here associated. which surround them. These masses are in all blende is surrounded by a somewhat confusedly with some finer tuffs, and a very subordinate amount of A considerable mass of meta-andesite breccia probability genetically related to the crystallized groundmass composed of epidote, massive meta-andesite. The latter appears to have been erupted as surface flows, generally contemporaneous with the and tuff occupies the southeast corner of the dis­ great quartz diorite, granodiorite, and secondary feldspar, quartz, and muscovite. In tuffs and breccias. Along the western edge of the area occur trict, forming Buckhorn Flat. This is probably granite areas 01i> thei higheri n sierra slope,i . such a rock it is often difficult to determine conglomeratic facies, which, by the addition of non-volcanic pebbles, often pass into the siliceous conglomerates of the a residual portion of a much larger area of similar and belong to the same general period of intru­ which minerals are primary and which are second­ Calaveras formation. On the northeast the contact between rocks of which the greater part have been trans­ sion. In the Jackson and Sonora folios this ary. In many cases it is believed that accessory the meta-andesite and the Mariposa clay slates is usually formed into aniphibolites. relationship was expressed by mapping all of titanite and apatite, and possibly some of the sharply enough defined. Yet the occurrence within the meta- andesitic area of long, thin lenses of dark clay slates, (often too Meta-hornUende-andesite (" Hornllende-porphy- these areas as " granodiorite," although it was real­ hornblende, are all that remain of the original small to map), which resemble in every way the known rite" of earlier folios). Meta-andesitic rocks in ized at that time that as a whole they are far more igneous constituents of the rock. It is thus seen Mariposa slates, and the projecting tongue of Mariposa slates and sandstones extending northward from the Grwin mine up which the principal dark mineral is hornblende closely related to the diorites than to the granites. that the meta-diorites, even where not superficially Black Gulch, all point to a close age relationship between the instead of augite, are not common in well-defined In the present folio it has been thought best to decomposed, are much altered from their original Mariposa rocks and at least a portion of the meta-andesites. masses. One area of such rock occurs on Moccasin map them as meta-diorites, or altered diorites, dioritic character. They have undergone deep- A second regular belt of meta-andesitic rocks lies just north­ Creek not far from its mouth, while another lies reserving for the text the explanation that they seated alteration throughout the Mother east of Jackson, Sutter Creek, and Amador. This is composed O Alteration. in- large part of rather fine tuff which is so intimately asso­ just southwest of Moccasin Peak. Both areas are are probably somewhat basic facies of the same Lode region, and it is probable that ciated with the Mariposa slates east of Amador as to show that clayey silts, which were ultimately to form the slates, were composed largely, if not wholly, of indurated magma which has solidified to the eastward as this alteration is connected with the rock-fissuring laid down simultaneously with the tuffs. breccia. Hornblendic facies also occur at various typical granodiorite. The intrusive character of and the passage of solutions through the country Numerous lenses of meta-andesitic tuffs and breccias occur other places within the meta-andesite areas, but the meta-diorite is sufficiently shown by the very rock during the period of ore deposition. In part in the broad belt of Calaveras rocks in the northwest corner of the district. Not uncommonly these lenses are composed of do not occur in sufficient quantity, or with suffi­ irregular outline of the areas, together with evi­ also the alteration is due to mechanical stresses. waterworn pebbles of volcanic rock. It is difficult or impos­ ciently definite boundaries to make it practicable dent local disturbance and contact metomorphism It is evident through a close examination of the sible in many cases to discriminate sharply between the purely to map them separately. of the invaded rocks. As a rule the intruded meta-dioritic masses that the forces igneous conglomerate and the ordinary siliceous conglomerates Schistosity. of the Calaveras. The two grade into each other, and were Meta-diabase. Bj meta-diabase is understood masses grow larger with depth, and if the surface which have produced the prevailing deposited at the same time and under like conditions. These an altered rock of the gabbro-basalt family pos­ of the country were more deeply eroded than at schistosity of the Bed-rock sediments continued relations are well seen south and west of Sugarloaf. sessing a holocrystalline ophitic structure. Such present, the existing areas of meta-diorite would to be operative subsequent to the dioritic intru­ It will be seen from the foregoing that the rocks are composed essentially of basic plagioclase be much larger, and in all probability some masses sions, or perhaps recurred, for the latter are rneta-andesites can be assigned neither wholly to (labradorite or anorthite) and augite, with some­ would be uncovered which are now still concealed. frequently schistose, particularly near their peri­

Calaveras nor wholly to Mariposa times, Age of meta= times olivine. The prefix metar indicates that The area of rneta-diorite near Algerine. Algerine., . pheries or where they send out relatively narrow but that their eruption took place both andesites- these minerals may be largely altered to secondary is particularly instructive in illustrating area- tongues into the surrounding schists. Excellent in the Carboniferous and in the Juratrias period. minerals. the points just referred to. Its outline is curiously examples of this may be seen northeast of ISTo certain criterion has yet been discovered for The meta-diabases of the Mother Lode district irregular. This is largely due to the fact that Amador, north of Butte City, three-quarters of a discriminating between the meta-andesites of these are closely associated with the meta-andesites and erosion has only partly stripped the Calaveras mile northeast of Middle Bar on the Mokelumne two periods. It seems probable that the broad belong to the same general period (or periods) of schists from an intruded mass which grows rapidly River, and on Jackass Creek, a tributary to Moc-. belt of tuffs and breccias lying west of Jackson, eruption. They appear to have been flows, or larger with increasing depth. The contact of the casin Creek, in Tuolumne County. In some of Sutter .Creek, Amador, and Plymouth may consist intrusive masses or sheets. meta-diorite with the siliceous schists of the Cala­ these cases it is difficult to distinguish the schist­ veras is well exposed at several places. One of ose hornblendic facies of the meta-diorite from the in part of rocks erupted in Carboniferous time, The area about Chinese Camp consists of a medium-grained and in part of those erupted in Juratrias time. rock composed of an ophitic aggregate of plagioclase, augite, the best of these is half a mile northeast of Alger­ ordinary amphibolite-schists of the region. and biotite. The rock is much altered and con­ ine, where slaty schists of the Calaveras formation The belt of meta-andesitic tuff which separates the Mariposa tains abundant secondary hornblende, chlorite, campSarea. Most of the separate areas of ineta-.diorite call for little and Calaveras rocks west of Carson Hill consists of a hard, quartz, and carbonates, as well as a serpentinous can be seen striking generally into the meta-dior- special description. The Mokelumne Hill area is notably fine-grained rock containing occasional fragments of black alternation product which may possibly represent former itic area, but becoming curled and twisted as the irregular in outline and the meta-diorite shows considerable variety. Surface exposures are Description shale. A rather fine indurated tuff is the prevailing rock of olivine. It was not found practicable to separate this massive contact is approached and so indurated as to this area, although some portions of it may possibly be mas­ intrusive meta-diabase by a definite line from the ineta-ande- usually decomposed, and the rock is markedly detail?8 m sive. Toward the northwest the strip becomes narrow, and sitic breccias to the southeast. resemble compact quartzites or horn stones (horn- schistose along its western border. South of passes into an area of amphibolite-schist. On the right bank of the Tuolumne River, just above Jack­ fels). The Calaveras rocks were evidently soft­ Butte City a facies occurs consisting almost wholly of horn­ On the extreme edge of the area mapped, southwest of sonville, is a small area of fine-grained meta-diabase which was blende, and is in contact with Calaveras schists showing some­ Angels Camp, a narrow area of meta-andesite is shown. This either a surface flow or a sheet intruded at no great depth ened near the contact, and were kneaded like what more crystallinity than usual. The contact between the is merely the eastern edge of the Bear Mountain mass and a beneath the then existing surface. putty. Ragged fragments and wisps of the schists meta-diorite and the Calaveras schists is well exposed in the spillway at the head of the Blue Lakes Water Company's pipe portion of the broad belt west of Jackson already described. The third area which has been separately mapped as meta- are included in the intrusive rock, and the intru­ It is largely a breccia, made up of fragments of porphyritic diabase lies in the extreme southern corner of the area map­ line, one mile southeast of Jackson Butte. The intrusive meta-andesite. ped, in Hunter Valley. This is a fine-grained green rock, sion was accompanied, at least to some extent, by nature of the contact is here beautifully shown. The meta- Between the Stanislaus River and Table Mountain, several which the microscope shows to be an altered diabase. It was a fusion and assimilation of the invaded schists diorite is a rather hornblendic facies, and locally carries red lenticular areas of meta-andesitic breccia and tuff occur in the probably a lava flow. garnets. The siliceous schists of the Calaveras are not con­ Mariposa slates. They are moi'e numerous than shown, as by the molten magma. Contact phenomena may spicuously metamorphosed at the contact. The microscope many of them are mere narrow streaks, too small to map. e. -^^ name granodiorite has been also be well studied on Rancheria Creek, 2 miles shows the schists to consist chiefly of quartz in very irregular interlocking grains, small pink garnets, a little mica, and an The large lens crossing Mormon Creek is composed in part of used in the earlier Gold Belt folios to designate east of Drytown. rather coarse breccia, and in part of finer tuffs. There is also undetermined mineral of high refractive index. Just north of a massive porphyritic facies exposed where the creek cuts a granular igneous rock mineralogically and chem­ Most large bodies of intrusive igneous rocks Mokelumne Hill and on the west slope of French Hill occurs through the lens, which occurs in what are apparently thin ically intermediate between the granites and the are composed of rocks of more than one kind. a fine-grained green meta-diorite which is apparently merely contemporaneous flows interbedded with the tuffs. This rock a hornblendic facies of the coarser meta-diorite of the area. is full of chlorite and other secondary minerals, but appears diorites, and as later defined, very close to quartz- There is usually a facies or variety The considerable area of meta-diorite east and southeast of to have consisted of labradorite-feldspar and augite, with monzonite. Rocks . of this general character which makes up the bulk of the mass San Andreas is interesting in that at one point erosion has spared a small mass of the Calaveras schists which probably probably some glass, and may be a meta-basalt rather than a occupy wide areas in the higher portions of the and constitutes its central portion. But meta-andesite. The second considerable area, on Bear Creek, at one time covered the intrusive body. A small lens of lime­ is composed chiefly of tuff and breccia, often alternating with range, but they are not characteristic of the associated with this facies, particularly around the stone within the patch of schist has been metamorphosed to thin streaks of clay slate. The mass contains also some altered Mother Lode district, as will be presently .shown. edges of an area of eruptive rock, there often marble. andesitic or basaltic (meta-andesites or meta-basalts) dikes, East of Angels Camp, and about Parrott Ferry most of the which also cut the adjoining Mariposa slates. The name has been retained, however, for the area occur, in minor amounts, rocks which are very dif­ meta-diorite is of the usual character, with frequent highly which just enters the district about 2 miles north­ ferent from the main type of the mass and yet hornblendic facies occurring in patches, especially about its perimeter. In the canyon of the Stanislaus at and above The largest area of meta-andesitic rocks in east of Algerine, because it is directly connected which really belong to it grade into it in fact. Parrott Ferry there occurs a facies so little altered that it may the district extends from the vicinity of Chinese with the main intrusive masses of the higher Such variations of the principal type are com­ be termed a diorite (quartz-hornblende-mica-diorite). This is Camp across the Tuolumne River, and thence south­ sierra. As only a small peripheral poriion of the a medium-grained gray rock showing abundant crystals of monly called peripheral facies. The great masses hornblende and biotite, and rather fresh triclinic feldspar. eastward, constituting a region of much rugged- mass appears in the present map, it was deemed of granodiorite to the eastward present many such The microscope shows that the rock consists of fresh labrador­ ness, in which Moccasin Peak, the Penon Blanco best not to revise, on so slender a basis, the name local variations ranging from granite to gabbro. ite-feldspar, quartz, hornblende, and biotite, with accessory muscovite, apatite, zircon, and iron ores. The structure is Ridge, and the Horseshoe Bend and Hunter by which the mass as a whole has been indicated The smaller, and usually isolated masses of the the usual one found in granitic rocks. In spite of the general Valley mountains are the dominant features of in the Sonora folio. As indicated in that folio this meta-diorite which occur in the Mother Lode freshness of the rock, epidote is a conspicuous constituent, topographic relief. Indurated breccias of the whole mass is exceedingly variable in character often cleanly and sharply intergrown with the primary miner­ region may perhaps be regarded as somewhat als, and it might be regarded by some petrographers as a character already described occur with monoto­ and passes into dioritic and even gabbroitic facies. basic facies of the great granodioritic intrusion. primary constituent of the rock. nous persistency throughout the area. Almost Quartz-diorite. The village of Big Oak Flat But they are in themselves by no means perfectly A mile and a half, south of Parrott Ferry an augitic facies occurs which may be termed meta-augite-diorite. Near the equally abundant are finer indurated tuffs, with lies near the middle of a basin-like hollow floored uniform or homogeneous. Each mass has its local southern border of the area the normal meta-diorite passes frequent rough schistosity, and sometimes distinct with granitic rock and surrounded by B. Oak plat variations from the type of rock which may be into dark, coarse-grained, hornblendic facies a very common bedding. hills of Calaveras schists. This granite- area- regarded as characteristic of the mass as a whole. modification in this and in the other areas of the meta-diorite. West of Penon Blanco, near the contact with the Mariposa like rock has not undergone deep-seated alteration Thus the meta-diorite frequently becomes very The Algerine area of meta-diorite is petro- slates, the meta-andesite contains irregular grains of quartz, to anything like the extent of the originally simi­ rich in hornblende near contacts, so much so that graphically interesting on account of the very visible with the unaided eye. West of Coulterville, on the road to Horseshoe Bend, a small lens of Mariposa slate is lar rocks described under the next heading as the rock sometimes becomes almost a pure horn- coarse hornblendic facies which abound shown within the meta-andesite area. The exposures along meta-diorites. It is a somewhat altered quartz- blendite. Disregarding, however, for the present near Algerine, particularly just south b£aild?chorn= this portion of the road afford good examples of the close diorite, consisting of feldspar, quartz, hornblende, such minor facies, the meta-diorite areas . of the village. The rock at this point relationship frequently observed between tuffs and clay slates. are all composed7 of very similar rock. Mainmeta-<"orite. type of The two rocks are so intimately interbedded hi the neighbor­ and biotite. It is traversed by numerous quartz contains crystals of dark-green hornblende up to hood of this slate lens that neither rock can be mapped sepa­ veins, and its upper portion, which is weathered This is an evenly granular rock /of moderate 2 inches in length lying in a white, finely granular rately. to a crumbling mass 30 or 40 feet deep, was coarseness, which is popularly ternied " granite." matrix made up of nearly colorless epidote with Fine, even-bedded tuffs which pass on the west formerly extensively hydraulicked for gold. Such The unaided eye usually recognizes hornblende, a little quartz and muscovite. This aggregate has into banded cherty rocks of chocolate, red, white, rotten rock in place has been called saprolite. feldspar, and sometimes quartz as the most promi­ probably resulted from the complete alteration of

and °green colors occur on the southwest cFine. tuffs, Quartz-diorite also occurs at Parrott Ferry, but nent constituents. The rocks are thus, strictly the original feldspars. In the creek bed, half a slope of Hunter Valley Mountains. andcherts- as it is here merely a small part of a larger area speaking, meta-quartz-diorites. The feldspar has mile northeast of Algerine, fragments of a similar These cherts are mapped separately. Similar of more altered rock, meta-diorite, it is described almost invariably the dull-white appearance which coarse hornblendic facies can be seen included in cherts occur about 5 miles west of Coulterville. under the next head. denotes alteration. Biotite, or black mica, some­ the normal medium-grained meta-diorite, showing Several small areas of meta-andesite breccias Meta-diorite (chiefly altered quartz-diorites). times occurs, but is often lacking, and is always that" the coarse basic facies was an early and local and tuffs, including also a little massive nieta- The meta-diorites of the Mother Lode district are subordinate in amount to the hornblende. Exam- crystallization from tjie main molten magma. The large area of meta-diorite lying between Big Oak Flat cases apparently intrusive into the rather basic NEOCENE PERIOD. such areas is determined by the probabilities of and Coulterville is usually a rather dark rock of moderate meta-diorites with which it is in contact, while in the case as deduced from general position, eleva­ coarseness showing abundant hornblende, dull white feld­ spars, quartz, and usually chlorite and epidote. The feldspars other cases dike-like masses of similar meta-diorite Auriferous river gravels. The oldest Neocene tion above the present stream beds, and the char­ are generally completely altered to an aggregate of epidote traverse the serpentine. The conclusion is that gravels are those which lay in the stream-ways acter of the gravel itself. (or zoisite) and white mica. Apatite is sometimes an abundant accessory mineral. the meta-diorite areas do not represent wholly before the outbreak of the Neocene rtOrigin, , of the The material included in the Neocene gravels Three miles a little south of west from Jackson is an area of simultaneous intrusions, and that they and the volcanic eruptions. Such gravels rep- gfavels- varies considerably in the size of the separate par­ intrusive rock which probably belongs with the meta-diorites. serpentine belong to the same general intrusive resent the material which had accumulated along ticles. In the gravel proper the peb- Materialsof It is composed of a grayish-green granular rock of medium grain which the microscope shows to Decomposed, period. the stream beds in the course of the long erosion bles may range from diameters of a the «ravels' bei apparentlyj.i a much1,1.1. altered j diorite.j- -j. Themi feld-f u areas,rather basic Soda-syenite-granophyre and related rocks. of Cretaceous, Eocene, and earliest Neocene times. fraction of an inch up to a foot or more. As a spars are clouded and decomposed, and calcite, chlorite, and epidote are abundant. The exact original char­ These rocks occur as dikes, of which the most During that period a high, rugged mountain rule, large and small pebbles occur in the same acter of the rock is therefore somewhat in doubt. remarkable lies «liust east of Moccasin Moccasin,_ range had been worn down to a range of compar­ bed, not infrequently mixed with sandy or clayey Another area of somewhat similar kind occurs west of the Creek. This dike has a length of about Creekdike- atively low relief, with a broad, gently graded material. These gravels are sometimes irregularly Rawhide serpentine. The rock composing this area is gener­ ally considerably decomposed. It varies from gray to green 6 miles and a width varying from a few feet up western slope. Thousands of feet of rock had interbedded with light-gray or yellowish quartzose in color and is usually fine-grained and traversed by veinlets to about 200 yards. It is easily visible from been disintegrated, reduced to sand and silt, and sandstones, or impure rhyolitic tuffs. Cross bed­ of epidote. The whole area, however, shows much streakiness and variability, and some coarse hornblendic facies occur, most points along the road up Moccasin Creek, carried down by the streams to fill the depression ding is a characteristic feature of such deposits. recalling the rock already described at Algerine. on account of its conspicuous white outcrop along of the Great Valley. The rock masses which In some localities, where the gravels rest upon Northwest of Horseshoe Bend is a cluster of rounded hills the steep hill-slope. In the main it follows the have been worn down and removed as sediments the upturned edges of the Calaveras formation, covered with grease wood, surrounded on the north, east, and south by the higher and more rugged ridges of meta-andesitic contact between serpentine on the west and were traversed by quartz veins carrying gold. In their lower portion contains large, breccias. The rock which underlies this area of gentler amphibolite-schist on the east, although some part these were the upper portions of veins which imperfectly rounded masses of the sili- topography is chiefly an altered, rather fine-grained diorite a meta-diorite, usually deeply decomposed. The freshest serpentine occurs to the east of the dike near the are being mined to-day, but in part also they were ceous Calaveras schists, often several specimens obtainable show clouded, decomposed feldspars, mouth of Grizzly Gulch. small " pocket veins," containing occasional masses feet in diameter. Such material may have been green hornblende, and sometimes augite, with secondary The variations in the texture and mineralogical of free gold. As the rocks decayed and were deposited during the time of rapidly changing quartz, epidote, chlorite, amphibole, and calcite. The medium-grained rock is often intimately associated with composition of different portions of this long dike washed down into the streams, the gold also found conditions at the opening of the volcanic period ; dark-green fine-grained facies, as well as with coarse horn­ are remarkable. At its northern end it is com­ its way more slowly into the channels. Thus in but it is difficult to conceive exactly the conditions blendic varieties. Sometimes the contacts are sharp, but in general there is an intimate mingling, as if the entire mass posed of a rather coarse granular rock consisting the course of time the gold originally scattered in which must have governed such deposition. Such had solidified from a heterogeneous or streaky magma. The chiefly of albite, quartz, and niuscovite, and may countless veins, of all sizes and degrees of richness, blocks may be well seen in a patch of gravel, meta-diorite is probably intrusive into the meta-andesites, be termed a soda-granite. About a mile farther became concentrated in the beds of the streams. which has been extensively hydraulicked, one but owing to the prevailing decomposition of .both rocks near the contact, and the depth and similarity of the result­ southeast the rock is a soda-syenite-granophyre, Rock fragments, when subjected to constant mile northeast of Fourth Crossing. The gravels ing soils, the line separating the two is only approximately consisting chiefly of albite and segerine with acces­ grinding* and solution in rapidly running water, at this point are 30 to 40 feet in thickness and correct. This rather basic and augitic meta-diorite can be well studied near the mouth of Picture Gallery Gulch, at sory zircon, titanite, and apatite. Still farther do not all behave alike. The softer and more rest upon a rather coarse-grained and massive Horseshoe Bend. southeastward, about a mile south of Priest, nius­ soluble kinds are soon worn to sand or dissolved amphibolite-schist. They are light-colored, the covite and quartz are abundant and the rock is a by the water and carried away. Ordinary pebbles being quartz, acid porphyries, rhyolite, Serpentine. This rock has resulted from the soda-granite-porphyry. It is possible that the weathering is also effective in disintegrating many granitic rocks, and chert. A lens of soft white

alteration of basic gabbros, or of ultra basic rocks dike at this point may be a multiple dike, i. e., of' the pebbles, especially if the gravels are for a rhyolite / tuft',' about 12^ feet in maxi- Rhyolite_. ... tuff... of the peridotite family, which were originally a dike consisting of two or more branches, but time deposited and exposed to the atmosphere. mum thickness, containing fragments of !n sravels- intruded into the rocks which now inclose the the exact relations are obscured by the amount of Finally, pebbles composed of the most resistant silicified wood, is interbedded with the gravels serpentine. In general it occurs in elongated sliding that has taken place on the steep slope. rocks come to far outnumber all other kinds in the above the large subangular blocks of schists. dike-like masses of irregular shape and extremely Smaller dikes of similar soda-syenite-granophyre bed of a long established stream. Thus, as might Similar masses of schist were observed in the varying width. occur north of the Tuolumne River and eastward be expected, the earliest Neocene gravels are com­ gravel areas north of Lower Calaveritas, where posed chiefly of well-rounded pebbles of quartz, they may have been derived directly from the The process of alteration whereby serpentine from Jacksonville. Of the three dikes Dikes near results from basic rocks rich in magnesian silicates here shown on the map the two nearest Jacksonville. compact quartzite, siliceous schists, close-grained underlying bed rock. They can not be regarded is essentially one of hydration. The amount of Jacksonville are composed of nearly white rocks granitic rocks, or hard, dense porphyries derived as of morainal character. water thus chemically combined with the serpen­ with minute faint-blue mottlings. The microscope from small acid dikes. The most extensive development of auriferous tine renders it considerably more bulky than the shows that the chief constituent of both dikes is The deposition of gravels did not cease with gravels within the area is found along the former original igneous rock, and the increase in volume the soda feldspar, albite, and that the blue mot­ the beginning of the rhyolitic eruptions which course of a Neocene river channel Gravels near must be accommodated by squeezing and move­ tlings are due to beautiful little tuft-like clusters opened the period of volcanic activity. which extends across the district in Moakeeiumnlr ment throughout the mass. Thus serpentine of slender crystals of blue amphibole. The rock Beds of rhyolitic tuff often occur in the a northeast-southwest direction, past siliceous gravels, sometimes as small eruptlons- Mokelumne Hill and Golden Gate Hill. In the masses are usuallyJ full of irregularO sur- Internal. , also contains segerine, apatite, zircon, and specks faces along which crushing and move- m°vements. of iron ore. The most easterly of the three lenses, and sometimes as beds of considerable neighborhood of Mokelumne Hill these early sili­ ment have taken place. It is almost impossible dikes is more decomposed than the others and horizontal extent. These relations indicate that ceous gravels are overlain at many points by for regular open fissures to be formed within such darker in color. It consists chiefly of albite and at the beginning of the volcanic period there was rhyolitic and andesitic tuffs. Where covered by a rock, and it is accordingly a most unpromising biotite, the latter in great part decomposed to considerable shifting and rearrangement of the a hard capping of tuff they have been extensively material in which to look for continuous and pay­ chlorite. earlier formed siliceous gravels, due either to an exploited by drifting, and by hydraulicking when ing veins. Other dikes closely related to the syenitic dikes increase in the grade of the streams or an aug­ the absence of a capping, rendered this method The serpentine of the district is fairly constant just described occur at various other points, as, mentation in their volume, or to both causes com­ practicable. On the ridges inclosing Spring in character and the separate areas call for no bined. These causes alone would ultimately have Gulch, Chili Gulch, and Old Woman Gulch an for example,A ' about 14-& miles northwest Other_.. local-. , special petrographic description. The rock is of Penon Blanco Point and about one ities- swept the streams clear of the greater part of upper stratum of the gravels has been cemented usually readily recognized by the miners and mile south of Coulterville, but they are small and their accumulated gravels had they not been into a hard conglomerate stained with oxide of given its proper name. A dark facies with usually considerably altered. Special interest hampered, clogged, and finally buried by the iron. A number of relatively small patches of glistening cleavage faces of bastite is rather com­ attaches to the Moccasin Creek dike, and to those products of the rapidly succeeding volcanic erup­ gravel testifies to the former continuity of the mon and the serpentinization as seen in hand east of Jacksonville, from the fact that they are tions of the late Neocene. Mokelumne Hill gravels with the large area 3 specimens is usually complete. In nearly every locally auriferous. In discussing the geology of the western slope miles northwest of San Andreas known as the serpentine area of any size, however, there occur Miscellaneous dike 'rocks. Dikes, usually of of the Sierra Nevada the Neocene is generally Central Hill area. All these small patches have isolated masses of gabbro and of peridotitic rocks. small size, are fairly abundant in the Mother considered as being brought to a close with the been worked in the past by the hydraulic method In most cases these seem to represent facies of the Lode district. Like the other intrusive rocks, cessation of the great volcanic eruptions. It was and evidently proved remunerative. Good expos­ original basic igneous mass which have escaped however, they are nearly always considerably a period of rapid accumulation, chiefly of volcanic ures of these gravels can be studied on the road complete serpentinization. altered and are not of great petrographical interest. materials, during which the streams, overburdened to Mokelumne Hill, about 4^- miles northwest of The original rocks from which serpentine has Certain of them, from their close association with and choked with the amount of debris thrown San Andreas. Some lenses of rhyolite tuff may been derived were intrusive into the sediments, the gold-quartz veins, have great importance, as into them, were unable to establish stable chan­ be seen inclosed in the gravels left standing by nels. At intervals, however, .there were lulls in the miners. These gravels, in spite of the con­ volcanic rocks,' and amphibolite-schistsA., Character_. , of, will be more fully shown when these latter are of the Bed-rock complex. The intru- intrusions- described. The greater number of the dikes are the volcanic activity, during which some streams siderable area of country that they Weludefined sions seem to have had a very general tendency altered diorite-porphyries, or fine-grained diorites. were able to erode channels of considerable depth formerly covered, were in places laid cnannels- to take place along contacts. This is illustrated The serpentine areas, however, are sometimes cut and in some cases to cut down into the Bed-rock down in channels possessing steep and well- by the large area of serpentine west of Rawhide, by dikes of more basic character, and by nearly complex. With the close of the Neocene began defined banks cut in the rocks of the Bed-rock but, best of all, by the almost continuous line of white dikes. One of the latter, cutting coarse, the process of erosion which has continued with­ complex. Much of the gravel in these smaller intrusions extending from the Tuolumne River rotten gabbro within the serpentine, about half a out general interruption to the present day. A areas contains more fine material than usual and

down past Coulterville. . mile northwest of Penon Blanco Point, was found classification of the auriferous river Classification is often red and rusty in color. The gravel of the of the aurif­ There is usually little or no evidence of contact to consist of a nearly colorless amphibole (tremo- gravels is possible in the light of this erous river large Central Hill area is light colored and of the metamorphism about the peripheries of the ser­ lite), epidote, and albite. The rock is altered, and history. Such gravels as occur beneath gravels- usual siliceous character. It has been extensively pentine masses, the conditions being rarely favor­ all the minerals may be secondary. volcanic accumulations are manifestly of Neocene hydraulicked in several places, and is now being able to its detection. There has usually been age, as that age is understood for the Sierra > exploited by drifting. movement along the contacts subsequent to the THE SUPEBJACENT SERIES. Nevada. Moreover, gravels which, although not Extensive gravel deposits occur north and intrusion, and the rock in the immediate neigh­ This series consists of late Cretaceous, Eocene, at present covered by lava, occur in positions northeast of Angels Camp. These gravels are of borhood is generally decomposed and soil- Neocene, and Pleistocene sediments, together with where they could not have been deposited by the usual siliceous character and are sometimes covered. volcanic rocks of the same periods, lying uncom- streams belonging to the present cycle at some capped by and sometimes interbedded with rhyo­ It does not appear from a study of this district formably upon the Bed-rock complex. In- the earlier period of their history, are also classed as lite tuff. Gravels and rhyolite are in several whether the serpentines preceded or followed the Mother Lode district the marine sediments of the Neocene gravels. Probably in most cases such places overlain by andesitic conglomerates and great dioritic and granitic intrusions Relativea e late Cretaceous and the Eocene are wanting, and gravels were once covered by volcanic materials breccias. About half way between Altaville and which are so prominent a feature of the andemeta!lne the Superjacent series is locally represented by which have been carried away by erosion. Occa­ Dogtown the siliceous gravels pass upward into region lying northeast of the Mother the Neocene auriferous gravels, the Neocene vol­ sionally masses of gravel are met with in such impure andesitic gravels which might perhaps be Lode. The detailed evidence, as far as it goes, is canic rocks, and the Pleistocene gravels and allu­ positions as to make the question of their Neocene as appropriately mapped with the auriferous river rather conflicting. The serpentine is in some vium. or later age a matter of doubt. The mapping of gravels as with the andesites. It was in a shaft Mother Lode.

TmSj""'" '' sunk beneath the rhyolite of Bald Mountain, cene age, and was washed into the nearly inclosed basin in from the andesitic breccias with which they are forms termed globulites. There is always a little half a mile northeast of ^Itaville, that the which it now lies. so closely connected, and as they are rarely if accessory magnetite and apatite present. Three famous Calaveras skull was found in TheCalaveraseaav Andesite. The post-Neocene erosion of the ever auriferous,7 it seems best to include _Relation . of ' chemical analyses of specimens taken from por­ 1866. If there were no doubt of the Mother Lode district has been so extensive and them with the volcanic series. Similar thelSffe?" tions of this lava flow 15 to 20 miles apart, show- authenticity of this and similar discoveries in the general in its character that comparatively few deposits, in which the andesitic mate- ousgraves< it to contain from 56.1 to 59.8 per cent of silica, Neocene auriferous gravels, they would prove that fragments remain of the mantle of clastic andesitic rial was laid down chiefly as sand and gravel, 5. to.6.5 per cent of lime, 2.5 to 3.9 per cent of man had inhabited the earth for a vastly longer rocks which once nearly covered the region. occur northwest of Mpnteziima and at a few other soda, and 3.4 to 5. per cent of potash. period than there is conclusive evidence for in the Thus flat-topped ridges capped with andesitic tuff places in the area. The largest area of latite in the district is that other portions of the world. and breccia are less prominent features of the Massive andesite is known at only five points forming the dark massive cap of Table Mountain. South of the Stanislaus River the auriferous topography than they become a little higher up within the district. It forms the upper portion Other areas occur to the northward in river-gravels are of much less importance as the slope of the range. .The ordinary fragmental of the peakJ: known as Jackson Butte.7 Massive_. . the vicinity of Parrott Ferry. These regards area and former economic value than the andesites of the region may be divided occurs on Golden Gate Hill, and caps andesite- separate, massesJ were atX one time-L- con­ deposits farther north. A well-defined channel is into breccias, conglomerates, and fine andesTtic three smaller hills about 1^- miles northeast of nected and formed part of a single long lava known to exist beneath the greater tuffs. Of these the breccias are per­ Golden Gate Hill. The highest of the latter is stream which poured from some unknown vent part of the massive black lava (latite) Mountain haps the most common and characteristic. They sometimes known as Tunnel Hill. near the crest of the range down to Knights capping rr\1 able-i i -»/rMountain. mi1 nese gravels,i channel. are composed of fragments of very unequal size Jackson Butte owes its conspicuous and pictur­ Ferry on the edge of the Great Valley a distance however, are much older than the dark lava which firmly cemented by a matrix of finer andesitic esque outline to the mass of andesite, 400 to 500 of more than 60 miles. Portions of this flow must caps the mountain. They represent an early particles. The fragments are frequently porphy- feet in thickness,' which forms its sum- Jackson. . Butte_ . . have attained a thickness of about 300 feet, as Neocene stream channel which was buried under ritic, in which case the phenocrysts may be either mit. The rock itself is rather light andesite- shown in the columnar cliffs which constitute a andesitic conglomerate and breccia. During a hornblende, augite, or plagioclase. The micro­ colored, with a slight reddish tint, and shows picturesque feature of portions of Table Moun­ pause in the volcanic eruptions a new stream cut scope shows that augite and hornblende are small porphyritic crystals of dark hornblende. tain. a channel in the andesite and flowed for a por­ usually both present in varying abundance. The Under the microscope it is seen to be a fresh PLEISTOCENE PERIOD. tion of its course above the old buried gravels. groundmass consists of labradorite or andesine hornblende-andesite, consisting of labradorite-f eld- This stream was busy eroding its bed, and did feldspar, augite, bronzite, glass, and the usual spar, bright-brown hornblende, and glass, with a In the Mother Lode district, the Pleistocene, not deposit auriferous gravels. It was this newer accessory magnetite and apatite. It is believed little magnetite and apatite. The groundmass or, more strictly, that portion of the- Pleistocene channel which was found and usurped by the that these breccias were deposited as floods of shows flow structure. The form and isolated which Professor LeConte has proposed to call liquid lava, now congealed and forming the cap thick mud, which poured down from volcanic position of this andesitic mass have suggested that the Sierran epoch, has been essentially a time of of Table Mountain. Thus the fact that the lava vents situated to the eastward, near the crest of it may be a volcanic plug which has been intruded erosive activity. Such deposits as occur are has covered and protected the more ancient gravel- the range. Whether the material issued from the from below through the Calaveras rocks. The merely incidental to the action of the streams in filled stream bed may be looked upon almost in vents as mud, or was thrown out and subsequently latter, however, appear to be undisturbed in the carving and shaping the surface of the land into the light of a curious coincidence. Above this washed down by the floods which often accompany vicinity of the mass, and it is known that gravels its present form. They comprise small areas of deep gravel occur deposits of fine light- p| e=cla and volcanic outbursts, is not known. The latter is and fine tuffs occur beneath at least a portion of alluvium bordering some of the streams or washed colored silts, " pipe-clay " of the miners, fossil leaves- believed to be more probable. the lava. It is probable, therefore, that the ande­ down into valleys from the surrounding hills, as site is a remnant of a thick flow, and merely rests which have been found at many points within the The conglomerates are composed of well- well as patches of gravel left by the streams 1 mountain to be rich in fossil plant leaves. The rounded, waterworn pebbles of andesite, cemented upon the Calaveras rocks. above their present high-water mark as they have study of these plant remains, and of those from by andesitic sand. The pebbles vary in size from Golden Gate Hill consists in the main of sunk their beds deeper into the bed rock. These other similar deposits in the Gold Belt, has coarse sand up to bowlders a foot in diameter. andesitic breccia and conglomerate of the ordinary last, like the Neocene gravels and the gravels of afforded the best evidence yet adduced of the Such conglomerates are old stream gravels which type, capped by a thick mass of horn- QoIdenGa,e the present streams, are usually auriferous. Miocene age of the auriferous gravels which differ from the ordinary auriferous gravels merely blende-andesite similar to that of Jack- Hlil andesite- immediately underlie these silts. In Tuolumne in the nature of their material, which is andesitic son Butte, although somewhat richer in porphy­ GEOLOGICAL STRUCTURE. Table Mountain the silts grade upward without throughout. These conglomerates are usually ritic crystals of hornblende. Although included The problem of the detailed structure of the any stratigraphic break into coarser andesitic found to underlie the breccias, but in a thick with the massive andesite to distinguish it from Bed-rock complex is an exceedingly intricate one, sediments. series they may occur at various horizons interca­ the ordinary clastic andesites, this rock is in part and has never been solved. It is known that the North and east of Montezuma are considerable lated with the breccias. They sometimes contain a breccia of rather peculiar kind. The fragments long belts of slaty and schistose rocks have been areas of gravels, which, although not capped by quartzose pebbles and so grade into the non- fit closely together, and are held by a matrix of folded from nearly horizontal into nearly vertical volcanic material, are probably of Qravelsnear volcanic auriferous gravels which they often the same general character as the fragments. The positions and closely compressed by forces acting Neocene age. They are composed Montezuma- brecciated portions grade into parts which are generally in a northeast-southwest direction. But overlie. The andesitic conglomerates indicate !$! chiefly of pebbles of quartz and chert, with con­ that within the volcanic period there were inter­ perfectly massive, and the phenomenon is probably the internal structure of the belts has yet to siderable light-yellow quartzose sand. As shown vals during which temporary, shifting streams to be explained as a case of partial brecciation be fully unravelled. There is no doubt that in the extensive pits left by hydraulic workings wandered over the desolate lava-covered plain, as during the movement of the lava while still hot prior to their final compression the sediments and they rest upon an uneven erosion surface of the if striving to rediscover their buried channels or and semi-fluid. Some of the hornblende-andesite volcanic rocks of the Bed-rock complex were nearly vertical Mariposa slates. A similar but to establish new ones. The finer andesitic mate­ is also exposed on the south slope of the hill, intricately folded, and not, as formerly supposed, smaller mass of gravels occurs just east of Chinese rial occurs in beds of sand or tuff made up largely underlying the ordinary andesitic breccia and con­ simply tilted as a whole into their present position. Camp. of small, angular particles. Between these beds glomerate. It is probable, although not demon­ Evidence of such folding is found in a careful Mhyolite. Ehyolite does not occur in the and the breccias and conglomerates no hard and strable, that the hornblende-andesite of Golden study of the geological map. Actual minor folds Mother Lode district in the form of massive lava fast line can be drawn. As a rule these finer beds Gate Hill was erupted in place through the may be seen in the excellent exposures of Mari­ flows, but as volcanic ashes or rhyolitic tuffs. occur at the base of the andesitic series, but are underlying rocks. posa rocks along Woods Creek. But such data These tuffs are white or pink and are sometimes not everywhere present. They usually consist in Tunnel Hill, 1^ miles north of Golden Gate have not yet been sufficient to reconstruct the consolidated into a firm though somewhat porous part of andesitic sand consolidated into a porous Hill, is capped by a very light-gray hornblende- original folds in detail and to determine whether rock. The microscope shows them to be com­ crumbling rock. Scattered andesitic pebbles, as andesite which under the microscope resembles a given belt of rock now possessing nionoclinal posed of crystals and crystal fragments of sanidine, well as lenses of andesitic conglomerate, occur in the hornblende-andesite of Jackson Butte and structure was evolved from an original anticline plagioclase, quartz, and biotite in an abundant this sand. Below the sand there is sometimes Golden Gate Hill. The hornblende, however, is or an original syncline, or from a combination of groundmass made up of glass spicules and shreds found a deposit of fine silt, the so-called pipe-clay less abundant and is greenish instead of brown. synclines and anticlines. The lack of character­ of pumice. The groundmass is usually more or of the miners. This material often directly over­ Similar hornblende-andesite caps the two hills istic and persistent stratigraphic horizons, joined less altered by secondary silicification. lies the older auriferous gravels and is noteworthy half a mile east and half a mile northeast of Tun­ with the compression and metamorphism of the The rhyolite tuffs were thrown out' in the first as sometimes containing fossil leaves. Collections nel Hill. Hornblende-andesite of the character beds and with the effects of igneous intrusions, _ , ... volcanic eruptions of the Neocene period. They of these leaves have been made at various points described at these five localities has not Relative age has hitherto baffled efforts to interpret the detailed appear never to have been so wide- Originaldis= and they are considered by paleobotanists as indi­ been recognized in the ordinary frag- structure. A long course of patient observation, spread as the later andesitic breccias tCfrfyolite cating Miocene age. mental andesites of the region. The andesites- with analysis of a high order, can alone solve this and tuffs, but probably were confined date of eruption of the massive andesites, with the problem, which, however, has no immediate eco­ All three kinds of andesitic deposit are well exposed in Table to the broad stream- ways of the early Neocene. Mountain, particularly along its southwestern side. Two miles possible exception of the small mass on the south nomic bearing. The andesites, on the contrary, appear to have a little south of west from Jamestown, the cap of side of Golden Gate Hill, is later than that of the The part played by igneous intrusions in the the mountain is a flow of solid latite about 60 feet Jne Table clastic andesites of the vicinity. completely filled the old valleys and to have in thickness.,-, . , Below-r. i the.LI latitei j-'±. occurs a bedIT ofj. Mountainsection. Bed-rock complex has already been sufficiently buried the greater part of the region beneath their rather coarse Andesitic breccia in which many of Latite. The name latite has been given to a described and need not be repeated. cover. Between Mokelumne Hill and Golden the fragments are subangular. Then come 200 feet of andesitic group of lavas which in petrographical character The structural relation of the Superjacent series conglomerate, sand, and pipe-clay. The conglomerate is most Gate Hill the rhyolite tuffs overlie the earliest abundant near the top, arid the pipe-clay near the bottom, stand between the andesites and the trachytes. to the older complex should also be sufficiently auriferous gravels and are in turn overlain by the but they are interbedded somewhat irregularly, and grade They correspond to the granular intrusive (plu- clear from what has gone before. more or less into each other. The fine silt is here probably andesites. The rhyolitic tuffs were thrown into derived chiefly from andesitic material, as it is interbedded tonic) rocks called monzonites, which occupy a the air as volcanic ashes from volcanoes near the with and passes into undoubted andesitic sands. But in some position between the syenites and diorites. The GENERAL GEOLOGICAL HISTORY OF THE MOTHER crest of the range, washed into the streams, and other localities the so-cajled pipe-clay is probably chiefly of lava capping Table Mountain, although commonly LODE DISTRICT. rhyolitic origin. The andesitic beds at the point described deposited by them over the earlier quartzose rest upon the edges of the Mariposa slates, but it is known called a basalt, belongs to this group. It is The oldest rocks known in the Mother Lode stream gravels. The streams of that time prob­ that gravels underlie the pipe-clay in many portions of the characterized by abundant porphyritic crystals region are those comprising the Calaveras forma­ mountain. (phenocrysts) of labradorite-f eldspar lying in a ably flowed slowly, in broad valleys, and may tion. They > are,' in *-partf at least,' of Carbonifer-_ . have been choked by the volcanic debris and Most of the areas of clastic andesite call for no dark-gray fine-grained matrix. Vesicular facies Carboniferous age. It is possible, how- oustiine- forced to spread locally into broad, lake-like special comment. North of Altaville much of the occur in the upper portion of the flow, the vesicles ever, that the Calaveras formation as a whole may expansions in which the tuffs were laid down as material mapped as andesite is an andesitic gravel being often partially filled with clear colorless include some rocks older than the Carboniferous, sediments. There is evidence that the tuffs thus or conglomerate which contains a rather large opal (hyalite). Examined under the microscope, but such have not yet been definitely recognized. deposited were considerably eroded before the proportion of foreign pebbles, and might therefore thin sections of the Table Mountain lava show Little is known of the origin of these oldest rocks eruption of the andesites. with almost equal propriety be mapped as phenocrysts of labradorite, augite, and usually a of the district. They must have been derived Neocene gravel. Strictly speaking the andesitic little olivine in a groundmass of lath-shaped from a tions appear to have been unfavorable for luxuri­ face once more into a high mountain range. The long after the low hills, which once confined it, lode. Most of the large mines of the Mother Lode ant marine life. Unlike the Paleozoic sea, it .did grad,e of the streams was thereby increased and have been washed away by rain and streams. This district are upon leads of this character, and their not deposit limestone. The volcanic eruptions of their erosive power augmented. But their cutting erosion has gone so far that ,the old stream-bed veins are really aggregates of many veinlets. After Juratrias time were, however, very considerable. was again interrupted by the andesitic eruptions, now stands above the surrounding country as a a fissure has been formed and filled with ore or In the enormous masses of ancient andesitic and which far exceeded the rhyolitic outbursts in the mountain, while to the north of it the modern gangue, it is usually still a line of weakness, and is basaltic lavas, breccias, and tuffs which are colored volume of the material thrown out. The stream- Stanislaus River has cut a canyon a thousand feet very likely to be again partly opened by renewed upon the geological map as meta-andesites, there ways were filled with andesitic detritus, partly in in depth. movement. This process may take place several has been preserved a record of vigorous and the form of flows of thick volcanic mud, partly as The erosion of Pleistocene time (Sierran epoch) times, and explains the formation of i n i / -i n T 11 a i -i Banded ore. prolonged volcanic activity. water worn andesitic conglomerate and sand, and did not proceed with perfect uniformity. North much of the " banded ore " of the dis­ At the close of the Juratrias occurred the great partly as fine tuff. The Mother Lode region must west and south of Table Mountain there are trict. The action of the ore-bearing solutions is geological revolution whereby the Calaveras and have been part of a great desolate plain of frag- stretches of nearly level country incised by the not limited to the deposition of material within Mariposa rocks were folded by forces Post.jurassic mental andesitic material. Beneath the lava were present streams. Thus Mormon Creek has cut its the walls of the fissures. Rocks are porous, and acting along northeast-southwest lines foldin«- buried the low hills and shallow valleys of the channel below a former surface of near 1500 feet the solutions circulating in the fissures penetrate and gradually raised above sea level until the for­ pre-Neocene topography, above it projected an elevation, remnants of which remain at French to a certain extent the inclosing walls and pro­ mer sea bottom had become a lofty mountain occasional monadnock of hard rock which had Flat, Rawhide, and Tuttletown. Similarly, south duce chemical changes in the country rock. Thus range. This folding was so intense and the folds never been worn down. Over this plain the of the mountain, Jamestown, Quartz, Montezuma, the country rock of a vein is always an so closely compressed that the sediments were trans­ streams, choked and encumbered with volcanic and Chinese Camp are all situated upon a surface altered rock, and may consist almost country00 of formed into slates and schists possessing cleavage debris, flowed in shifting courses, sometimes which would be almost a plain were it not for the wholly of secondary minerals, such as and schistosity generally approximately parallel spreading into lake-like expansions in which finer ravines of Woods and Slate creeks which inter­ calcite, sericite, and quartz. Such altered wall to their original bedding and dipping northeast­ material came to rest, and again gathering into sect it. These surfaces show that at one time rock is often highly impregnated with metallic erly at a high angle. Accompanying and fol­ temporary channels and forming deposits of ande­ during the Pleistocene the streams draining this sulphides, especially pyrite, deposited by the solu­ lowing the folding there were great intrusions sitic gravel. During the period of andesitic erup­ portion of the area were interrupted in their ener­ tions which have soaked into it from the fissure. of igneous rocks granites, granodiorites, diorites, tions, the crest region of the sierra was getic downward cutting (or corrasion) and began When, as is sometimes the case, the altered wall- and gabbros into the folded sedimentary and probably gradually rising, thus increas- to erode laterally with the aid of their tributaries, rock adjacent to the fissure is impregnated or volcanic beds. These were deep seated intrusions, ing the grade of the western slope. producing local peneplains. The cause of this enriched in such a way as to constitute ore, it is originally far beneath the surface, and it is only There were pauses within the general period of' interruption is not certainly known, but it is convenient and customary to include it within the by enormous later erosion that these intrusive andesitic eruptions, during one of which a stream probably connected with some obstacle formerly term " vein." The wall-rock sulphurets in this masses have been exposed to our view and to-day draining the high sierra eroded its channel in, encountered by the Stanislaus and Tuolumne district, however, are usually worthless, and are constitute part of the surface of the sierra slope. and in part through, the andesitic cover. This rivers in the process of corrading their channels never so rich as those found in the vein itself. By these intrusions and through subsequent channel w'as usurped, however, by the long flow to the westward of the Mother Lode district. When worked, they usually afford a bullion con­ erosion the rocks of the Calaveras formation have of lava (latite) the remnants of which now form With the removal of this obstruction, Mormon, taining considerable silver. It is a fact of much

been cut off from the shore line to the east, against Tuolumne Table Mountain. Subsequently this Woods, and Slate creeks again began to cut scientific and economic importanceJ- >that Gold ,. confined... which they were probably laid down. These rocks lava was covered, in the high sierra, by fresh rapidly downward, and carved their present can­ the gold of this district has been chiefly *° veins- are, as far as known, everywhere bounded on the accumulation of clastic andesites. In the Mother yons and ravines. deposited within the walls-of the fissures and has east by the eruptive contact of the granitic rocks. Lode region proper, however, it is the youngest In connection with the topography the fact not migrated to any great extent into the porous It is certain that, at many places, when those lava of which any remnants are preserved. With which is of most importance in an intelligent country rock. intrusions took place, the adjacent Calaveras the cessation of volcanic activity the streams, no comprehension of the conditions governing the Not all portions of a vein are equally produc­ sediments had already been closely folded and longer overladen with debris, were free to begin occurrence of gold is the salient one that the hills tive. Those portions, often of irregular form, cutting down into the tilted lava-mantled plain. which contain ore sufficiently rich to rendered approximately as schistose as they are and valleys of to-day are in no sense due to local Pay shoots. to-day. Moreover the intrusive rocks themselves As the slope of this plain to the southwest was uplift or disturbance, but are the result of the pay for extraction are termed pay are often locally schistose. Thus 'we see that the now considerably increased, the streams cut ener­ very processes of atmospheric etching or carving shoots. In the Mother Lode district these pay geological processes did not act suddenly and getically through the volcanic covering and have which are now slowly transforming the rocks into shoots usually extend to great depths (in the simultaneously over large areas. The elevation of since developed the topography which we see soil, washing the loose materials into the streams, Kennedy mine 2500 feet) without any appreciable the range was probably slow, and the erosive to-day. and carrying them down to the sea. regular or progressive change in tenor. They are forces attacked it at once without waiting for it It is interesting in looking back over the geo­ usually nearly vertical, but may pitch or " rake " to reach its full altitude. The folding and com­ logical history of the region, to note the similarity ECONOMIC GEOLOGY. slightly to the northwest or southeast. The fissures which the veins fill were formed pression of the beds probably occupied very-many in the chief volcanic products of the GOLD. thousand years, and the granitic and other intru­ Paleozoic, the Mesozoic, and the Tertiary of after the post-Jurassic folding of the Bed-rock i rru ',-, (v i products in The gold of the Mother Lode district occurs complex and after the granitic and dio- ' sions may have taken place at any time during volcanoes. 1 he ancient tufts associated ical period. that period. While considered geologically as with the Calaveras rocks are now trans­ both in the gravels of the Superjacent series and ritic intrusions. They have probably fissures- one period of intrusion, considerable time may in formed in great part to amphibolite-schists, but in veins in the Bed-rock complex. As in other continued to be a zone of movement and read­ reality have intervened between the intrusion and these schists grade into less altered tuffs and portions of the Gold Belt, the gravels were the justment ever since their first dislocation, and solidification of different masses as for example breccias of meta-andesitic character, which are first to be exploited and were formerly actively such movements are still in progress. It should the Algerine area of meta-diorite and the Parrott in no noticeable way different from those of worked. At the present day gravel mining has not be forgotten that when the veins now accessi­ Ferry area. Juratrias time. During Mariposa (Juratrias) almost ceased and the available capital and energy ble to us were formed the surface of the country Thus at the close of the Juratrias, the Mother time andesitic tuffs and breccias were piled up are chiefly directed to the development of mines was far different from what it now is. The out­ Lode region was part of a high and rugged moun­ in thick accumulations and they form the meta- located upon the gold-quartz veins. crops of to-day were then several thousand feet tain range, with lofty peaks and narrow V-shaped andesitic areas of to-day. Lastly, the Neocene Gold quartz veins. Internal stresses in the below the surface. canyons. was marked by the eruption of enormous quanti­ earth, due primarily to imperfectly understood The Mother Lode is not a single vein, but is a All through Cretaceous and Eocene time this ties of andesitic tuff and breccia. Were these causes, produce coinpressive strains in the outer remarkable linear system or chain of interrupted Mother Lode. 8 and overlapping veins which traverses the district The larger stringers are frequently banded, as a to expand from a thin seam of gouge (country-rock ground pyrite, galena, and a silvery fine-grained mica (sericite or embraced in the accompanying map from end to result of reopenings and renewed filling of the up by movement along the fissure and reduced to the phys­ margarodite). The last three are not generally abundant. ical condition of clay) into larger lenticular ore bodies consist­ The pyrite often occurs in bunches of rather large crystals, end, and extends beyond its boundaries. vein. Each time such a widening of the fissure ing of many large stringers with crushed and impregnated especially in the massive white hanging-wall quartz, but such Toward the south, veins belonging to Lodea°sys- takes place, a little of the slate wall rock adheres slate. The middle portion of these lenses is generally occu­ large crystals are usually not of much value as ore. Galena ' ° ° tern of veins. pied by a large and solid quartz vein. The ore of the Key­ is regarded as a good indication. The pay is generally near tnis great system extend down past to the portion of the vein already formed, and stone contains quartz, pyrite, free gold, and a white sericitic the walls, particularly near the foot wall, where there is a Benton Mill, Bear Valley, and Mount Bullion to finally forms a narrow, irregular slaty seam in the mica. persistent black gouge derived from the crushed slates.* near the town of Mariposa. Toward the north widened vein. As the richest portions of these The South Spring Hill mine adjoins the Keystone on the The Argonaut mine adjoins the Kennedy on the south, and south, and is mining the main contact vein probably here is on the same vein, which here, as in the Kennedy, outcrops the line of veins running up past Placerville to veins are as a rule near the walls, it is evident of somewhat higher grade than in the Keystone. in meta-andesite ("diabase"), and dips about the Middle American River, and perhaps beyond, that such banded ore is almost invariably of more Some of the quartz shows ribbon structure, and 40° E. Just above the 290-foot level the vein ^^aat contains pyrite, free gold, and a little zinc blende. passes eastward into the Mariposa clay slates, undoubtedly belongs to the same system. It is than average value. The ores of the slates are The banding is partly due to the original deposition of the and its dip rapidly increases to 68° or 70°. At the point where erroneous to suppose, as some do, that a continu­ free milling, and a large percentage of the gold is quartz and pyrite. The proportion of free gold to gold con­ the vein passes into the slates it can be seen that the fissure ous drift might be run along the whole length of usually caught by amalgamation. tained in the sulphurets averages about five to one. was originally opened by a thrust fault of at The ore of the Medean mine, east of the South Spring Hill, least 120 feet throw, the hanging wall having the "lode" without losing sight of the vein. The veins in the amphibolite-schists resemble contains a little chalcopyrite, but this mineral is been thrust upward over the foot wall to at least Medean mine. Although one of the veins may be traced for those in the slates by being in the main complex never abundant in the ores occurring in the that distance. As far as known, this is the only place along miles, yet it will finally die out entirely and be stringer leads. But the country rock Mariposa slates. the Mother Lode where the displacement can be actually seen The Wildman mine lies in the northeast corner of Sutter and measured. The meta-andesite becomes an amphibolite- succeeded by an interval where no noticeable veins is usually much more altered, and may amphiboiite- Creek on a vein whose general course is about N. 19° W., true. schist near the contact with the clay slates. occur; or else its place will be taken by a second 1,be neavilyt, -1 impregnatedx J with-J.T, pyrite / ^hists. The ore-body as a whole is in the black Mariposa The Argonaut vein is on the whole a remarkably regular slates, but is associated with some amphibolite. and continuous body of quartz which in the lower levels is perhaps overlapping vein lying some distance to near the vein. It is often changed to a soft gray­ It is divided by the miners into three distinct from 8 to 14 feet in thickness. Much of it, especially on the the east or west of it. In some cases there may be ish rock consisting chiefly of carbonates of lime portions, viz., the "granite bowlder," the "main lead," and hanging-wall side, is a firm, white quartz showing little several approximately parallel veins a mile or and magnesia, with sericite, and sometimes a little the "magpie." The main lead is a well-defined vein of solid sulphurets. Some of this unpromising-looking material is quartz, with some banding near the walls. These banded said to yield as much as $16 per ton, mostly in free gold. The more apart, as near Angels Camp or Coulterville. chlorite. Such altered and pyritized country rock portions are generally richer than the rest of the vein. Fre­ richest ore is a firm, banded quartz containing gold, pyrite, The usual tendency of the miner or engineer who is too poor in gold to pay for working alone, but quent irregularities are shown and the vein sometimes passes galena, and zinc blende, the last two in small amount and into a stringer lead, or locally pinches. The '' granite bowlder " lying next to the foot wall. There is no sharp separation, wishes to represent the Mother Lode system in is often run through the mills for the sake of the lies on the foot wall, or western side of the main vein, between however, between this rich ore and the poorer white quartz diagrammatic form is to exaggerate a continuity rich stringers which intersect it, and from which the latter and the foot-wall slates. It is a body of amphibolite- next the hanging wall. in itself sufficiently remarkable and draw imagi­ it can not be economically separated. These veins schist which is traversed in all directions by stringers of quartz and is itself locally impregnated with sulphurets. Explicit In the vicinity of Jackson, the veins of the nary connections and " branches," where there is are usually richer in carbonates than those in the statements as to the value of this portion of the lead were not Mother Lode system are divisible into two nearly no evidence for their existence. black slate areas, and in certain parts of the dis­ obtainable; but it has been extensively stoped, and is un­ parallel secondary or subordinate svs- , doubtedly a valuable portion of the ore-bearing ground, « > t f <> Veins near The veins of the Mother Lode system are not trict are rich in tellurides. With the exception although varying much in tenor. The "magpie "is a some­ terns. A western line, comprising the Jackson- confined in their occurrence to rocks of any of these last, they are all free-milling ores. what similar mass of stringers and country rock, lying on the Kennedy, Argonaut, Alma, Anita, Amador Queen particular kind or of any particular Veins occurring in or alongside serpentine areas hanging-wall side of the main vein. The impregnated coun­ try rock in this case, however, is black slate. This portion of No. 2, Mammoth, and others, follows the narrow age. Nevertheless a study of the Mt&lfLoae are characterized by conspicuous outcrops of mas­ the lead is of little value, running only about 50 cents per ton. strip of Mariposa slates already described as pass­ i -i -i i i , -i . system due sive white quartz. These quartz masses, Veinsin The ore of the Wildman is generally low grade, and is said to whole district veryj stronglyo j suggestsoo toweak a belt rocks. of ing west of Jackson and extending down through however, constitute but a part of the serPent|ne- cost less than $2 to mine and mill. Some rich ore occurs, that the position of the Mother Lode usually in streaks scattered through the ore body, with no Murphy Gulch to the Mokelumne, which it is determined by the general character of the whole vein, which may be 200 or 300 feet in regular arrangement so far as known. The vein minerals crosses at Rich Gulch. A second line, locally rocks which its veins traverse. Briefly, the veins width, and usually consists of a more or less observed were quartz, pyrite, free gold, and small quantities of chalcopyrite, zinc blende, and galena. Some streaks of called " the east vein of the Mother Lode," is that of this system are confined to a belt of weak schistose aggregate of dolomite, mariposite (a sericitic mica occur occasionally in the quartz. upon which the Jackson Gate, Zeila, and Amador rocks. This structural weakness is due to a com­ bright green chromium mica), and talc. The In the Mahoney mine, which adjoins the Wildman on the Queen No. 1 are located. This system of veins quartz occurs as intersecting stringers or great, northwest, the ore body is a stringer lead in black slate with bination of at least three main causes: (1) The a foot wall of amphibolite, or altered meta-andes­ is confined to the vicinity of the contact between prevailing slaty cleavage or schistosity of the thick lenticular bunches in this dolomitic mass, ite tuff. This latter is probably continuous with a narrow strip of slaty Calaveras schists on the rocks, itself a manifestation of strain; (2) their which is without much doubt an alteration the arnphibolite-schist of the "granite bowlder" in the Wildman, and broadens out into the considerable area west and the main belt of amphibolite-schist on occurrence in narrow belts and in small masses, product of the serpentine adjacent to the original of greenstone and greenstone tuff (meta-andesite) forming the the east. Passing just west of Jackson Gate, it giving many contacts, which are seldom if ever as fissure. No paying mine has yet been worked on hill just northwest of the town. The ore of the Mahoney is runs east of Jackson, just west of Scottsville, and a vein wholly within the serpentine, although very similar to that of the Wildman. strong as homogeneous rock; and (3) the great Northward from the Mahoney the lode appears to divide, down Hunt Gulch to the Mokelumne River, length of this individually and collectively weak rich ore bodies occur in contact veins of Avhich the Lincoln mine, now being reopened, being Lincoln which it crosses near the bridge. There is abso­ zone of rock masses and its general parallelism the serpentine forms one wall. Even within the located upon the western and apparently more mine. continuous branch. lutely no trace of any direct connection between with the structural features of the range. It serpentine areas it will usually be found that the South of Sutter Creek lies Hayward's old Eureka mine, once these two sets of veins by "spurs" or branches, appears highly probable that much of the move­ large veins are accompanied by one or more the deepest mine in the State (2200 feet) and famed for the and they are an excellent example of the essen­ great quantity of high grade ore that was decomposed dikes. Otherwise probably no open j. j. j £ -j. mi, -j-u J.T, Eureka mine. ment which has affected the Sierra Nevada since extracted from it. This mine, with the associ­ tially compound character of the Mother Lode the close of Jurassic time has been localized fissure would have formed, or remained open, for ated claims constituting the Amador Consolidated, has been system as a whole. along this weak zone, and has resulted in the a time sufficient for the deposition of a quartz idle many years. The mines are in Mariposa slates, but these contain many thin lenses of meta-andesite tuff (greenstone). The Zeila mine, which has been worked nearly continuously linear fissure system of the Mother Lode. vein. Some of the larger ones are shown on the map, but many are for thirty years, lies about half a mile southeast of Jackson The veins of the Mother Lode, without any Contact veins (i. e. veins occurring between too small to indicate. on the eastern line of the Mother Lode veins. rocks of different kinds and origins) are an The general pitch or rake of the ore shoots in the Sutter The vein is essentially a stringer lead in amphibo­ known exception, dip northeastward, generally at Creek district is said to be to the south. lite-schist, and has a general dip of 50° to 60° E. It is important feature along the Mother Contact a flatter anglec> than the . inclosing slaty Character , of, The South Eureka mine, a little more than a mile south of separated from the black slates (Calaveras formation) of the rocks. They usually strike nearly with faultins- Lode, and are often thick plates of veins> Sutter Creek, lies in Mariposa slates and is on the northern foot wall by a heavy gouge. The width of the vein where extension of the Oneida, Kennedy, and Argonaut stoped is 40 to 50 feet, but the general average is somewhat the inclosing slates or schists; but there are quartz, with bold outcrops. Such massive veins lead. There are no croppings at this point, the Eureka less. A fine-grained, altered dike, from 4 to 6 feet wide, prob­ exceptions to this general rule. The dislocations are usually of low grade, and it is a very common slates of the Bed-rock complex being covered by ably originally a diorite, accompanies the vein. The ore is thing to find that mines originally located upon Neocene andesite. The mine was located by means of sur­ low grade less than $4 per ton. Pyrite is the principal by which the fissures were originally opened, veys, and through certain shrewd deductions drawn from the sulphide, but there is sometimes a little molybdenite, and were of the kind known as thrust faults, i. e. the these prominent croppings are really working a record of mines north and south of it. It is an example of small quantities of galena and zinc blende are said to occur, hanging wall moved up relatively to the foot wall. smaller vein, or a stringer lead, on one side or the bold yet legitimate and successful prospecting. The strike of and to indicate good ore. Calcite is quite abundant, both as the vein is N. 18° W. true, and the dip easterly, with the slates. stringers and crystallized with the quartz. The present structure of the veins shows that the other of the large " bull " vein as it is sometimes For a short distance north of the shaft, however, the vein has South of the Mokelumne Kiver, the Gwin mine is situated original displacement was followed at intervals called by the miners. Small dikes also play an a strike of N. 3° W. true, and locally cuts across the more in a belt of Mariposa slates which is directly continuous with important part in some of the ore deposits, as will westerly strike of the slaty cleavage. The real fissure vein in the_, slates~ . of the , Kennedy.. *, and,, Argonaut, ° , mines.. Gwin_ , mine. by further movement of the same kind. There this portion is about 10 inches wide, but the slates are impreg­ The Gwin vein strikes with the slates and is gen­ has very probably been subordinate displace­ be more fully shown in the notes on the principal nated with pyrite and filled with stringers for a width of 4 erally parallel with their cleavage. It is accompanied by ment of reverse character, i. e. a downward move­ mines which follow. These notes are necessarily feet. Occasional thin lenses of amphibolite-schist or meta- stringers, and varies in width, but on the whole is entitled to brief. andesite tuff (greenstone) are met with in cross cutting the be called a simple vein rather than a stringer lead. The vein ment of the hanging wall relative to the foot black slates. The pay ore is usually found near the foot wall, minerals are quartz, pyrite, arsenopyrite, free gold, and a little wall, producing local crushing of earlier-formed The Empire, Pacific (Plymouth Consolidated), Lucile, and the hanging wall being frequently a large body of nearly chalcopyrite, galena, zinc blende, albite, sericite, and calcite. veins, and resulting in ore bodies of irregular and Pioneer mines all lie in the broad belt of Mariposa slate south barren (50 cents to $1 per ton) solid white quartz. The ore is Galena and zinc blende, when present in small amount, are of the town of Plymouth. Their ore is of the generally high grade (by this is usually meant ore running considered to indicate good ore. The arsenopyrite occurs in brecciated character. There is evidence that this usual character found in the black slates, and Empire, over $10 per ton). The component minerals are quartz, both large and small crystals. The former are particularly latter movement is still in progress, producing shows, some banded, , , quartz,, containing, . . pyrite,., Pacific,Lucile, and pyrite, arsenopyrite, gold, with a little galena and zinc blende. prized, as they inclose beautiful arborescent masses of crystal­ gold, calcite, and a fine-leaved, pearly- white These last are regarded as indicating good ore. lized gold. * All the free gold in the ore is coarse, and is easily the gouges and slickensided surfaces which mica, which is a variety of muscovite (sericite or The Oneida vein lies in Mariposa slate, near the contact caught on the plates after passing a No. 16 screen on the accompany most of the veins. "With the possible margarodite). These mines were closed in 1898, and their with the meta-andesite breccia on the west. The general mortars; the tailings are said to average only 12 cents per ton. workings were not accessible. The first two were formerly strike of the vein is about N. 27° W., true. The If true, this indicates a saving of about 98 per cent of the gold, exception of serpentine there is no indi­ Oneida mine. large producers. average dip is about 62° E. This mine was then which is a remarkably high average. Common pyrite is by cation that the character of the wall Influenceofwall rock on A number of claims have been located on Dry Creek, north closed after being worked to a depth of 1350 feet on the incline. far the most abundant sulphide in the ore. The average tenor. rock has any direct influence upon the of New Chicago, near the northern termination of the Bunker It is now being reopened by a vertical shaft sunk to the east­ value of the ore is said to run from $5 to $7 per ton. The Hill area of meta-andesite ("diabase"). The Cosmopolitan ward of the old shafts. At the 1500-foot level in this new shaft, slates near the vein are impregnated with sulphurets, but richness or poverty of the vein. Neverthless, the has been extensively worked, and is said to have a curved a crosscut of 250 feet has been run west to the vein, which here these sulphurets are not so rich as those in the vein and are veins traversing different rocks possess certain vein following the contact with the meta-andesite. There are shows banded ore consisting of quartz, pyrite, and free gold. said to contain a larger proportion of silver. A conglomerate distinctive features which will be briefly outlined. not at present any producing mines in this vicinity. Adjoining the Oneida claim on the south is the Kennedy, similar to many such beds occurring in the Mariposa forma­ Between Dry Creek and Amador, the principal vein of the one of the most important mines in the district. This mine tion, lies on the foot-wall side of the vein, and is said to carry The veins occurring in the dark clay slates of Mother Lode system follows in a general way the contact has been a steady producer of high-grade ore for gold up to 50 cents per ton, as shown by several assays. The the Mariposa formation or the slaty schists of the between the Bunker Hill area of meta-andesite many years and is now the deepest mine on the m'ine1.edy ore shoot in the Gwin appears to be pyramidal in form and and the principal belt of Mariposa slates lying Amador, Mother Lode (2250 feet). The Kennedy vein out­ to be nearly vertical as far as exploited. The present depth Calaveras formation are usually com­ to... the west., Several~ , important. , , mines, such, as andBunker May- Hill, crops in meta-andesite ("diabase") but soon passes into a of the mine is 1400 feet.f plex stringer leads, without conspicuous Sne'ntar'y the original Amador, the Bunker Hill, and the narrow belt of black Mariposa slates, within which the dip of surface outcrops. The vein material is Mayflower, were formerly extensively worked, the vein becomes much steeper. The vein is usually strong In the neighborhood of San Andreas the origi­ but are now idle. and well defined but varies considerably and is sometimes a chiefly quartz, with a little calcite or dolomite, The Keystone mine in Amador has been worked almost stringer lead. Near the south end of the 2100-foot level the nal fissuring, which elsewhere resulted in a more and sometimes nests of albite. The metallic com­ continuously from the early fifties, and is said to have pro­ vein consists of from 4 to 5 feet of banded quartz, with a heavy or less linear and parallel arrangement of the duced from $12,000,000 to $13,000,000. It is lenticular mass of solid white quartz on the hanging-wall side. pother Lode veins, appears to have been irregular ponents of the ore are pyrite and free gold, with located on a contact vein of massive white The banded quartz is worth about $20 per ton but the white small amounts of galena, zinc blende, arsenopy- quartz, from 12 to 200 feet in thickness, lying quartz is very poor. The sulphurets in the banded quartz rite, and chalcopyrite. The ore body is generally between meta-andesite ("diabase") on the east and Mariposa are most abundant in and near the thin slaty seams which * Since this was written a new vertical shaft has been started slates on the west. The latter are here unusually narrow. produced the banded or ribbon effect. The vein follows in 1950 feet east of the vein, designed to cut the latter at a depth a complex mass of stringers of varying sizes, This vein carries from a few cents up to $7 per ton, but is not the main the eastern contact of the clay slates with the meta- of 3500 feet. mingled with slate, and separated by heavy at present worked. The ore bodies now being mined are a andesite ( "greenstone" or "diabase" of the miners), but does t Since the Gwin mine was visited the 40-stamp mill has series of parallel stringer leads lying in the foot-wall slates, not strictly conform to it at all points. been increased to .one of 80 stamps, and other extensive gouges of crushed slate from the country rock, and dipping eastward into the main contact vein at angles of The pay shoots in the Kennedy are considered to pitch improvements have been made. The shaft also has been sunk which usually shows little chemical alteration. about 47°. When carefully followed up, these leads are found northward. The ore consists of quartz, pyrite, gold, arseno- to about 1800 feet in depth. - , ' 9 in character. Veins were formed, but they are general character of its vein is very similar to that of the Utica Alameda and the Rawhide mines, the fissure apparently resembles the latter in the general character and position of neither continuous for a long distance nor strik­ and Stickles mines. In 1897 it was simply- a traversed the serpentine, and no vein has yet been found. its ore body. There is ^a similar relation of prospect 200 feet in depth, but in 1898 it was ]n1jn£ightner At the Rawhide mine the main vein practically follows the stringers of quartz and calcite, black slates, and nl\m amphibolite-schist and Calaveras rocks between Amador Silver 9,357 13,895 8,008 5,230 280 1,089 3,767 3,477 courses of these veins usually differ from the gen­ Priest and the Tuolumne River. The gold occurs Total 1,469,309 1,409,857 1,218,391 1,511,204 1,332,196 1,393,018 1,527,118 1,327,950 eral trend of those composing the Mother Lode, usually in thin leaves, often showing triangular Gold 618,821 738,884 794,531 1,669,193 2,119,365 1,717,916 1,546,398 1,439,861 Silver 2,499 4,861 24,441* 122 5,182 77 500 1,745 and in some cases they constitute separate and octahedral faces on the surfaces of the flat plates. Calaveras Total 621,320 743,745 818,972 1,669,315 2,124,547 1,717,993 1,546,898 1,441,606 local systems of fissures. They may or may not The largest amount taken out of any one pocket along this contact is said to have amounted to Gold 1,500,629 1,384,950139' 1,092,549 354,734 547,448 666,754 1,070,142 1,809,572 have been formed simultaneously with the long Tuolumne Silver 13,062 911 1,329 1,072 312 328 1,696 system of Mother Lode fissures, but it is probable $2500. There has also been considerable pocket Total 1,513,691 1,385,089 1,093,460 356,063 548,520 667,066 1,070,470 1,811,268 that their genesis was not separated from that of mining in the amphibolite-schists south and east Gold 124,265 84,414 . 81,012 164,116 153,707 216,622 335,637 451,427 the latter by any very great interval of time. of Coulterville, in the meta-andesite east of Horse­ Mariposa Silver 22 67 307 88 7 180 660 About 2 miles northeast of Amador a conspicu­ shoe Bend, and in Hunter Valley. Total 124,287 84,414 81,079 164,423 153,745 216,629 335,817 452,087 ous vein outcrops on the southern slope of Quartz A few generalizations, based upon a Totals for district 3,728,607 3,623,105 3,210,902 3,701,005 4,159,008 3,994,706 4,480,303 5,032,911 Mountain (not the Quartz Mountain in Tuolumne study of the Mother Lode veins of the tff^p'pn" * . cable to the * $22,000 from Satellite copper mine. Campo Seco. County) with so flat a dip (25°) that its hanging area, and not necessarily applying to JjSfflS Lode t The silver is quoted for this year at its commercial value. The figures for previous years are coining values. wall is largely eroded away and the vein forms other areas, may be formulated as fol­ rides, the universal practice is to employ rapidly In the case of Amador and Calaveras counties the northern slope of a spur of the mountain. lows : dropping, moderately heavy stamps (850-1000 the production of precious metals, as given in This heavy mass of quartz, 10 to 15 feet thick, 1. The ore bodies of the Mother Lode occur, Ibs.). The pulp passes through rather coarse the table, probably does not greatly exceed the has been extensively prospected, but it has not almost without exception, in true fissure veins. screens over the amalgamating plates and thence output from such portions of these counties as been profitably worked on a large scale. The 2. Paying veins may be found in any rock of to concentrators. The concentrates are chlorina­ are within the limits of the area mapped. De­ vein is of interest on account of its northeast- the Bed-rock complex with the possible exception ted. The cyanide process is occasionally used on ductions must be made, however, for a few southwest strike and low northwesterly dip. of serpentine. The most favorable rocks for the tailings. About 3 or 4 tons of ore per stamp is mines lying to the eastward of the district, The Baliol and Potazuba mines, about I J- miles formation of large productive ore bodies are those the usual daily capacity of a modern mill. The such as the Sheep Ranch mine, which pro­ northeast of Sutter Creek, are examples of mines possessing cleavage or schistosity. process is simple, and from the conditions in Cali­ duced nearly $131,000 in 1891, and is credited situated on minor fissure systems. The ore-bearing 3. In order to allow the deposition of ore, the fornia there has been evolved a standard method with $95,000 in 1892. The bulk of the prod­ ground of the former consists of quartz stringers country rocks must previously have been fissured of procedure known throughout the gold-mining uct of these two counties for the years given in of various sizes which traverse the country rock in such a way as to produce open spaces. But world. the table has come from the large mines devel­ on the hanging-wall side of a massive vein of bar­ many spaces of moderate size are more favorable Auriferous gravels. The occurrence and dis­ oped along the Mother Lode vein system, from ren quartz 12 to 13 feet in thickness. The veined to ore deposition than a single large one. Thus tribution of the Neocene and Pleistocene gravels Plymouth to the Stanislaus River. The Ken­ and impregnated country rock is a light-gray, the stringer leads and impregnated zones of the have already been described. Their economic nedy mine, which has been a very steady pro­ finely banded schist consisting of quartz, dolomite, fissile and schistose rocks are more uniformly importance at the present day is comparatively ducer for many years, had an output of more sericite, and albite. It is probably an altered productive than the large veins of solid quartz. insignificant. The Neocene gravels of the Mokel­ than $2,025,000 from 1890 to 1893 inclusive, facies of the amphibolite-schist series, and was 4. Very large solid croppings of white quartz umne Hill district were formerly extensively and the Utica group of mines (Utica, Stickle, originally a tuff. Stringers and pyritized country are usually low grade, and likely to be lenticular, worked by hydraulicking and drifting and were Gold Cliff, and Madison) afforded about $2,631,- rock are all sent to the mill, and the ore is low and to pinch out vertically or horizontally. They very rich. No work was in progress in 1898, 000 in the same period. The last-named group, grade usually less than $4 per ton. indicate, however, a strong line of fissuring and although preparations were being made for drift­ which is under one management, ranked in 1894 The Potazuba mine is a prospect in dark Cala- movement, and are often accompanied by smaller ing under the rhyolite near the head of Old as the largest producer of gold bullion in the veras slates. The general strike of the vein is paying ore bodies on the foot or hanging wall. Woman Gulch. Similar work was in progress United States. N. 43° "W., true, and the dip 62° NE. The ore 5. The pay ore is not uniformly distributed at two points in the Central Hill area northwest Tuolumne County is noted for its pocket mines, is unusually base, some of it assaying 'TO ounces through the veins, but is more or less concentrated of San Andreas, and some hydraulicking had some of which have been considerable, although of silver per ton. It consists of a quartz gangue into nearly vertical " shoots " or " chimneys," best recently been done about 2 miles southeast of San erratic producers. The output of the county is carrying pyrite, galena, zinc blende, chalcopyrite, referred to as pay shoots. Andreas. The gravels just southwest of Dog- therefore subject to fluctuations, and it is difficult and a little pyrrhotite. The vein is rich in spots, 6. Veins must somewhere pinch vertically as town were also being worked by the hydraulic to apportion the total yield among the different but small, and is irregular in tenor. well as horizontally. There is no evidence that method. Drift mining under the lava of Tuol­ mines. The large product in 1897 was probably The Esperanza (or Boston) mine, about 1J- any mine on the Mother Lode which has ever umne Table Mountain has been practically aban­ due chiefly to the general activity and develop­ miles northeast of Mokelumne Hill, is on a 60- paid has been really worked out. A productive doned, and never proved a remunerative venture ment of the larger and more permanent mines, foot vein, dipping about 50° E. Altered diorite pay shoot followed down to a pinch will probably on the whole. A little work is at present in such as Rawhide, Shawmut, Jumper, Dutch, and forms both foot and hanging walls. The ore is reappear again at greater depth. The extent to progress near Montezuma, and a few Chinese are at App, along the Mother Lode, and the Golden Gate, low grade. which it may be advisable to prospect for it must work along some of the present river and creek near Sonora. But numerous smaller mines col­ About 2 miles northeast of Jamestown there is be determined by a thorough knowledge and beds, but beyond these minor operations there is lectively make up a considerable part of the total, a sinall group of veins in amphibolite-schist, upon study of the portion of the vein already exploited, no gravel mining going on in the district. The while several important mines, such as the Black one of which the Golden Gate mine is operating. and by the economic conditions of operating. Pleistocene gravels along Moccasin Creek were Oak, lie to the eastward of the Mother Lode This vein dips about 66° E. The gold occurs 7. The gold usually occurs chiefly in the veins formerly extensively worked, the material being district. chiefly in the pyrite, in a gangue of quartz and and stringers, not in the impregnated wall rock. raised by derricks and sluiced. But they are It is impossible to say how much of the com­ calcite. 8. Quartz and pyrite are the only vein miner­ said to have barely paid expenses, and their paratively small output for Mariposa County can An interesting set of veins occurs near Big Oak als always associated with the gold. When the poverty is an indication that there are probably properly be apportioned to the district described Flat. These are most abundant within the quartz- pyrite occurs in large crystals of cubical form it no very productive veins in the vicinity. The in this folio. The development of the Mary Har- diorite ("granite"), but some veins lie Big0akFlat is usually not auriferous. Zinc blende and galena former importance, of the auriferous gravels rison mine, near Coulterville, is probably respon­ at the contact between the dioritic rock veins- in small quantities appear to indicate good ore. diminished to the south in this district, and the sible for the considerably increased production of and the inclosing schists, while others traverse both The free gold may occur embedded in pyrite, region as a whole never possessed such extensive the last three years. arsenopyrite, tellurides, tetrahedrite, quartz, dolo­ areas of rich Neocene gravels as were found far­ rocks. The Mississippi mine, just west of Big Oak QUICKSILVER. Flat, is on a vein of solid milk-white quartz 3 to 4 mite, calcite, talc, or mariposite. There is but one ther north. feet wide. As shown on the map, this is a curved way to determine the value of ore from, a new Cinnabar, or sulphide of mercury, occurs in SILVER. vein, and traverses both the quartz-diorite and the prospect, and that is by assays. Experience is small quantities at Marsh Flat, and in good crys­ Calaveras schists. It dips at angles varying often sufficient to distinguish the signs of rich ore No silver ore is mined within the district, the tals in a quartz vein east of Horseshoe Bend. (See between 25° and 35° NE. Northeast of this vein in a given mine, but the same indications may not appearance of this metal being merely incidental Sonora/?folio). Neither deposit is of economic are numerous small, approximately parallel veins hold in a mine a few miles away. to the output of gold. The amount of silver is in value. 11

MANGANESE. GLOSSARY OF ROCK NAMES. Granite. A granular intrusive rock composed altered andesite. The andesites are the effusive of soda or potash (alkali) feldspar, quartz, mica, equivalents of the diorites. A little psilomelane occurs in a small patch of The. sense in which the names applied to and sometimes hornblende. Granites usually con­ Latite. An effusive rock, usually of porphyritic serpentine about three-quarters of a mile west of igneous rocks have been employed by geologists tain some soda-lime feldspar or plagioclase. A structure, which is mineralogically closely related French Flat. The deposit is not likely to be of has varied, and it is likely to vary as knowledge soda-granite is one in which albite or other soda- to the andesites, but contains more potash and economic importance. increases. The senses in which the names are rich feldspars predominate. soda. The latites are the effusive equivalents of employed in this folio are as follows: Diabase. An intrusive or effusive rock having the granular intrusive rocks called monzonites. LIME. Peridotite. A granular intrusive rock gener­ a mineralogical composition like gabbro, but dif­ The latite of Table Mountain consists essentially Most of the limestone of the area makes a fair ally composed principally of olivine and pyroxene, fering from the latter in structure. The feldspars of labradorite-feldspar, augite, olivine, and glass. lime, and has been burned at several points to but sometimes of olivine alone. are lath-shaped, and the rock is often porphyritic It contains about 56 per cent silica, 6.5 per cent supply local demand. Pyroxenite. A. granular intrusive rock com­ and usually less coarsely and evenly crystalline lime, 2.5 per cent soda, and 4.5 per cent potash. posed chiefly of pyroxene. than gabbro. A meta-diabase is an altered dia­ Hhyolite. An effusive rock, generally nearly QUARTZ. Hornblendite. A granular intrusive rock con­ base in which the pyroxene is often changed into white in color, consisting of alkali feldspars, Large loose crystals of clear quartz, up to sisting chiefly of hornblende. fibrous green amphibole (uralite) and the feld­ quartz, and usually hornblende or biotite, in a 19 inches in length, and suitable for cutting Serpentined- A rock composed of the secondary spars into epidote and other secondary minerals. groundmass which is often glassy. The rhyolites into ornamental objects and lenses, have recently hydrous mineral serpentine. Serpentine is an Diorite-porphyry. A rock possessing the gen­ are the effusive equivalents of the granites. been discovered in the Green Mountains gravel alteration product of rocks of the peridotite and eral chemical and mineralogical character of a I Amphibolite and amphibolite-schist. A massive mine, near Mokelumne Hill. They occur abun­ pyroxenite series, and sometimes contains unal­ diorite, but with porphyritic instead of granular or schistose rock composed principally of green dantly in one of the Neocene gravel channels, tered remnants of pyroxene or olivine. structure. Porphyry is merely a structural term. amphibole, with quartz, feldspar, epidote, chlorite, and were found after drifting 900 feet from the Gabbro. A granular intrusive rock consisting Syenite-porphyry and syenite-granophyre. A calcite, and other minerals, and usually derived surface. of lime-rich feldspars (labradorite or anorthite), rock having the chemical and mineralogical char­ by metamorphic processes from andesites and dia­ pyroxene (more rarely hornblende), and some­ acter of a syenite, but with porphyritic instead bases (often in the form of tuff), diorites, and BUILDING STONE. times olivine. of granular structure. A syenite-granophyre is other igneous rocks. There is practically no quarrying done within Diorite. A granular intrusive rock consisting a syenite-porphyry in which the groundmass is Tuff. A rock composed of small fragments of the district. White marble of fair quality occurs chiefly of lime-soda feldspar (oligoclase, andesine, finely granular. lava (basalt, andesite, rhyolite, etc.) originally in a small patch of Calaveras schists included in or labradorite) with hornblende or pyroxene, and Basalt. An effusive rock containing basic lime- thrown into the air by explosive action of volca­ the meta-diorite northeast of San Andreas, and sometimes biotite. If quartz is present the rock soda feldspars, much pyroxene, generally olivine, noes, or erupted from volcanic vents in the form has been used to a small extent for building in is called a quartz-dwrite. A meta-diorite is a and frequently considerable glass. The silica con­ of mud flows. The finer tuffs are often bedded, the latter town. About 2 miles northeast of partly recrystallized or altered diorite. tent is usually less than 56 per cent. The basalts and may become cemented into a hard rock. Angels Camp a rhyolite tuff has been quarried in Granodiorite. A name used in the Gold Belt are the effusive equivalents of the gabbros. Breccia. A rock composed of angular frag­ the past. It affords an easily worked and fairly folios to designate a granular intrusive rock stand­ Andesite: An effusive rock, usually of porphy­ ments embedded in a finer clastic matrix. The durable stone. An inexhaustible supply of excel­ ing mineralogically and chemically between a gran­ ritic structure, composed essentially of soda-lime breccias of the Mother Lode district are almost lent building stone might be derived from the ite and a quartz-diorite, but closely related to the feldspars (chiefly andesine and labradorite) and all of volcanic materials, and may be regarded as granitic masses just to the eastward of the area, latter, and yet more closely to quartz-monzonite. ferromagnesian silicates (hornblende, pyroxene, or very coarse tuffs. If the fragments are waterworn but there is at present no demand for such mate­ Syenite. A granular intrusive rock composed biotite), in a groundmass of feldspar microlites and rounded the rock becomes a conglomerate. rial. A banded and ornamental serpentine has chiefly of alkali feldspars with amphibole, pyrox­ and magnetite, with some glass. The silica is F. L. RANSOME, been quarried on a small scale 4 miles southwest ene, or mica. A soda-syenite is one in which a ordinarily above 56 per cent. The andesites are Geologist. of Plymouth. soda feldspar is prominent. usually gray in color. A meta-andesite is an April, 1899. Mother Lode.