116 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4TH SER. 
abound in all parts of the Neocene series, excepting possibly 
the latest. In the Mt. Diablo Range diatomaceous and other 
organic shales often make up a large percentage aggregate 
of the Monterey and later groups, and they occur also in the 
Etchegoin group, included by Ralph Arnold in ne so-called 
“McKittrick series.” 
The opinion has been unequivocally expressed’ that in the 
Coalinga field the real source of the petroleum is in the Eocene 
shales underlying the Neocene, and that migration of the 
petroleum upward through the strata has brought it into its 
present repositories in the Neocene oil measures. That petro- 
leum, in some parts of the Mt. Diablo Range and elsewhere, 
has originated in the Eocene cannot be denied, and it is also 
now found there in many places. But to conclude that all or 
any of the Neocene oil-measures have derived their supplies 
from the Eocene is illogical and unnecessary. The Neocene 
beds themselves contain the same organisms in even greater 
abundance than does the Eocene, and this is particularly true 
in the Mt. Diablo range. And there is no reason to suppose 
that the oil found in the Neocene measures has not originated 
in the Neocene strata themselves. 
The view here expressed is that the oil found in any Neocene 
group has more probably originated in that group, and that 
migration would be far easier along the planes of bedding and 
lamination than at right angles to the same. That thick beds 
of clay and shale often restrain oil, water, and gas, is quite 
well demonstrated in California, and even within the Temblor 
basin the upward transverse migration of these substances 
under enormous pressure has been successfully resisted by 
certain impervious beds, possibly clays. 
Naturally in the sedimentation of any basin the sandy detri- 
tus usually remains near shore, and the finer materials are 
carried away to other localities to be deposited. Also if 
Diatomaceae and other delicate organisms form any apprecia- 
ble deposits they will more probably be formed off shore. In 
subsequent regional deformations of the strata, the organic 
deposits are apt to be left occupying the position of synclinal 
depressions, bounded by the sandy shore line deposits left lying 
1U. S. Geol. Surv. Bull. No. 357, p. 73. 
