CHAPTER III. 



GENERAL DISCUSSION OF THE DESICCATION QUESTION. 

 Section I. — Lrtrodmtory . 



It will be proper, at the present stage of our investigation, to recapitulate 

 what has been set forth and shown to exist, in connection with the prob- 

 lems of climatic change, in the preceding chapters of this work, and in a 

 former one (the Auriferous Gravels), to which frequent reference has already 

 been made, and to which the present volume may properly be considered as 

 supplementary. 



As one of the important results to which we were led by our extended 

 examination of the detrital formations resting on the western slope of the 

 Sierra Nevada, we have the — as it appears to the present writer — un- 

 doubted fact presented to us of the former existence, throughout that region, 

 of much larger rivers than tliose whose channels now furrow that slope. 

 These rivers had, however, on the whole, pretty essentially the same gather- 

 ing-grounds which their present much diminished representatives have ; 

 hence it follows, as a matter of course, that the former precipitation must 

 have been very considerably larger than it now is. This condition of things 

 prevailed during a portion of the Tertiary epoch, probably during a large 

 part of the Tertiary, and certainly during the later Pliocene. If it cannot 

 be definitely stated that this increased precipitation also prevailed during 

 the earlier Tertiary period, it is only because Eocene fossils have not been 

 distinctly recognized as occurring in the detrital formations in question, so 

 that the means of going back to the beginning of the Tertiary epoch are 

 not furnished. There is no evidence, however, that the precipitation over 

 the region in question was, during the earliest portion of the Tertiary, any 

 smaller than it was later on in that period ; on the contrary, there is some 

 reason for supposing it to have been larger. So much has been set forth in 

 detail in the Auriferous Gravels. 



Pursuing our investisrations still farther, we have seen, as described in 

 the first chapter of the present volume, that long after the deposition on the 



