128 BRUCE CLARK 
HISTORICAL SKETCH 
The most detailed work on the Eocene of the West Coast, 
since the time of Gabb, is that of Dickerson.t He recognized 
only two stratigraphic units as belonging to this period, the Mar- 
tinez (Lower Eocene) and the Tejon (Upper Eocene). As the 
result of his studies of the so-called Tejon, he outlined four faunal 
zones, thought to occupy four successive horizons in that group. 
These, beginning with the lowest, were the Turbinolia zone, the 
Rimella simplex zone, the Belanophyllia variabilis zone, and the 
Siphonalia sutterensis zone. The first three zones were described 
from the Eocene section in the vicinity of Mount Diablo, and the 
fourth from the Eocene bordering the foothills of the Sierra 
Nevadas, the beds of which apparently form part of the Ione 
formation. 
In the paper in which Dickerson first outlined his ideas of 
the faunal succession of the Tejon we find the statement? 
A study of the relationship between zone 3, Mount Diablo region, and the 
Siphonalia sutterensis zone and their geographic position suggest that the 
uppermost strata of the Marysville Buttes and Oroville were deposited by a 
transgressing sea, and that only in favored places along the western borders of 
the Sierra have the latest Eocene sediments been preserved from erosion. 
Lava caps such as that of the older Basalt of South Table Mountain have 
preserved these youngest Tejon sediments which have heretofore been regarded 
as Ione. 
The first writers to suggest that there are more than two 
distinct stratigraphic units or groups in the Eocene of the West 
Coast were Dr. Ralph Arnold and Mr. Harold Hannibal in a 
joint review of one of Dr. Dickerson’s papers. These writers, 
in their study of the Eocene of Washington and Oregon, recognized 
three horizons, the Chehalis (at the base), Oliqua, and Arago 
or Ione (at the top) formations, all higher than the Martinez 
(Lower Eocene). Their Chehalis horizon they referred to the 
1R. E. Dickerson, ‘Note on the Faunal Zones of the Tejon Group,” Univ. Cal. 
Pub. Bull. Dept. Geol., Vol. VIII (z914), No. 2, pp. 17-25; ‘‘Stratigraphy and Fauna 
of the Tejon Eocene of California,” Univ. Cal. Pub. Bull. Dept. Geol., Vol. TX (1916), 
No. 17, pp. 262-534, Pls. 36-46. 
2R. E. Dickerson, “Note on the Faunal Zones of the Tejon Group,” Univ. Cal. 
Pub. Bull. Dept. Geol., Vol. VIII (1914), No. 2, p. 24. 
