44 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
also least covered by detrital and volcanic materials, a sketch of the structure 
of the region between the Merced and the Stanislaus rivers may here be in- 
troduced. This is a district which has been of great importance on account 
of the quartz mines worked at numerous points along the line of the Great 
Quartz Vein, and it was also the scene of much rich placer-mining during 
the years when this class of deposits was so productive. 
The width of the auriferous slate belt in this region is, on the average, 
about eighteen miles. The foot-hills at the base of the range are here made 
up of horizontally stratified Tertiary sandstones, eroded away so as to leave 
the rock in the form of low flat-topped elevations. Turning from the lower 
edge of the foot-hills wp into the mountains, we meet at once, at a very 
moderate elevation above the sea-level, with the metamorphic belt of the 
Sierra, which at its lower edge is largely made up of slates of various com- 
position, chiefly talcose and chloritic. The strata stand nearly vertical, usu- 
ally but little covered with detritus, and the peculiar weathered outcrops 
of the slaty rocks, turned up on edge, and projecting a few feet above the 
surface at intervals along regular lines, are familiarly known to the miners as 
“ ovave-stone slates.” Soon the character of the rock begins to change, and 
we strike a broad belt of compact, dark grayish-green, fine-grained rock, 
which frequently assumes a porphyritic structure, and hence was called by 
the Survey “porphyritic green slate,” although it never assumes the finely 
laminated character of the argillaceous slate series. This rock occurs in high 
rocky ridges, generally much broken, and running about northwest and 
southeast. It forms the first noticeable range of elevations at the base 
of the Sierra, in the region at present under discussion. Mount Bullion, 
Juniper Ridge, Bear Mountain (on the Merced), and Merced Mountain are 
made up of this rock. This material, which was a great puzzle to us in the 
field, as to whether it was of volcanic or sedimentary origin, appears, from 
Mr. Wadsworth’s (not yet completed) examination, to be a diabase tufa, a 
much metamorphosed volcanic deposit. It appears very probable that the 
lithological investigations now going on in regard to the Sierra Nevada rock 
will prove the existence in the bed-rock series of a large amount of material 
originally volcanic, but at present so much altered by those metamorphic 
agencies which have been so conspicuous in their action on the purely sedi- 
mentary beds, that their original character can no longer be recognized, 
except by the aid of microscopic investigation. This belt of altered volcanic 
rock occupies a width of from two and a half to five miles on the surface 
