302 RESUME AND THEORETICAL DISCUSSION. 
Nevada County northward to the extreme end of the gold region we may 
expect to find the auriferous gravels resting on rocks quite unlike those 
; 
The slate belt of the Sierra begins to appear as having some development 
from which they were abraded. 
a little south of the southern border of Mariposa County, and rapidly ex- 
pands on entering it, so as to occupy a belt of over twenty miles in width, 
as already explained.* No sooner do the slates set in, in this manner, than 
powerful quartz veins begin to make their appearance; and those of Mariposa 
County are numerous, and have been extensively worked, although with 
varying fortunes. The conditions indicated for Mariposa are continued in 
Tuolumne County, near the northern boundary of which we have for the 
first time — proceeding from the south towards the north — a lava-flow, ex- 
tending entirely across the auriferous belt, and covermg an ancient river 
channel. ‘To the south of this lava-flow (already in previous pages described 
as the Table Mountain of Tuolumne),t we find no proper deep gravels or 
hydraulic mines. The surface mines have, however, been quite important 
over various irregular areas, the localities of which have already been suffi- 
ciently designated.t These areas are chiefly in the vicinity of the Great 
Quartz Vein; and the affiliation of the deposits of paying surface gravel 
with the slaty portions of the bed-rock series, and with auriferous quartz 
veins, is a very marked feature of the region. It is in these counties and in 
the adjoining ones on the north, Calaveras and Amador, that those remark- 
able limestone belts, the surfaces of which have been corroded into such 
gigantic riffles, have their principal development, as previously described :§ 
these have been worked for many years, although probably by this time 
very nearly exhausted. 
From the Tuolumne Table Mountain north, voleanic matter occupies, as a 
general rule, more and more of the surface on the higher portions of the divides 
between the streams; and, with the development of the volcanic, the gravel 
deposits also increase in importance. In Calaveras they have already 
acquired in places a considerable thickness, although by no means to be 
compared in this respect with those occurring farther north. The paying 
portion of the gravels in this county, however, is usually very thin, and the 
total development of pipe-clay, sand, and gravel only occasionally reaches as 
much as a hundred feet. The area occupied by the detrital deposits is also small; 
* See ante, p. 43. Tt See ante, pp. 181-137. 
t See ante, pp. 137, 138 § See ante, pp. 138, 139. 
