GEOLOGY OF THE SIEERA NEVADA. 



47 





direction. In some localities, however, the mica schists and quartzites are 

 interlaminated with each other close up to the granite. Along the Merced 

 River this belt of rock is very wide ; but it narrows towards the northwest. 

 East of Sonora there is a large area of granite enclosed in the mica slates, 

 occupying most of the space between the Tuolumne and the South Fork of 

 the Stanislaus. It is this to which the mining district about Soulsbyville 

 belongs. There is a much smaller insulated area of granite at Big Oak 



Flat. 



In this belt of mica schists and quartzites are found interstratified belts of 



metamorphic limestone, or saccharoidal marble, belonging to the limestone 

 series mentioned before. On the Merced River there are two bands of this 

 marble, one of which is almost in contact with the granite ; the other at 

 about three miles from it. Neither of these has been traced more than a 

 few miles to the northwest, and the range of this rock seems to be broken 

 until just before reaching the Tuolumne River. Here, almost in contact with 

 the granitic mass mentioned before as occurring in the vicinity of Soulsby- 

 ville, there is an outcrop of limestone which extends along in a curved line, 

 and before reaching the Stanislaus widens out, so as to form a belt between 

 one and two miles wide where it crosses that river. In the vicinity of 

 Sonora, this rock is cut through by numerous dykes of diorite. 



The region of which the lithological characteristics of the bed-rock series 

 have just been sketched is, in general, quite bare of overlying gravels and 

 volcanic materials. Just at its northwestern border, however, and before 

 reaching the Stanislaus River, we meet with the lava-flow known as the 

 Sonora Table Mountain, to be described farther on. There is also another 

 How which comes down from above, and branches out in various ramifica- 

 tions over the granitic mass mentioned as occurring in the Soulsbyville 

 district. After crossing the Stanislaus, we find in following the auriferous 

 belt along the flanks of the Sierra to the northwest, that volcanic materials, 

 in the form of continuous flows, or in that of isolated patches, become more 

 and more frequent, and that these and the associated gravels cover more 

 and more of the bed-rock surface, the mining localities at the same time 

 becoming more numerous and important. It is not necessary or possible at 

 the present time, to go into any minute description of the bed-rock in this 

 region. Some general statements may, however, be made in regard to the 

 order of succession of the different groups of rocks, in the auriferous belt 

 north of the Stanislaus. In Mr. Goodyear's notes on the Volcanic and 





