General Topographical Structure.—In
the extreme northwestern part of the State the general structure of
the Sierra Nevada prevails—an axial mass of granite associated with
hard, crystalline rocks forming a high range. Coming south, and into
the northern part of the Coast Range belt (west of Trinity and Klamath
rivers), the structure is modified, the granite disappears, the old
crystalline rocks are replaced by newer and softer strata, the
elevations decrease, and the ranges become more numerous and
indistinct, although as far as Clear Lake there is still one dominating
range, quite well defined and parallel with the coast line.
South
of Clear Lake the ranges are very much intermixed, the hills are lower
and more rolling, and the valleys are wider. The average elevation
decreases steadily to the vicinity of San Francisco Bay, the point of
maximum depression.
Further
south, to the bay of Monterey, there are two distinct ranges, that of
Mount Diablo on the east and the Santa Cruz mountains on the west, with
the southern part of the bay of San Francisco and the important valley
, of Santa Clara between.
South
of the bay of Monterey, as far as San Luis Obispo County, the country
becomes more mountainous and confused. The general elevation increases
and the valleys become narrow and small. There can be distinguished,
however, three equally plain systems: the continuation of the Mount
Diablo range, east of the San Benito River; the Gavilan range
(connecting with the last at its southern extremity), between the San
Benito and Salinas rivers; and the Palo Escrito hills and Santa Lucia
range on the west.
From
the northern boundary of the belt to the south of this region the
ranges have, in general, a sufficiently well marked northwest and
southeast direction, as seen*' by the courses of the principal streams.
Here, however, a change occurs, the coast line, and with it the mountain
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