512 RALPH ARNOLD 
alternated. It is obvious, therefore, that the legends on the maps are 
very general. Only in those instances where the body of water 
indicated as fresh remained fresh throughout practically the whole 
of its existence is it indicated as a freshwater area on the map. 
The periods chosen for representation and as units for discussion 
are neither of equal length nor of equal importance, and the lines 
separating them are in some instances arbitrary; but it is believed 
that they serve the purpose of systematizing the discussion better than 
any other plan of subdivision. The data are incomplete and the 
conclusions admittedly tentative, and it is expected that future 
investigations will disclose new and important information, which 
will necessitate alterations, but the fact remains that general reports 
of this kind, based as they are on the present state of our knowledge, 
often point the way to more exact results in the future. 
ACKNOWLEDGMENTS 
The writer wishes to acknowledge his indebtedness to Messrs. 
Bailey Willis, J. S. Diller, T. W. Stanton, Robert Anderson, Chester 
W. Washburne, and several others for personal assistance in the 
preparation of the text and maps, and to express his thanks for the 
services rendered. In addition to the personal aid received, the 
literature relating to the subject of West Coast geology has been 
freely drawn on in the compilation of relevant data and in many cases 
proper acknowledgment for this is made in the text. 
THE EOCENE PERIOD 
RELATION OF THE EOCENE TO THE CRETACEOUS 
Before entering into the details of the geologic history of the 
Tertiary it is well to consider for a moment the relations existing 
between the earliest Tertiary rocks and those of the Cretaceous, and 
to note the conditions initiating the Tertiary, as implied by these 
relations. 
A widespread unconformity exists between the Eocene and the 
Cretaceous on the Pacific Coast of North America. Throughout 
Washington, Oregon, and certain parts of California, this uncon- 
formity is angular, while over considerable areas in California and 
at one locality in Oregon the unconformity may only be recognized 
by a more or less marked hiatus in the faunas. 
