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 

 from which they were abraded. 



The slate belt of the Sierra begins to appear as having some development 

 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 boundarv 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 covering an ancient river 

 channel. To the south of this lava-flow (already in previous pages described 

 as the Table Mountain of Tuolumne),! 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.* 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, volcanic 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. 



% See ante, pp. 137, 138 



t See ante, pp. 131-137. 

 § See ante, pp. 138, 139. 



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