42 SANTA MARIA OIL DISTRICT, CALIFORNIA. 
those observed under the microscope being only fragments, and this 
makes it probable that others are still more fragmentary and in a 
state of complete demolition. The likelihood, therefore, is that a 
greater proportion of the pure shale than 5 to 10 per cent, as roughly 
estimated by Mr. Keeley on the basis of visible forms, is composed 
of silica derived from diatoms. Radiolaria, which are scattered 
through the shale sparingly, have contributed somewhat to the organic 
silica. Whatever conclusion one should come to would apply to almost 
all of the soft unaltered shale of the siliceous type in the Monterey of 
the Santa Maria district, as this type is fairly constant. Locally it is 
varied by an increased proportion of argillaceous material, causing a 
greater similarity in appearance to ordinary clay shale, or by the 
presence of lime; but diatoms are visible in practically all of it and the 
general conditions of deposition seem to have been the same through¬ 
out. The conclusion is reached elsewhere (p. 47) that the same prob¬ 
able origin may be assigned to all the siliceous shales of the Monterey, 
whether hard or soft—or, in other words, to by far the greater part of 
the formation. 
The list of organic constituents of the shale is by no means exhausted 
by the small organisms of low order so far mentioned. Another 
important source of silica lies in the abundant sponge spicules, which 
are only second in number to the diatoms and which are scattered 
with remarkable persistency throughout the shale. In the slightly 
gritty beds of soft shale, which occur sparingly, these spicules even 
predominate over the diatoms, being possibly the cause of the grit- 
tiness. They seem also to be less easily obliterated than the fragile 
diatom shells and to have been preserved in places where slight alter¬ 
ation of the rock has destroyed the latter. One of the commonest 
and most characteristic features of both the unmetamorphosed sili¬ 
ceous and the calcareous shales is the presence of scales of fish, show¬ 
ing that fish remains found their way to the ooze at the ocean bottom 
in greater or less abundance. Locally the bones and nearly complete 
skeletons are also to be found. Delicate mollusk shells, usually of 
small size, are gathered thickly in some places in the Monterey shale, 
and at such points may be considered as constituting an appreciable 
proportion of the total volume of the deposit. As a rule they are 
crushed and poorly preserved, a fact that lends weight to the theory 
that a large part of the diatom frustules also have been destroyed. 
But mollusks are rare in the formation as a whole. Crab shells and 
claws are occasionally found, usually not whole but in small pieces, 
as if they had been subjected to conditions favorable to their destruc¬ 
tion before coming to rest. Seaweed impressions are not rare. In 
addition to organic remains of these kinds, the shales, especially the 
