2 28 RALPH ARNOLD 



The fourfold subdivision of the Tertiary is the one which seems 

 best to fit the phenomena of the Pacific Coast, although for con- 

 venience of discussion in the present paper the writer has separated 

 the upper from the lower Miocene on account of the diverse geologic 

 histories of the two. It is obviously impossible to make exact correla- 

 tions between the European and East American subdivisions on the 

 one hand and the faunal and stratigraphic subdivisions of the Pacific 

 Coast on the other, but by means of various direct and indirect 

 methods it is possible, however, to make approximate correlations, 

 and as the work progresses these approximations will be made to 

 approach nearer and nearer to the exact. Paleontology forms the 

 basis for the correlations, but other criteria, such as periods of wide- 

 spread diastrophism and volcanic activity and profound changes in 

 climate, have also been taken into consideration. It is well to men- 

 tion here that the total thickness of Tertiary and Quaternary sedi- 

 ments in California approximates 25,000 feet and that within the 

 Tertiary and Quaternary periods, relatively short, geologically 

 speaking, as compared with the earlier divisions of the time scale, 

 probably more distinct and profound movements have taken place 

 on the western border of our continent than have occurred over an 

 equal length of time in any of the preceding periods within the limits 

 of North America. 



Five maps have been prepared to elucidate the paper, each respec- 

 tively representing the supposed distribution of land and water along 

 the western border of the United States during the Eocene, the 

 Oligocene, the lower Miocene, the upper Miocene, and the Pliocene 

 and Pleistocene epochs. It is admitted that these maps are composites ; 

 that is, they represent the distribution not at any definite moment 

 but throughout a period of time during which the local conditions 

 usually changed but little relative to the changes taking place between 

 these periods. For instance, the areas shown as subject to deposi- 

 tion during the Eocene are the areas over which deposits were laid 

 down at one time or another during the Eocene epoch. In the case 

 of certain portions of Puget Sound and elsewhere, marine conditions 

 prevailed during the early Eocene, brackish-water conditions a little 

 later, and freshwater or river, and coal-marsh conditions toward the 

 close. In other portions of the same general area the conditions 



