302 J. D. Dana on the Origin of Prairies. 
gently distilling mists keep the ground thoroughly wet, and lose 
much less by running off. Hence, the regions of the greatest 
fall of moisture on a rain-chart do not necessarily correspond 
with those of the greatest extent of forests. The area of even 
‘45 inches” over the United States is not necessarily, therefore, 
an area throughout of forests. : 
gain, the effects of the falls of moisture in the shape of rain 
mists, &c. depend much on the heat of the region, because of 
the consequent difference in evaporation. With a cool climate, 
like that of the coast of Oregon, there is comparatively little 
evaporation; and the moisture lies long in the soil. But the 
St ane re 
_ America, and has been illustrated by reference to facts in the | 
_ States of New York and Maine. : : 
_,, tet, where there is only a moderate supply of moisture, the 
unfit for trees. When, on the first recovery of the land, grass 
takes possession of the surface, forests, in moist regions, ma 
gr wets and di ss it. 
Jrass-regions may encroach on forest-regions, or the reverst, 2 
recording to the dryness or moistness of the country.— W here the 
ground is permanently moist, the meadow seldom makes ne 
the sop 
the old forest. covers dry soil, the grass, or its precurs- 
‘ts, may gradually make an undergrowth among 
the nearer trees, and us commence encroachment. Droughts 
