12 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 



in altitude. Farther details in regard to the topographical character of this 

 part of the western slope of the Sierra may more conveniently be added 

 after a brief notice of the geological features of the range, since it is hardly 

 possible to understand the very considerable differences between the sur- 

 face features of different portions of the region in question, without having 

 previously learned something of its geological structure. 



Something should, however, here be introduced with reference to the 

 amount and distribution of the rain-fall on the western slope of the Sierra 

 Nevada, and on the borders of the Great Valley in general, and especially in 

 illustration of the manner in which precipitation is influenced by the topog- 

 raphy, since these are questions of much interest in connection with the 

 subjects to be discussed farther on in this volume. It is necessary in the 

 first place to lament the almost total want of accurate statistics of precipi- 

 tation, especially in the mountain regions. Some data may, however, be 

 given of a comparative kind ; and it must be borne in mind that the actual 

 amount of rain falling from year to year is extremely capricious, so that 

 many years' observations would be required in order to obtain an accurate 

 general average result.* 



The dominant fact in regard to California precipitation is, that it is almost 

 exclusively limited to less than half the year. In the latitude of 38°, for all 

 practical purposes, there is no rain during the six and a half months be- 

 ginning with the first of April. Almost half the total rain-fall of the year 

 occurs in the two months, November and December. As we go north from 

 the latitude of San Francisco there is a tendency to a slight increase of 

 total precipitation and to a summer rain-fall ; but, as a general rule, there 

 is a marked uniformity in the characteristics of the climate from one end of 

 the Great Valley to the other ; but in any cross section of the valley and 

 the ranges on either side, the influence of the topographical features of the 

 country on the rain-fall is very marked, 

 almost exclusively from the south and southwest, the west slopes of the 

 Coast Ranges receive a considerable share of the moisture which they 

 contain, and the eastern flanks of those mountains are very much drier. 

 An inspection of the map will show this at once, accurate statistics being 

 almost wanting. It will be noticed that on the western side of the Great 



The rain-bearing clouds coming 



* At San Francisco the range in twenty-two years was between 49.3 and 7.0 inches ; at Sacramento, in 

 twenty-four years, between 36.4 and 4.7 inches ; at Clear Lake, in six years, between 66.7 and 16.2 inches ; 

 at San Diego, in twenty-two years, between 14.8 and 4.5 inches. 



