/ 



134 



THE AURIFEKOUS GRAVELS OF THE SIERRA NEVADA. 



white clay made up, probably, in considerable part of volcanic mud and 

 ashes, often in very fine layers ; other portions are more like ordinary river 

 sand, not very closely compacted together, and usually disintegrating rapidly 

 on exposure to the air. The entire thickness of the sandstone and clay 

 under the centre of the lava flow is, in one locality at least, fully two hun- 

 dred feet. Under this stratified deposit is a distinctly marked river channel, 

 containing pebbles and boulders of metamorphic rock, intermixed with more 

 or less of fragments of trees, and presenting exactly the same appearances 

 as any of our present river channels. 



The peculiar features of the Sonora Table Mountain will be best made out 

 by examining the sections given on Plates E and P. Of these the one given 

 on Plate E, Fig. 1, by Mr. Eemond, was made at a point east of Mormon 

 Creek, about one and a half miles below Springfield, and in the vicinity of 

 Shaw's Flat. In this section only one of the " rims " of the channel has been 

 preserved, the other having been eroded away, together with a considerable 

 portion of the lava flow itself. This partial denudation of the region on one 

 side of the Table Mountain is what led originally to the discovery of the 

 channel at this point, or in its immediate vicinity, as already mentioned. 

 On the east side, the basaltic lava forms a perpendicular wall a little over a 

 hundred feet high, under which a few feet in thickness of sands and clays 

 are exposed, the position of the channel under these sediments being indi- 

 cated on the diagram. On the western edge of the flow, at this point, the 

 rim-rock rises nearly as high as the edge of the lava itself. There is under 

 the basalt a greater or less thickness of brecciated material, called by the 

 miners « cement," and which is probably chiefly made up of andesitic lava, 

 The thickness of this deposit seems quite variable, according as more or less 

 of it had been eroded away previous to the flowing down of the basaltic 

 mass which everywhere covers it, and in most places conceals it entirely. 



At the Buckeye Tunnel (Plate F, Fig. 1), a couple of miles farther down 

 the flow than the locality of the section just described, the following data 

 were obtained, by the writer, from observation and from information given 

 by the miners then at work there. The whole flow, so far as visible on the 

 surface, seems here to be of solid basaltic lava, which is very smooth and 

 flat on the summit and almost entirely destitute of vegetation. The width 

 of the mass was here found by measurement to be 1,700 feet, and its thick- 

 ness varies from 40 to 140 feet. Under the basalt is more or less " cement" 

 or brecciated (andesitic ?) material ; but this rock could not be well exam- 



