226 W. A. CANNON 



midst of the mountains, howev^er, as at Boulder Creek, in the Big 

 Basin, the amount of the annual precipitation may reach 95.76 

 inches, although the mean is 55.81 inches. 



A very important climatic feature of the coast regions as a whole 

 is the summer fog, although there appear to be little data on this 

 subject. Fogs may cover the shore for days at a time and reach 

 inland so far as 20 miles. They operate at once to lessen the 

 evaporation and also, in forests, as will be mentioned again, to 

 increase the precipitation through condensation by the trees. 



The mountains of the southern portion of the state have, for 

 parallel altitudes, less rainfall and higher temperatures than the 

 other mountain groups mentioned. Thus with high temperatures, 

 little rain, and no fogs, the southern mountains are relatively arid. 

 Hence, here we find a large development of xerophytic perennials, 

 and especially extensive areas covered with dwarf-forests, the 

 chaparral. 



THE VALLEYS 



A second extensive physiographic province is made up of the 

 larger valleys, which, holding many features in common, may, for 

 convenience, be considered together. 



The most important valley is the vast interior one, constituted 

 by the Sacramento valley to the north and the San Joaquin ^-alley, 

 to the south. This, the Great Valley, is relatively arid. Nearer 

 the coast, and separated from it as well as from the Great \"alley 

 by portions of the coast ranges, are also several important valleys. 

 In these the climate, although not so dry as in the Great Valley, 

 is nevertheless surprisingly arid. For example at Gilroy, in the 

 Santa Clara valley, the precipitation extremes are 7.98 and 38.67 

 inches, which may be taken as being fairly representative of the 

 rainfall of such valleys. One of the most striking features of the 

 rainfall of the valleys, particularly those near the coast, is the 

 great variation in amount which accompanies a relatively small 

 distance between stations. Thus a difference of 1 to 2 may 

 accompany a horizontal difference of stations not exceeding 4 

 miles. This feature is always in relation to the mountains, as 

 might be supposed. Another characteristic of the valleys, partic- 



