■• *-^p-j !■■ IWI 



GEOLOGICAL AGE OF THE GRAVELS. 



317 



been set forth, and as will be farther 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 ransre, 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- 

 sulcrable amount of erosion has taken place since the occurrence of the 

 very moderate uplift which raised the formation to its present position. 



