314 B. N. A. BOUNDARY COMMISSION. 

feathered seeds, capable of long aerial flights. The various species of — 
poplars and willows may be specially mentioned, and it is a fact which 
any one can observe, that the seeds of the birch are often found far from 
woods, drifting over the open country with the snow. There are pro- 
bably few large areas of the plains which are not every year more or less 
thickly seeded with, at least, willow and poplar. 
737. The excess of moisture in the soil, during some months, has 
also been brought forward, but certainly cannot apply to any consid- 
erable area west of the Mississippi and Red River. The dryness of the 
soil and atmosphere, as a cause of treelessness, has deservedly received 
more attention, and is undoubtedly a true cause over great areas. — Prof. 
Dana upholds this theory, and has very clearly shown the relation of 
prairies and regions of drought.* The seeds of trees and shrubs, and of 
all plants, except those suited to withstand the greatest desiccation, 
though they may be scattered broadcast, and may germinate in the 
spring, are, in many districts, within a few months killed by drought; 
or, even where the moisture may be sufficient for their growth, it may 
not be enough to afford protection from fire. In these regions a patch 
of forest once destroyed does not renew itself, and can only be restored 
by care and cultivation. 
738. The area of the interior plains can never, since the complete ele- 
vation of the Rocky Mountains, have been a very moist one. The winds 
with their prevailing direction toward the east, in passing over three to 
five hundred miles of serried peaks and ridges, loose a great proportion 
of their moisture; and a great part of the rain which should be spread 
over the entire west, is expended on a comparatively narrow strip of 
country along the coast. All the facts at present known, however, 
~ strengthened also by analogy with other countries, appear to show that 
the dryness of the plains is in great measure intensified by their treeless- 
ness. The very dryness of the atmosphere and soil, is thus brought about, 
to a great extent, by the destruction of forest areas by fire, and the effect 
is a cumulative one. When therefore it is asked—why, if the prairies 
were originally in great part forest-clad, they do not now show any ten- 
dency to revert to that condition, or even to renew the growth of trees 
over lately burned areas? It may be answered: that while the area of - 
prairie was still quite small, there would be an immediate effort in that 
direction, but that as the prairie with its hot, parched, and rain-repelling 
* See especially Am, Jour, Sci. and Arts, vol. xL., p. 297, 
