GEOLOGY. 
35 
It is in some localities banded with fine white laminae or with bands 
more translucent than the rest. These bands run parallel with the 
bedding, and commonly show intricate contortions. The flint frac¬ 
tures conchoidally. From the flint there is every step in the grada¬ 
tion through rocks of less hardness and flinty, compact character to 
soft white diatomaceous shale. The soft unaltered shale in which the 
constituent diatom tests are plainly to be seen occurs sparingly, how¬ 
ever, in the lower division. A striking example of its occurrence at 
that horizon can be found at the very base of the Monterey on the 
San Julian ranch, at the junction of El Jaro and Salsipuedes creeks, 
where it is pure, soft diatomaceous earth in thick beds, associated 
with flint and lime and overlying the hard fossiliferous, calcareous 
conglomerate of the Yaqueros. The specimen of analysis 3 (p. 45) 
is from this point. The varieties of shale are very numerous, but 
there is no departure from the general siliceous and calcareous types 
so peculiar to this formation. There is no common clay shale or slate 
derived from it, and only very locally is there an appearance of a 
sandy texture. In the San Rafael Mountains the series has a some¬ 
what different character, especially at the base, where a considerable 
amount of sandstone, in some places soft and in others quartzitic, is 
interbedded with the hard calcareous shales. Hard, coarse, yellow 
and grayish volcanic tuff of acidic nature is interbedded with the Mon¬ 
terey in the vicinity of Cuyama River (see PI. Ill, A), and elsewhere 
the lowest portion of the formation is marked by beds of tuff of local 
extent. At the east end of the Santa Rita Hills the Yaqueros grades 
into the Monterey through beds of coarse basic tuff composed of 
small fragments of glass and crystals of various kinds and of large 
fragments of pumice. Round bowlders or nodules of very fine grained 
basalt that look like volcanic bombs are included in this tuff. 
The series of hard shales of the lower division is commonly impreg¬ 
nated with bituminous material. The limy beds have almost uni¬ 
versally a bituminous odor and some of them contain pockets of tarry 
oil. The same is true of the flint with the difference, however, that 
the limestone is impregnated with petroleum, owing to its porosity, 
whereas the oil in the compact flint seems more commonly to be con¬ 
tained along lines of fracture or in cavities. The great mass of the 
hard, brittle shales has in general a similar odor or is discolored with 
oil. This hard shale series, especially the lower portion of it, and in 
places possibly the uppermost sandstone of the formation just below 
it, contains the principal oil-bearing zones in the developed fields. 
The fact that this shale is so brittle and fractures when folded has an 
important bearing on the storing of oil in this portion of the Monterey. 
The fracturing produces cavities in which the oil can collect while 
the softer unfractured shales adjacent remain more or less impervious 
to the oil. 
