FORMATION AND EXTENT OF DESERTS. I03 
in favor of the view that the present is a state of maximum aridity, which 
may become still more accentuated. Small areas may become suitable 
to desert types of vegetation by highly localized causes, such asan accumu- 
lation of salts brought to the surface by the capillary action of introduced 
water. 

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Fic. 6.—Western America, showing conceptions of American deserts currentin 1859. 
Copied from Warren’s Physical Geography, published in 1859. (Reprinted 
from Publication No. 6.) 
The chief cause of deserts, however, may be ascribed to the change from 
lower temperatures and accompanying greater supply of moisture in glacial 
times to the comparative aridity of the present. This change, however, 
may not be assumed to have been one constantly moving in the same 
