Reports of explorations and surveys, to ascertain the most practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean: Vol. 6, Pt. 3

BOTANY.
perature, being greatest toward the coast and northward, less on the southern than northern
coast, and least in the interior.
Geological structure.-The geology of that portion of California and Oregon under consideration has been given somewhat in detail in tie accompanying geological report. It will, therefore,
not be necessary to repeat what has already been said upon the subject. The general characteristics of the geology of this region may, however, be said to consist in the great prevalence
of comparatively recent volcanic rocks, and of a soil derived from their decomposition, in all
the mountain ranges. In the valleys, and on the flanks of the coast mountains, tertiary strata,
generally of sandstones, constitute the sub-structure, and give character to the soil. As compared with the valley of the Mississippi, and the more northern of the eastern States, the Pacific
coast is much more recent, the greater part of it having emerged from the ocean since the middle
of the tertiary period.
LOCAL BOTANY.
COAST MOUNTAINS.
Climate.-The climate of the immediate shore of the Pacific is quite unlike that of the interior,
a difference dependent upon its proximity to the evaporating surface of the ocean, the ocean
currents, and the prevalent winds. It is much more uniform, cooler, and more moist. The
uniformity of temperature which it exhibits is due, unquestionably, to the equalizing influence
of the nearly constant temperature of the wide expanse of open sea which lies adjacent to it,
and over which the winds blow inland, almost without intermission, throughout the year.
These winds, which are loaded with moisture, in summer usually blow from the northwest or
west; in winter, from the southwest or west. The temperature upon the coast scarcely ever
rises to what is called summer heat, and is never so high as to render other than woollen clothing
comfortable. From observations* made at San Francisco, Fort Humboldt, and Fort Orfordlocalities which may be supposed to present fair samples of the climate of the coast-we find
that the average temperature for the year, taking the mean of the observations of several years,
is, for San Francisco, 540.88; for Fort Humboldt, 52~.80, and, for Fort Orford, 530.62. At
San Francisco, the mean temperature for January being 49~.60; for July, 57~0.90. At Fort
Humboldt, for January, in 1854, 40~.83; for July, 56~.71. At Fort Orford, for January,
48~.38; for July, 59~.73. The low summer temperature of the Pacific coast of the North
American continent, like that of the Atlantic, seems to be due to the Arctic ocean current,
which constantly sweeps it. The amount of rain falling at the same points, as indicated by the
same tables, is, at San Francisco, 23.59 inches; at Fort Orford, 68.52.
Vegetation.-The coast mountains, throughout nearly the entire distance from San Francisco
to the mouth of the Columbia, are covered by a continuous forest, which is more dense towards
the north. Immediately north of San Francisco the forest is composed almost exclusively of the
red-wood, (Sequoia sempervireno,) and is limited to the valleys, especially such as open towards
the coast.'oing northward the trees become more numerous, and with the red-wood are found
the sugar and yellow pine, (P. Lambertiana and P. ponderosa.) In the vicinity of Crescent City
these trees combine to form one of the most magnificent forests in the world-the red-wood and
the sugar pine attaining nearly equal gigantic dimensions; trees of both species being not
uncommon 12 to 15 feet in diameter, and 300 in height. Near the line of 42~ a marked change
is noticed in the trees which constitute the forest, which is even, perhaps, more dense than
e Medial Statistic U. S. A.
11