THE BED-ROCK SURFACE AND THE CHANNELS. 341 
Again, the character of the erosion eflected by any stream depends, in a 
marked degree, on the permanence of the supply of water. Very different 
results must be produced, according as the stream is constant, subject to 
periodical fluctuations, or diminishing in volume. This is too obvious to 
need discussion. In point of fact, however, as the writer believes, there is 
abundant evidence that, throughout the world, rivers, although undoubtedly 
always more or less irregular in their regimen, have on the whole been 
diminishing in size at least during the later geological epochs.* 
It is apparent, then, that every valley beginning in a high mountain-range, 
and terminating in a plain, must exhibit, in cross-section, a variety of forms, 
and that these result from an intricate combination of physical conditions, 
the nature of which can be made out only by a careful study of both the 
geology and topography of the whole adjacent region. If we wish to form 
an idea of the changes of form which various portions of any such valley 
may have undergone in past geological times, we must not only study pres- 
ent climatic conditions, but endeavor to get, from some quarter or other, 
light upon the difficult question of what changes have taken place in these 
conditions during former epochs. 
It would appear that we have in the hydraulic mining-region of California 
a favorable opportunity for studying questions of the kind suggested in the 
preceding pages. It is doubtful if there has ever been anywhere so large an 
area of bed-rock surface underlying detrital material artificially exposed for 
study; and it is surprising how little could have been known about the most 
interesting points in the geology of the Sierra if mining operations had 
never been carried on there. It is not the intention of the writer to claim 
that the difficult questions presenting themselves in this connection can all 
be answered in full; on some of them, however, light can be thrown. 
The first, and perhaps the most important result which can be drawn from 
the study of the mode of occurrence of the gravel deposits, as described in 
the preceding pages, is this: that the character of the channels indicates on 
the whole, beyond question, that a much larger quantity of water passed 
through them than that which runs at the present time from the same region. 
Thus the conclusion, which would be naturally drawn from a consideration 
of the character and quantity of the detrital material itself, is corroborated 
by an examination of the form and size of the depressions in the bed-rock in 
which the gravel has been deposited. 
* See Climatic Changes, ete., Chap. II. 
