136 
GEOLOGY. 
The mountains having this transverse direction do not, however, end abruptly at the end of the 
Sierra Nevada, hut preserve their direction beyond it, towards Point Conception on the coast, thus 
crossing the southern ends of the Coast Mountains also. The chain is rendered distinct from 
the Coast Mountains, not only by its direction, but by its geological structure, it being granitic 
and metamorphic, while the r anges of the Coast Mountains are chiefly of more modern and sedi¬ 
mentary strata. The limits of the Bernardino Sierra, ou the west, may thus be considered to 
be at the termination of the high granitic ridges north of Santa Barbara, although, topographi¬ 
cally, it is believed, to be prolonged nearly to Point Conception. As, however, the topography 
of that region is but little known, the precise limits of the chain cannot be assigned, its rela¬ 
tions to the terminal ranges of the Coast Mountains not being determined. The whole length 
of the chain from San Bernardino to its eastern end is between 170 and 200 miles, it being 
about 200 from Point Conception to the mountain. The length, along the northern or interior 
slope of the chain, from the end of the Sierra Nevada, eastward, is about 120 miles. 
The mean or average direction may be considered as west 20° north, east 20° south; this 
being the direction of that part of the chain between San Bernardino Mountain and the end of 
the Sierra Nevada. 
These mountains, east of the Sierra Nevada, are the only wall of separation between the 
elevated surface of the Great Basin and the sea, being, in fact, the southern rim of the Basin, 
and holding the same relation to it as the Sierra Nevada on the west. There is no great range 
or chain of heights between the chain and the Pacific corresponding to the Coast Mountains west 
of the Sierra Nevada. Low hills and a line of intrusive rocks are found, but they do not form a 
prominent topographical feature, or exert a marked influence on the climate. The sudden and 
remarkable change in the direction of the coast-line found at Point Conception is produced by 
this line of mountains. From a general northwest and southeast trend of the coast, parallel 
with the Sierra Nevada and Coast Mountains north of Point Conception, the direction becomes, 
on the south, nearly east and west, conforming very nearly to the base of the chain. The slopes 
of the mountains thus face the south, and the climate of the region differs very greatly from 
that north of the cape. In the vicinity of Santa Barbara, a range rises directly up from the 
water, and is not flanked by a slope ; the shore-line is thus very clearly defined, and is nearly 
east and west in its direction. East of San Buenaventura, on the coast, the shore-line is no 
longer coincident with the base of the ranges, but trends more to the south, the base of the 
mountains continuing inland with nearly the same direction, leaving a broad slope between it 
and the beach. 
The difference of altitude between the upper margin of this seaward slope and the margin of 
the slope of the Great Basin on the other side of the chain is worthy of special notice. It is a 
marked feature in the relief of the region. The foot of the ranges at several points on the south 
side varies from 1,000 to 1,500 feet in elevation, while a corresponding line on the side of the 
Great Basin has a mean elevation of 3,000 feet or more. 
The mean elevation of the chain can be only approximately given, but the numerous surveys 
of the passes by the Expedition, and the reconnoissance along the northern slope, permit a close 
estimate to be made. The number of passes show the broken character of the chain, and the 
plotted results of the Survey exhibit an overlapping position of the ranges. The passes, with 
scarcely an exception, are oblique to the general trend of the chain, and are coincident with 
some marked geological feature, forming long valleys extending between the ranges. The 
