314 



BfeUME AND THEORETICAL DISCUSSION. 





wise there would be no room for the great body of tailings which accumu- 

 late as the work goes on. And still another favorable condition must be 

 present: above the gravels themselves which are to be washed there must 

 be a higher region, whence water may be obtained in large quantity, and 

 under sufficient pressure. All these conditions are united In the region in- 

 dicated as being par excellence the hydraulic mining district of California. 

 Proceeding still farther north from the head of the Feather River, we find 

 mining operations quite put an end to, because the covering of volcanic 

 matter over the bed-rock becomes an unbroken one, so that anything of 

 value, which might exist beneath it, would be entirely concealed. But there 

 is reason to believe that the auriferous character of the bed-rock series is by 

 no means so marked to the north of the Feather River as it is farther south. 

 For, in the first place, there is a gradual falling-off in this respect already to 

 be noticed, before the exclusively volcanic region has been reached. Plumas 

 has fewer productive quartz-veins and smaller areas of gravel than Sierra, 

 and the latter county is not equal in these respects to the region adjacent on 



Again, the slaty formations of the Sierra Nevada system seem to 

 emerge from under the lava covering in the extreme northwestern corner of 

 the State, but here they are only quite moderately auriferous. And, still 

 farther north, in Oregon, neither the Cascade nor the Blue Mountain 

 Range — one or the other or both of which must be, as it would seem, the 

 geological equivalent or continuation of the Sierra Nevada — can be coin- 

 pared in metallic wealth with the Californian division of the ran™. 



the south. 



Section IV. 



of 



region. Not 





# In the preceding chapter will be found a condensed account of (he prin- 

 cipal fossil remains which came under the notice of the Geological Survey at 

 vanous times, while engaged in the exploration of the gravel .. e ._. 

 much need be added in the present section to what has been already said, in 

 regard to the geological age of the detrital masses which form the subject 

 of this volume; but a few points seem to demand further elucidation 



The age of the bed-rock series can be set down with certainty as being 

 nowhere more recent than Jurassic, as has been already explained. It is 

 not impossible that a portion of the metamorphic belt of the Sierra mav 

 prove to be older than the Carboniferous, although, in the light of the evi- 

 dence collected up to the present time, it is quite improbable. But, on the 



the position and relations of the strata of Cretaceous age, all 



other hand 





i 



