REGIONAL DISTRIBUTION 



511 



and is merely accelerated by the coming of warm weather in 

 the spring. Most herbaceous plants cease to grow soon after 

 the summer drought has set in, so that the face of the country 

 is parched and in places seems almost lifeless. 



The northern portion of the Pacific slope, through the 

 states of Washington and Oregon, is divided by the Cascade 

 Mountains into a comparatively moist western region and a 



Fig. 381. Annual rainfall of the United States 



Darkest shade, over 80 inches; lighter vertical lines, from 40 inches to 80 inches; 



horizontal lines, from 20 inches to 40 inches ; hlank, from 10 inches to 20 inches ; 



dotted, less than 10 inches. After W. M. Davis, " Physical Geography " 



much drier (sometimes semi-desert) eastern region. This is 

 due to the fact that the Cascade Range, running in a general 

 north-and-south direction, causes the precipitation, in the form 

 of rain or snow, of most of the moisture brought from the 

 Pacific by the southwest winds. These mountains are heav- 

 ily timbered, especially along their western slope, and the 

 dense forests abound in such valuable coniferous trees as the 

 western white pine,i several species of true fir,^ and the west- 

 ern hemlock.^ West of the Cascades tliree qf the principal 



Pinus moniicola- 



2 Abies. 



3 Tsuga heterophylla. 



