GEOLOGICAL AGE OF THE GRAVELS. 5] 
~I 
been set forth, and as will be further explained in the course of the present 
chapter. 
It appears to result, then, from a careful study of the mining region, that, 
for the practical purpose of the study of the gravels and the old river-chan- 
nels, we may assume that orographic causes may pretty much be left out of 
consideration in the discussion of what has taken place since the gravel was 
deposited. The only exception: to this would be, as already indicated, in the 
extreme northern part of the region, where confessedly the phenomena are 
too obscure for present explanation. There have, it is true, been slight 
disturbances even in the central and most important portion of the gravel 
district, as will be noticed further on; but these have certainly not been of 
such a nature as to give rise to any important changes in the general direc- 
tion or grade of the ancient river systems. 
It seems then, to be, if not absolutely proved, at least rendered highly 
probable, that, while the marine Cretaceous strata were accumulating at the 
base of the Sierra, along its central portion the range itself had at least its 
present elevation; and that from the material eroded from it these marine 
deposits were, in large part, built up. If the conditions were as thus indi- 
cated, why should not there be strata containing fresh-water or land animals, 
or plants of Cretaceous types, found above the line which marked the sea- 
level of that epoch? That there are not, seems a well-established fact. The 
animal remains are of distinctly Tertiary affinities, and there is nothing 
among them which is related to the characteristic forms of the Cretaceous, 
while the whole aspect of the flora is distinctly that of an epoch as recent 
as the Tertiary. The reason for this condition of things may be gathered, 
perhaps, in part at least, from what follows in regard to the absence or very 
imperfect representation of the lower Tertiaries in the formation under 
discussion. 
That the western slope of the Sierra was above water, and in a position to 
be the recipient of detrital material, during the whole of the Tertiary epoch 
can hardly be doubted. All the geological conditions indicate this. At the 
same time marine strata were accumulating along the base of the range, the 
materials for which, as in the case of the Cretaceous, were derived, in large 
part, from the adjacent land to the east. There is no continuous belt of 
Tertiary, any more than of Cretaceous, along the foot-hills, because a con- 
siderable amount of erosion has taken place since the occurrence of the 
very moderate uplift which raised the formation to its present position. 
