12 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
deprest over 1,000 feet. The geological record for the latter region is in terms of degrada- 
tion rather than of deposition; and such deposits as have here accumulated are referable 
wholly to fluviatile, lacustral, and volcanic agencies. It is thus apparent that from 
the point of view of the stability of the earth’s crust, the Coast Range region has been 
very much more mobile than the Sierra Nevada. The long comparative stability of the 
latter was, it is true, interrupted at the close of the Tertiary by a very notable uplift, 
whereby it took the form of a tilted orographic block of great size and remarkable unity ; 
but this does not detract from the force of the contrast. The difference in behavior 
with respect to crustal stability makes the Coast Ranges a totally distinct and different 
geological province from the Sierra Nevada. 
- Between these two strongly contrasted provinces lies the great valley of California, 
one of the very notable geomorphic features of the continent. This valley is but one of 
a long series of similar depressions which lie along the western border of the North Ameri- 
can continent, between the coastal uplands and the western edge of the continental 
plateau. In the north it has its probable analogues in Hecate Strait, the Gulf of Georgia, 
Puget Sound, the Willamette Valley, the Ashland Valley, and the depression between the 
Sierra Nevada and the Klamath Mountains. On the south we see its analogues in the 
Colorado Desert, the Gulf of California, and in the valley which lies between the southern 
border of the central plateau of Mexico and the Sierra Madre del Sur. In the Californian 
region we must interpret the axial line of this depression as a tectonic hinge, upon which 
the mobile coastal region has swung in a vertical sense upon the edge of the interior 
plateau, here represented by the Sierra Nevada. Whether this tectonic hinge is a more 
or less flexible zone upon which movement has taken place without rupture, or whether 
it represents a zone of dislocation, is not clear; but that differential movement has 
taken place along the valley line is one of the salient facts in the geological history of 
California. 
STRUCTURE. 
A detailed account of the structure of the Coast System would involve a discrimination 
between features referable to the different orogenic movements which have affected 
the region at various periods of its history. Owing to this succession of movements, 
new structures have been superimposed upon older structures, or upon remnants of 
older structures, so often that the resultant effect is extremely complicated and not only 
difficult to unravel but difficult to state or describe in any simple way. In this sum- 
mary review of the subject, no such detailed discrimination will be attempted. The 
only effort will be to call attention to the salient features, which are for the most part 
referable to the orogenic movements of later Tertiary and post-Tertiary time. 
Marginal Features. — In a consideration of the structural features of the Coast System, 
its marginal lines on the east and west first claim attention. The eastern slope of the 
Coast Ranges rises from the floor of the Great Valley much more abruptly in general 
than does the western slope of the Sierra Nevada from the same valley floor. Turner ? 
has suggested that the Great Valley east of the Coast Ranges is determined by a fault. 
There is some warrant for this view and it is certainly true in part. The very precipi- 
tous mountain front which rises from the valley at its southern end is without doubt a 
degraded fault-scarp, tho whether or not this fault or a series of similar faults can be 
followed along the edge of the mountains to their northern end is questionable. It is, 
however, safe to say that the eastern margin of the Coast Ranges represents a line of acute 
deformation, with the probability of that deformation having taken the form of faults in 
certain places. No one has yet made a sufficiently careful study of the question to make 
a more precise statement possible. In general, this line of acute deformation is not 

1 Am. Geologist, vol. x1, p. 248. 
