BOTANY. 11 



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 considera- 

 tion has been given somewhat in detail in the accompanying geological report. It will, therefore, 

 not be necessary to repeat what has already been said upon the subject. The general charac- 

 teristics 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 com- 

 pared 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 Orford — 

 localities 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, 54°. 88 ; for Fort Humboldt, 52°. 80, and, for Fort Orford, 53°. 62. At 

 San Francisco, the mean temperature for January being 49°. 60 ; for July, 57°. 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, whicli is more den.se towards 

 the north. Immediately north of San Francisco the forest is composed almost exclusively of the 

 red-wood, (Sequoia sempervirens,) and is limited to the valleys, especially such as open towards 

 the coast. Joing 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 



o Medical Statistics U. S. A. 



