Page:California a guide to the Golden state-WPA-1939.djvu/50

14 along the California coast during the greater part of the summer. So dense and persistent are these coastal fogs that great areas south of San Francisco devoted to truck gardening require no other moisture during the summer months. The Coast redwood, as well as the plants which grow beneath it, is watered by the fog that condenses on its foliage.

In the southern part of the Central Valley, temperatures are often very high. Although the annual mean temperature of the inland is 64°, in Fresno and Bakersfield the mercury occasionally soars above 110°. The desert temperatures are still higher, the summer mean in Fort Yuma being 92°. In Death Valley, the average daily minimum for July, the hottest month, is 87.6°. But on July 10, 1913, it reached 134°, only slightly less than the highest natural air temperature hitherto accurately measured. In the mountain regions, on the other hand, summer temperatures are much lower and the winters are very severe. At the top of Mount Lassen, in the winter of 1932-33, the mercury registered 56° below zero.

Annual rainfall in the State varies from about 80 inches at Crescent City in the extreme north to about 10 inches at San Diego in the extreme south. At San Francisco the annual average is about 22 inches; at Los Angeles, 16 inches. The northern half of the Sierra and the northwest counties are covered by a heavy rain belt. In the high mountains precipitation, almost entirely in the form of snow, provides most of the run-off which supplies water for the cities and for irrigation. In the high Sierra the average annual snowfall is from 300 to 400 inches. At Tamarack in Alpine County the snowfall during the winter of 1906-7 was 844 inches, the greatest ever recorded for a single season anywhere in the United States. The belt of heavy rain shades off to a region of lighter rainfall which covers all the rest of the State except Inyo, Kern, San Bernardino, and Imperial Counties, and the eastern portion of Riverside County. The limits of this third region may, in dry years, include all of the State below Fresno and the entire Central Valley.

In general, rains occur in California only in the months from October to May. Even during this rainy season, the valley districts usually have no more than from 25 to 35 rainy days. Throughout the rest of the year excursions may be planned everywhere, except in some parts of the mountains, with considerable confidence that no rain will dampen the occasion.

GEOLOGY AND PALEONTOLOGY

Every major division of geologic time is represented in California by marine sediments, and many of them by continental deposits as well.