592 BRUCE L. CLARK 
differentiated in Washington are also represented in the same 
sequence in California, and tentatively we may consider the 
Oligocene series as being made up of two distinct parts, referred 
to in the correlation table as the San Lorenzo series. 
The aggregate thickness of the marine beds of the Upper and 
Lower Oligocene of the West Coast exceeds 10,000 feet. A large 
part of these sediments consists of shales and shaly sandstones. 
Correlation.—The evidence for the correlation of the West 
Coast marine Oligocene deposits is indirect. No molluscan species 
or even apparently related forms have been recognized as common 
to the Oligocene of the West Coast and the Gulf province. ‘The 
faunal evidence at hand seems to show that after the close of the 
Tejon epoch (Upper Eocene) there was no direct connection between 
the Atlantic and the Pacific Coast basins. 
Dr. Ralph Arnold was the first to announce the presence of 
Oligocene in California. The type section of the San Lorenzo is 
in the Santa’ Cruz Mountains of the Santa Cruz Quadrangle, 
California. Dr. Arnold concluded that this formation is of Oligo- 
cene age because of its stratigraphic position between beds generally 
recognized as belonging to the Upper Eocene and Lower Miocene 
(Vaqueros) age. He observed that the fauna of the San Lorenzo 
appeared to have both Eocene and Miocene affinities.‘ Later 
studies of the faunas of the Lincoln and San Lorenzo horizons 
have borne out Arnold’s original conclusions.? At the time Arnold 
did his work the Lincoln horizon had not been differentiated. The 
fauna of this horizon shows a,much closer relationship to that of 
the Tejon (Upper Eocene) than to that of the Lower Miocene, 
while the fauna of the San Lorenzo horizon, equivalent to Weaver’s 
Acila gettysburgensis zone, has a Miocene aspect, a fairly large 
number of the genera and species being common to the two. 
=R. Arnold, “Tertiary and Quaternary Pectens of California,” U.S. Geol. 
Surv., Prof. Paper 47 (1906). J. C. Branner, F. G. Newsom, and R. Arnold, U.S. 
Geol. Surv., Folio 163, Santa Cruz Folio. 
2B. L. Clark, “Occurrence of Oligocene in the Contra Costa Hills of Middle 
California,” Bull. Dept. Geol., Univ. Cal., Vol. IX (1915), No. 2, pp. 9-21; “San 
Lorenzo Series of Middle California,” Bull. Dept. Geol., Univ. Cal., Vol. XI (1918), 
No. 2, pp: 45-234. B.L.Clark and R. Arnold, “Marine Oligocene of the West Coast 
of North America,” Bull. Geol. Soc. Amer., Vol. XXIX (1918), pp. 297-308. 
