134 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
white clay made up, probably, in considerable part of voleanic 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 Eand F. Of these the one given 
on Plate E, Fig. 1, by Mr. Rémond, 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- 
