584 BRUCE L. CLARK - 
idea of the problems involved, the task will have been well worth 
while. 
There are several factors, aside from the lack of a sufficient 
number of trained workers, that have hindered the progress of 
correlation of West Coast horizons. These factors may be con- 
sidered under the following headings: (1) temperature differentia- 
tion (2) geographical isolation, and (3) poor preservation. 
1. It is well known that the marine faunas living on the Pacific 
Coast can be separated into distinct faunal and geographical 
provinces. In this respect the West Coast of North America is 
typical of the whole Pacific border. For example, the fauna found 
off the coast of Panama is very different from that living along the 
coast of southern California, and the latter has very little in common 
with that off the coast of Alaska, while faunas from some of the 
intermediate areas are almost equally distinct.t It is generally 
recognized that Pleistocene and Recent times mark one of the 
maximum periods of emergence of all the continents. While this 
is not true in so great a measure of all the periods of the Tertiary, 
it is well known that the North American continent was sub- 
merged only on its borders, and that during a large part of 
this time the Pacific and Atlantic oceans were disconnected. ‘The 
study of the Tertiary faunas along the coast discloses marked 
evidences of temperature differentiation; the faunas of the north 
having a more boreal aspect than those of the south. This 
differentiation was most exterme in the Pliocene and Upper 
Miocene, and it is undoubtedly because of this that there has been 
so much confusion in the past in the correlation of the deposits 
from various sections along the coast now referred to those horizons. 
There is good evidence of temperature differentiation during the 
Oligocene and Middle Miocene, and what is more interesting is 
that accumulated evidence seems to show that this differentiation 
of the faunas had its effects even as far back as Eocene times. 
«W.D. Dall, Summary of the Marne Shellbearing Mollusks of the Northwest Coast 
of America, Bulletin 112, United States Natural Museum (1921), pp. 1-213. 
2 J. P. Smith, ‘‘Climatic Relations of the Tertiary and Quaternary Faunas of the 
California Region,” Proc. Cal. Acad. Sci., Fourth Series, Vol. IX (1919), No. 4, pp. 
123-73. 
