520 RALPH ARNOLD 
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 
