THE CLIMATIC CHANGES 



OF LATEU GEOLOGICAL TIMES. 



CHAPTER I. 



THE GLACIAL AND SURFACE GEOLOGY OF THE PACIFIC COAST. 



Section I. — Introduction. 



This chapter and the following one form the basis on which the present 

 voliitne rests. Tliey were originally intended to be included in a work en- 

 titled •' The Auriferous Gravels of the Sierra Nevada of California," a part of 

 which has been already published, while the appearance of the remaining 

 portion will not l)e much longer delayed.* The reason why it has been 

 decided to make a separate work of the glacial geology is, that the quantity 

 of matter was found to be unexpectedly large, so that a volume which should 

 contain the whole results of the investigation would be inconveniently bulky. 

 The gravel deposits of the Sierra Nevada do, indeed, form a special subject 

 of inquiry, and a single volume may most properly be devoted to their ex- 

 clusive consideration. Moreover, by separating the discussion of the glacial 

 phenomena from the description of the auriferous gravels, a wider range 

 could be given to the former, which coiu'se seemed, after the Avriting out 

 of the work for publication had been taken seriously in hand, to be most 

 desirable. 



The study of the mode of occurrence of the formations described in the 

 gravel volume led the writer to form certain conclusions with regard to the 

 physical conditions prevailing during the epoch of their deposition.! It 

 seemed to him to be most clearly made out that a precipitation, greatly in 

 excess of that which is now taking place in the same region, was the most 

 important agent in bringing about the accumulation of those great masses 



* That work will be cited in the present one as the "Auriferous Gr.avels," or simply as the "gravel volume." 

 It forms the sixth volume of the Memoirs of the Museum of Comparative Zoology, 

 t See t'lia[iter IV, seetious 5 and 6 of the Auriferous Gravels. 



