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THE BED-ROCK SURFACE AND THE CHANNELS. 335 
September. At times of unusually copious and long-continued rainfalls, — 
such as that of the winter of 1861-62, — or, still more, when very warm 
weather suddenly sets in after a heavy fall of snow, these streams are, for 
a short time, enormously swollen; but even then they do not rise so as to 
make any approach to entirely filling the cafons in which they run. More- 
over, such heavy floods occur but rarely. The detrital material in the bottoms 
of these rivers, where they have not been filled with tailings, is apparently 
of quite moderate depth. It is true that the bed-rock is almost everywhere 
concealed, and the business of fluming the streams, so as to work the débris 
in their channels, — once so profitable, — is now entirely abandoned, so that 
opportunities for seeing the bed-rock under the present rivers are of ex- 
tremely rare occurrence. Even as early as the time of the beginning of the 
Geological Survey (1860), this kind of mining had nearly come to an end.* 
The present natural regimen of the streams throughout the auriferous belt 
of the Sierra is so masked by the vast mass of tailings poured into them, 
that it is not easy to make out exactly what was their condition before min- 
ing began in that region. 
After a careful review of all the circumstances and conditions connected 
with the occurrence of the gravel deposits, the writer has come to the con- 
clusion that the essential differences between the epoch of the high gravels 
and the present one depend for their existence on one all-important condi- 
tion; namely, that during the Tertiary or Gravel epoch there was a much 
larger precipitation than there is at the present time. How this greater 
rainfall of former ages can be shown to have existed, and how its results 
were combined with those produced by other but secondary agencies, such 
as the rise, culmination, and decline of the volcanic period, cannot be set 
forth in full detail within the limits of the present volume; but the attempt 
will now be made to elucidate a few of the more important points suggested 
by this theory, at the same time referring the reader to another work, in a 
measure supplementary to the present one, where some of the matters which 
must here be passed over in haste will meet with a fuller discussion.t 
One of the most interesting circumstances connected with the gravel 
deposits of the Sierra is this: that we have offered to us in this region an 
exceptionally good opportunity of ascertaining what the work of water 
* In such old photographs of river workings as have come under the writer's notice, the detrital material 
appeared to be thin and the surface of the bed-rock uneven. 
7 See Climatic Changes of Later Geological Times. Mem. Museum of Comparative Zovlogy. Vol. VII. 
Part 2. 
