214 Dr James Hector on the Physical Features of the 
formed of plastic and sandy clays much impregnated with 
sulphates, and yielding little or no soil that can support vege- 
tation. In some localities sandstones prevail, which disinte- 
grate with facility, and give rise to immense wastes of blown 
sand, that are continually, though slowly, travelling before 
the prevailing winds. 
The whole prairie-slope of the continent is divided into two 
regions by a low watershed, which traverses it from east to 
west, nearly following. the political boundary, which is the 
49th parallel of latitude, and throwing off the drainage, 
south to the Gulf of Mexico, and north to the Arctic Ocean and 
Hudson Bay. This watershed is very indistinctly marked, and 
has been formed entirely by denudation of the soft strata, being 
quite unconnected with any disturbance of the rocky frame- 
work of the basin. The prairies are traversed by several large 
rivers, but, excepting the sudden carrying off of the surface 
water when the snows melt in spring, these rivers can hardly 
be said to drain the country through which they flow, as their 
waters are derived throughout the greater part of the year 
from the Rocky Mountains; and the excess of evaporation 
over the rain-fall is shown by the drying up in summer of 
those streams that do not rise in the mountains or from the 
swampy region along their eastern base. 
There is one physical feature, which has an important bear- 
ing on the question of botanical distribution, the effect of which 
will be alluded to in an after part of this paper. This is the 
manner in which the plains are traversed by deep and narrow 
valleys, with abrupt slopes, and cut into the otherwise level, 
or only slightly undulating surface of the country. Those 
troughlike valleys, by favouring variety in the exposure, soil, 
and drainage, have afforded continuous lines for the migration. 
and diffusion of plants through tracts of country, where the 
prevailing conditions are quite unsuited for their support. 
The plains rise gently as the Rocky Mountains are ap- 
proached, and at their western limit have an altitude of 4000 
feet above the sea level. With only a very narrow intervening 
belt of hilly country, the mountains rise almost abruptly from 
the plains, and present lofty precipices that frown like battle- 
ments over the level country to the eastward. 
