6 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
GEOLOGICAL HISTORY. 
The Coast Ranges of California have had a long and varied geological history. Their 
structure is complex and the sequence of formations differs at different points. Several 
of the more important groups of sedimentary rocks contain, so far as known, but few 
fossils or none at all. Only in recent years have the topographic maps necessary for 
an adequate study of the stratigraphy and structure of the region become available, 
and then only for limited areas. Nevertheless the general outlines of the geology of 
the Coast Ranges are known, and in some of the localities which have been topograph- 
ically mapped, a considerable body of detailed information is at hand. 
The oldest sedimentary rocks of the Coast Ranges are of unknown age. They com- 
prize impure and somewhat magnesian limestone, quartzites, and various crystalline 
schists. The limestones are usually in the form of coarse marble varying in color from 
dark gray to white and containing frequently some graphite and less commonly lime 
silicate. The quartzites are thoroly indurated, as a rule, sometimes to the extent of be- 
ing vitreous, and usually show well-marked stratification. The schists have as yet been 
little studied, and no adequate observations upon their character in detail have been 
put on record. They are known, however, to comprize both micaceous and hornblendic 
varieties. 
These marbles, quartzites, and crystalline schists are known only in more or less 
fragmentary form, associated with considerable bodies of granitic rocks which have 
invaded them as batholiths. The most common occurrence of the marbles, quartzites, 
and schists is in the form of limited belts and isolated patches embedded in the granitic 
rocks, or in limited areas flanking the margins of the batholiths, and showing evidence of 
contact metamorphism. It is evident in most cases, and is probably generally true, 
that the granite of the Coast Ranges is of later date than the metamorphic sedimentary 
rocks-associated with them. While the age of these pregranitic sedimentary formations 
is at present unknown, the age of the granite is suggested by its seeming identity with the 
granite of the Sierra Nevada. The latter is a vast batholith known to be intrusive 
in Paleozoic and Mesozoic strata as late as the Upper Jurassic. This granite has been 
followed thru the Sierra Nevada to Tehachapi and Tejon Pass, where the range curves 
sharply around and passes into the Coast Ranges. Passing northerly thru the Coast 
Ranges, granite identical in character with that of the Sierra Nevada, and carrying 
identical inclusions of older sedimentary rocks, is traceable in more or less extensive 
areas from the upper reaches of the Cuyama River to Bodega Head on the coast north of 
the Golden Gate. It thus seems probable that the granite of the Coast Ranges, like 
that of the Sierra Nevada, is of late Jurassic or post-Jurassic age. The granitic rocks of 
the Coast Ranges, together with the pregranitic rocks into which they are irruptive, 
constitute a complex which is thus the probable analogue of the Bedrock Complex of the 
Sierra Nevada. 
This Coast Range Complex was subjected to vigorous erosion and then submerged to 
serve as the sea floor upon which the series of rocks known as the Franciscan was 
deposited. This series consists for the most part of medium coarse, dark gray or 
greenish-gray sandstone, strongly indurated, with subordinate shales and conglomerates. 
Intercalated with these sandstones are important horizons of foraminiferal limestone 
and radiolarian chert and admixtures of volcanic rocks, chiefly basaltic in character. 
In the vicinity of the Bay of San Francisco, whére the series is best known, it falls into 
seven stratigraphic divisions. These are in ascending order: . 
(1) A group of arkose sandstones with some conglomerates and shales reposing uncon- 
formably upon the Montara granite and with an aggregate thickness of about 800 feet. 
(2) A formation of light-gray, very compact and fine-textured foraminiferal lime- 
stone ranging in thickness from about 60 to possibly a few hundred feet. 
