GEOLOGY OF THE COAST SYSTEM OF MOUNTAINS, 

DEFINITIONS. 
In common with many other mountainous tracts the world over, the Coast System has 
limits which are difficult of precise definition. The criteria which serve to discriminate 
one tract from another are various and have different values in different cases. Any 
attempt at precise definition must be more or less arbitrary. An outline of the extent 
and subdivisions of the system will, however, be presented in summary fashion. 
On the north the Coast System extends to the northern end of Humboldt County, and 
in that county and in southern Trinity County the last typical ridge is South Fork 
Mountain. This is a remarkably linear ridge beginning near the coast and extending 
with a northwest-southeast course to the vicinity of North Yallo Bally Mountain. Be- 
yond South Fork Mountain to the northeast lie the Klamath Mountains, a group more 
nearly allied in the history of its uplift and in its constituent rocks to the Sierra Nevada 
than to the Coast Range. On the south the Coast System is sometimes regarded as 
ending in Santa Barbara County; and the mountains of Southern California, thence 
east-southeast and south to the Mexican boundary, are regarded as a distinct system, 
being viewed as a northerly prolongation of the orographic axis of the peninsula of 
Lower California. The chief consideration favoring this distinction is the change in 
trend of the mountain ridges, which becomes apparent just north of the Santa Barbara 
Channel. Other facts favor this discrimination, such as the prevailing absence of the 
Franciscan formations in the mountains of southern California and the greater abun- 
dance of granitic rocks; but more especially the greater incisiveness of the structural 
lines, indicating, on the whole, more intense orogenic action. But these considerations 
are largely offset by the unmistakable continuity of the tectonic lines of the northern 
ranges into the mountains of southern California, and by the fact that the movements 
to which their larger features are due date from the close of the Tertiary. It would 
seem, therefore, that there is sufficient unity of character in these coastal mountains, in 
spite of their change of trend, to warrant their being classed as the Coast System from 
South Fork Mountain south to the Mexican boundary and beyond. ‘That term may 
be used in a comprehensive sense, significant of the genetic and structural unity which 
runs thru them. 
It will nevertheless be very convenient to recognize three subdivisions of the Coast 
System thus outlined. The first of these subdivisions extends from South Fork Moun- 
tain on the north to the Valley of the Cuyama River on the south, and may, in accord- 
ance with popular usage, be referred to simply as the Coast Ranges, the term “system” 
being used only when it is intended to express the more comprehensive view. The second 
subdivision is a broad chain extending from Santa Barbara County to the far side of the 
Colorado desert with a general trend of west northwest-east southeast, and including the 
San Rafael, Santa Ynez, Santa Susannah, Santa Monica, San Gabriel, and San Ber- 
nardino Ranges, and also, perhaps, the Chocolate Range. This chain is sometimes 
referred to as the Sierra Madre, tho the full application of the term in popular usage is not 
clear. The third subdivision embraces the mountainous country south and southeast 
of the valley of Southern California, the principal ranges of which are the Santa Ana and 
the San Jacinto. These have the northwest-southeast trend of the Coast Ranges and, 
in accordance with the suggestion of some of the earlier writers on Californian geology, 
may be referred to as the Peninsular chain. 
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