THE MARINE TERTIARY OF THE WEST COAST 605 
ably upon the top of the Santa Margarita. Thus it would seem 
that if we can trust the correlation on the basis of the vertebrates, 
there can be very little doubt as to the age of the Santa Margarita 
group which comes between two vertebrate horizons, one not 
earlier than Middle Miocene, the other not later than Lower 
Pliocene. 
Climate.—The paleontological evidence seems to show that, 
beginning with the Upper Miocene, there was a temperature 
differentiation on the West Coast that was even more marked 
than that existing today. 
The Briones and Cierbo groups (lower and middle San Pablo) are 
not found in southern California, and because of their limited dis- 
tribution give us very little evidence of temperature differentiation. 
The fauna obtained from the Santa Margarita (upper San 
Pablo) in middle California may be regarded as approximately 
warm-temperate, and if it were now living it would probably not 
be found south of Santa Barbara County. This conclusion is based 
upon the large percentage of recent species found in the faunal 
assemblage and common to the fauna now found living between 
San Francisco Bay and Santa Barbara. The presence of certain 
recent species and the absence of certain genera found at northern 
localities indicate that the Santa Margarita horizon in southern 
California represents a warmer facies than that found in middle 
California. There is, however, a sufficient number of distinctive 
species common to the two horizons to establish their correlation, 
though the faunas are on the whole very different. 
The fauna of the Montesano formation of Washington, described 
by Dr. C. E. Weaver,’ is apparently Upper Miocene in age, but 
just what part of the San Pablo series it represents has not been 
established. This fauna, judging from the recent genera and 
species in the assemblage, is boreal and consequently very different 
from that of the San Pablo. If the correlation of the Montesano 
formation with the San Pablo series is correct, there was at that 
time a temperature differentiation comparable to that found 
between the recent faunas of middle California and Alaska. 
tC, E. Weaver, “‘Tertiary Formations of Western Washington,” Wash. Geol. 
Surv., Bull. 13 (1916), pp. 1-327. 
