THE BED-ROCK SURFACE AND THE CHANNELS. 343 
Pliocene Yuba must have been, in places, over a mile; at Columbia Hill the 
channel is estimated as fully a mile and a half across. While, therefore, there 
can be no doubt that the amount of water carried off from the basins of the 
Yuba and American by the Pliocene rivers was enormously larger than that 
conveyed away from the same areas by the present streams, yet numerical 
data are entirely wanting. It is impossible, for instance, to say whether the 
present flow of these rivers is five or fifty times less than that of Pliocene 
times, for we know too little of the form of the rock-bottom of actually 
existing large rivers. After weighing, however, the conditions favorable to 
the idea of a large flow of water which are apparent in the great width of 
the main channels of the gravel epoch, and the steepness of their grade, no 
one can hesitate to admit that the evidence is clear in favor of an enormous 
diminution of volume in recent times. This is particularly true with refer- 
ence to the portion of the gold region included in the basins of the American 
and the Yuba. Similar conditions prevail both to the north and south, but 
not on so extensive a scale. The Pliocene representatives of the Stanislaus 
and Mokelumne, for instance, were undoubtedly larger than the present rivers ; 
but there does not seem to be sufficient proof that the difference was as great 
as it was in the case of the other rivers mentioned. 
The enormous thickness of the detrital deposits in the large channels of 
Pliocene age is a condition so’ intimately allied with and dependent on the 
size of the channels themselves, that similar conclusions may unhesitatingly 
be drawn from one as from the other set of facts. Indeed, it would seem 
hardly necessary to occupy space with insisting on the abundance of proof 
of the former greatly increased flow of water, as offered by the size of the 
deposits which this water has left behind as the evidence of its former pres- 
ence. There is no other way of accounting for the existence of these masses 
of gravel than by admitting that the rivers which did the work of abrading 
and depositing this detritus were of a size corresponding to the magnitude of 
the results produced. To endeavor to substitute length of time in the place 
of energy of action, and to maintain that small streams operating for an in- 
definite period might bring about such results as are here manifested would 
be quite unreasonable, in view of the fact that the rivers of the present day 
are showing us exactly what kind of work they are able to do, and this. as 
already set forth, is something very different from that accomplished during 
the Pliocene epoch. 
We may now take up the next most important feature of the gravels, 
