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. The rain-bearing clouds coming 
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 
* 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. 
