2 GLACIAL AND SURFACE GEOLOGY OF THE PACIFIC COAST. 



of detrital materials whose geological peculiarities as well as whose richness 

 in gold has made them so interesting to scientific and practical men. 

 That the climate, during this period of greater rain-fixll, was wanner than it 

 now is, appears also to have been proved by the character of the vegetation 

 prevailing during deposition of the gravel. 



But in the course of the investigations of the California Survey it long 

 ago became evident that the present epoch is one of desiccation ; or, in other 

 words, that there has been, during the most recent geological times, a con- 

 stant diminution in the quantity of the water standing and flowing upon the 

 surface, throughout the region of the Sierra The same condition of things 

 has been proved to exist in the central and eastern portion of the Cordilleras 

 by the Fortieth Parallel and otlicr Surveys wliicli have more recently been 

 extended over that region. 



Again, we have to go but a short distance back in geological time whi3n 

 we find that a portion of the area enibrace<Ll within the field of our observa- 

 tions was covered by snow and ice ; so that we have offered to us, as occurring 

 between epochs of great precipitation and of rapidly increasing desiccation, 

 a period of ice extension, limited, it is true, to the higher portions of the most 

 elevated ranges of the Cordilleras, but still general enough to imply tlie 

 existence of some peculiar conditions at the time of its occurrence. 



Thus, then, we find ourselves led, by the observation of phenomena 

 connected with the geology and physical geography of the region west of 

 the Roclcy Mountains, to recognize at least three well-marked stages of 

 climatic change; and the question which naturally presents itself in this 

 connection is, whether a similar succession of events can be shown to have 

 occurred in other regions, or generally throughout the world. If, as appears 

 highly probable, this is the case, then we are led to inquire whether this 

 sequen(!e of changes is merely a series of disconnected events, in no way 

 interdependent and not capable of being referred to an^^ common cause ; or 

 whether, on the other hand, they are all linked together, as manifestations 

 of one common cause, which has been acting through all geological time, and 

 Avhich will continue to act in the future. The inquiry thus hinted at forms 

 the main object of the theoretical discussion which follows the presentation 

 of facts occupying the first two chapters of the present volume ; and the 

 writer hopes that he has been able to throw some light on the cliaracter of 

 the climatic changes which have taken place during the later geological 

 times. 



