TERTIARY FAUNAS OF THE PACIFIC COAST 237 



Porter near Grays Harbor, in western Washington, are believed to be 

 the oldest of the definitely known Oligocene. In these assemblages 

 are several species showing distinct Eocene affinities; in the later 

 Oligocene the forms are decidedly more closely allied to Miocene 

 forms. The climatic conditions prevalent on the west coast of the 

 United States during the Oligocene are believed to have been transi- 

 tional from the subtropical of the Eocene to the more temperate of the 

 lower Miocene. 



THE LOWER MIOCENE PERIOD 

 CONDITIONS INAUGURATING THE LOWER MIOCENE 



The Oligocene period of elevation and moderate erosion was 

 followed by diastrophic movements of a most interesting and important 

 character. It was during this post-Oligocene period of disturbance 

 that definitely recognizable movements along what is now termed 

 the great earthquake rift and associated rifts of California first took 

 place. Although profound regional subsidence was the rule in 

 central and portions of southern California, local movements along 

 the faults mentioned elevated blocks of the pre-existing formations 

 into islands, usually of considerable relief, in the region now occupied 

 by the Coast Ranges. It is in a study of details such as the distribu- 

 tion of the land and water in these fault zones that composite maps, 

 such as those accompanying this paper, become entirely inadequate 

 and sometimes misleading. Suffice to say that beginning with the 

 pre-Vaqueros (pre-lower Miocene) period of disturbance many of the 

 major blocks within the general fault zone of the Coast Ranges, and 

 to a lesser extent, the minor blocks within the major masses, were 

 seldom at rest for more than relatively short periods up to the present 

 day. Some folding took place during the pre-Vaqueros period, but 

 it was local in character, such as that exhibited in the Coalinga 

 district, and of minor importance as compared with the vertical 

 movements of the large masses. One of the most significant facts in 

 connection with the lower Miocene subsidence was the retention of 

 its position above sea-level of the Sacramento Valley region at a 

 time when the San Joaquin Valley to the south was subjected to 

 marine conditions. This discordance of movement between the two 

 ends of a continuous basin, which in the discussion of California 



