rTTfiJtri^^M ■ ■ ii 





46 



THE AUBIFEROUS CKAVELS OF THE SIERRA NEVADA. 



mentioned, it occurs in detached portions, all of which, however, are nearly 

 in one line, the direction of which is that of the trend of the formations 

 making up the bed-rock series. In the region between the Merced and the 

 Stanislaus, the Great Quartz Vein is almost continuous; it runs uninter- 

 ruptedly between French Gulch and the main branch of Moccassin Creek, 

 forming by its outcrop in the Pena Blanca a very conspicuous object, and 

 having there a width of 261 feet, measured horizontally across it, its inclina- 

 tion being to the northeast at an angle of about 60°. Beyond Moccassin 

 Creek it disappears for a time, but is seen again below Stevens's Bar, on the 

 Tuolumne, and at numerous points between there and the Stanislaus. This 

 powerful lode is made up of irregularly jjarallel plates of white compact 

 quartz, and crystalline dolomite or magnesite, # more or less mixed with 

 green talc ; and these plates, which somewhat resemble the " combs " of ordi- 

 nary lodes, are either in contact or separated from each other by intercalated 

 layers of talcose slate. The quartz is chiefly developed in the central portion 

 of the vein ; and, from its color and resistance to decomposition, it gives rise 

 to a very conspicuous outcrop, forming the crest of the hills, so that it can be 

 readily seen from a distance of several miles. The dolomitic or magnesitic 

 portion decomposes somewhat readily, and it becomes a kind of "gossan," 

 or a cellular, ferruginous mass, of a dark-brown color, often traversed in every 

 direction by seams of white quartz. The quartz is the auriferous portion of 

 the lode, although it is far from being uniformly impregnated with gold. 

 Most of the mines which have been worked, between the Merced and the 

 Stanislaus, are on the northeast side of the Great Quartz Vein, either in con- 

 tact with it, or in some parallel band of quartz subordinate to, or at a little 

 distance from it. The talcose slate bands in the vein are often, themselves, 

 more or less auriferous. 



Beyond the belt of metamorphic volcanic rock, which lies to the northeast 

 of the band of argillite and serpentine just described as containing the Great 

 Quartz Vein, there is a wide belt of mica slate, which extends to the gran- 

 itic nucleus of the Sierra. This is, in the region between the Merced and 

 the Stanislaus, the second great series of metamorphic schists belonging to 

 the bed-rock system. These mica schists are more or less quartzose on the 

 southwest, and they pass into pure quartzites as we approach the granite, 

 the proportion of argillaceous material in them gradually diminishing in that 



* 



In the only specimen which has thus far been chemically examined, the supposed dolomitic portion 



proves to be an intimate mixture of quartz and magnesite. 



