GEOLOGY OF THE COAST SYSTEM OF MOUNTAINS. 9 
comprizes about 2,000 feet of sandstones, portions of which are green sands, together with 
some shales. These make up the Martinez group. Its distribution, so far as known at 
present, is quite limited and is confined to the middle Coast Ranges on their eastern side, 
between Clear Lake and Mount Diablo. The upper division of the Katquines is known 
as the Tejon group, and comprizes also about 2,000 feet of sandstones, often somewhat 
ferruginous and weathering reddish, but very strongly cemented. The Tejon strata 
are apparently conformable upon the Martinez, but the sharp contrast in the faunal 
contents of the two groups suggests rather widespread physiographic changes at the 
close of the Martinez which may be regarded as indicative of unconformity. The Tejon 
strata are much more widely distributed than the Martinez, a fact which suggests the 
enlargement of the Karquines basin of deposition by subsidence of the coast during the 
progress of Eocene time. 
The next succeeding group of rocks, belonging to the Oligocene division of the Ter- 
tiary, has been named the San Lorenzo Formation.’ It is known in Santa Cruz County, 
where it attains a thickness of 2,300 feet, made up chiefly of gray shales and fine sand- 
stones. Its stratigraphic relations to the Tejon are not yet known, but its fauna is said 
by Arnold to contain many species which appear to be closely related to Tejon forms. 
It may thus be considered as following the Tejon conformably. It is in certain sections 
known to be unconformable beneath the oldest formation of the Miocene, known as the 
Vaqueros Sandstone, indicating that after the deposition of the San Lorenzo formation, 
the region of the Coast Ranges was disturbed and uplifted into the zone of erosion; 
and the following facts regarding the transgression of the Miocene Sea indicate that this 
uplift was a very extensive one. Such an uplift in time immediately preceding the 
Miocene is further indicative of a much closer relationship between the San Lorenzo 
and the Tejon than between the former and the Monterey. 
Miocene time in the Coast Range region was characterized by a progressive subsi- 
dence with oscillations of the coast. The Miocene sea gradually transgrest the conti- 
nental margin from the southwest, and as it did so spread a formation of arkose sands and 
conglomerates over the greater part of the southern Coast Ranges. This was followed, 
as the water deepened with progressive subsidence, by a remarkable deposit of bitumi- 
nous shales. These shales are usually whitish or cream-colored, tho often of a purplish 
or other dark tint, and may be either of a soft chalky consistency, or opaline, or hard and 
flinty. It is thruout an essentially siliceous formation and is largely diatomaceous in 
character, tho more or less admixt with volcanic pumiceous ash. In some portions 
the ash is a prominent constituent, and in San Luis Obispo County there is a deposit 
ageregating about 1,000 feet in thickness of well-stratified volcanic tuff and agglomerate. 
In San Mateo County there are basalts which were erupted at this period. Interstratified 
with these siliceous shales, thin beds of more or less ferruginous and somewhat mag- 
nesian limestones are by no means uncommon. They are, however, lenticular or non- 
persistent, and are of a very compact texture and usually nonfossiliferous. There are 
also in some places thin but persistent beds of a peculiar, very hard, fine-grained, light- 
colored sandstone intercalated with the shales. In the southern portion of the Coast 
Ranges the bituminous shales accumulated to a thickness of several thousand feet, but 
in the middle Coast Ranges, in the vicinity of the Bay of San Francisco, the Miocene sea 
was characterized by an oscillatory or to-and-fro migration of its eastern shore line, due 
to alternate uplift and subsidence of the coast, quite analogous to that described for 
the Franciscan period. This gave rise to an alternation of shallow water in which sand- 
stones were deposited, and deep water in which siliceous ooze accumulated with but 
little admixture of terrigenous material. We have thus in the territory between Mount 
Diablo and the Bay of San Francisco an alternation of four formations of bituminous 

1 Arnold, U.S. G, 8S. Professional Paper No. 47, p. 16. 
