316 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
300 feet thick, followed by other shales which are hard, very siliceous, 
and light gray or white. Overlying these is a second sandstone, | 
somewhat similar to the first, but less concretionary, and this again 
is overlain by other shales siliceous in tendency, but generally more 
earthy and friable, and of a brownish color. The correlation of this 
series in its entirety is in doubt, but the lower, highly siliceous shales ! i 
are unquestionably of the Monterey type, while the sandstones and 
the associated shales may also prove to be of the same formation. 
Unconformable upon the foregoing rest several thousand feet of con- 
glomerates, sandstones, and clays, which carry fossils that identify 
the beds as of the San Pablo horizon in the Middle Neocene. Youngest 
of all are some late Pliocene or Pleistocene conglomerates along the 
slopes of the Santa Clara Valley. 
The structure of the region is that of an anticline of very consid- 
erable proportions, modified by subordinate folds and faults of the 
utmost intricacy. Its axis has a somewhat irregular trend, varying 
from N. 70° W. to N. 80° E., the principal curvature occurring in 
the hills opposite the town of Pirn. The heart of the anticline lies 
in Topa Topa Mountain and is occupied by the series of Eocene 
quartzites and shales. Around these circle successively the Sespe 
red beds, the Lower Miocene shales, the Monterey, and the equiva- 
lents of the San Pablo, the last occupying vast areas extending from 
15 to 30 miles or more east of the heart of the fold. On the north the 
anticline is limited by other folds of equal importance, On the south 
the flexure is modified by a succession of sharp folds of greater or 
less extent, and by faulting, an especially important line of fracture 
passing east and west in front of San Cayetano Mountain, extending 
westward into the Ojai Valley, and eastward, perhaps, crossing the 
Santa Clara Valley. Numerous branches are given off from this 
fracture, particularly toward the west. 
It is in such an assemblage of strata, with the intricate folding 
to which they have been subjected, that the oil wells of the region 
under discussion occur. In horizon the oil is drawn from the lower, 
middle, and upper portions of the Sespe red beds, from the rusty 
series at the base of the Lower Miocene, and from sandy measures in 
the overlying shales; from the great sandstones which succeed and 
are associated with the shales of Monterey type; and, finally, from 
the equivalents of the San Pablo beds themselves. In addition, oil 
is known to occur in the Eocene quartzites forming the heart of the 
anticline. Fifteen thousand feet of strata, therefore, yield petroleum 
at one point or another in this field. The distribution of the devel- 
oped oil areas is either in a broad sweep about the axis of the main 
anticline itself, in close proximity to the axes of some of the subor- 
dinate folds, or along one or more of the great fault lines of the terri- 
tory, such, for instance, as that south of the San Cayetano Mountain 
and extending westward through the Silverthread district into the 
