604 BRUCE L. CLARK 
group ina former paper. The correlation is based upon an analysis 
of the molluscan fauna by the percentage method and the evidence 
afforded by the occurrence of vertebrates in beds immediately above 
and below the San Pablo. The following quotations are taken 
from the above-mentioned paper: 
The percentage of Recent molluscan species in the San Pablo of middle 
California as listed by the writer is 23 plus; as based upon the gastropods 
the percentage is only 11 per cent. If we use the percentages as applied to 
the east coast Neocene and if we can rely upon the equal refinement in the 
determination of the species, the San Pablo may be considered to be upper 
Miocene in age, possibly lower Pliocene. 
Probably the best evidence showing the age of the uppermost beds of the 
San Pablo of middle California comes from vertebrate material obtained in 
the fresh-water beds which in middle California overlie unconformably the 
San Pablo group. This material was described by Professor J. C. Merriam 
in his paper ‘‘ Vertebrate Fauna of the Orinda and the Siesta Beds in middle 
California.”* His conclusions as to the age of these beds as shown by the 
vertebrates are as follows: “‘The mammalian remains known from both the 
Orindan and Siestan up to the present time all represent forms such as might 
be expected in the late Miocene or in the earliest Pliocene, but it will be neces- 
sary both to have better material from the Orindan and Siestan and to have 
well known faunas of western Miocene and Pliocene for comparison before 
the last word on the age determination can be pronounced. 
‘‘Considering the indefiniteness of all the factors concerned, one would not 
seem justified in being more definite than to state that the Orindan and Siestan 
faunas are near a late Miocene stage. When the faunas of the two formations 
are better known, it may appear that more than one stage is represented.”? 
The reader will remember from the discussion of the age of the 
Temblor that the vertebrate fauna obtained from the Big Blue 
formation, which is apparently intercalated with the Temblor 
deposits in the north Coalinga region, was determined by Dr. J. C. 
Merriam as being not earlier than Middle Miocene. In this same 
section the Santa Margarita formation is found unconformably 
above the Big Blue and marine Temblor beds. Also, as will be 
brought out in the discussion of the Pliocene, a Lower Pliocene 
vertebrate fauna was found in land-laid beds which rest unconform- 
«J. C. Merriam, “‘ Vertebrate Fauna of the Orindan and Siestan Beds in Middle 
California,” Bull. Dept. Geol., Univ. Cal., Vol. VII (1913), No. 19, pp. 373-85. 
2B.L. Clark, ‘The Fauna of the San Pablo Group of Middle California,” Bull. 
Dept. Geol., Univ. Cal., Vol. VIII (1915), No. 22, p. 439-42. 
