314 RESUME 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 
the south. 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 com- 
pared in metallic wealth with the Californian division of the range. 
Section IV. — The Geological Age of the Gravels. 
In the preceding chapter will be found a condensed account of the prin- 
cipal fossil remains which came under the notice of the Geological Survey at 
various times, while engaged in the exploration of the gravel region. Not 
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 may 
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 
other hand, the position and relations of the strata of Cretaceous age, all 
