6 ‘MOUNTAIN SYSTEMS. 
From most portions of this interior mountain belt, the waters have been able to force their 
barriers and escape to the ocean. The valleys thus drained are, those of the southern 
tributaries of the Upper Missouri, that of the North Fork of the Platte, and its tributary the 
Sweet Water, between the first and second systems; that of the U,per Rio Grande del Norte, 
in the first system ; that of the Great Colorado of the West and its tributaries, between the first 
and second systems ; those of the waters of the Bay of San Francisco and of the Klamath river, 
in the third system; and that of the Columbia river and its tributaries, between the second 
and third systems. Some of these streams, as well as others in the enclosed basins, have in 
places worn for themselves, through the solid rock, the most stupendous chasms or cafions, 
often 2,000 feet in vertical height, many of which it is imposs.ble to follow or to cross. 
The position of this belt of mountain region, stretching from north to south, gives rise to a 
peculiarity of climate and soil. Fertility depends principally upon the degree of temperature 
and amount of moisture, both of which are much affected by increase of elevation; and the 
latter also depends on the direction of the wind. The upper or return current of the trade- 
wind, flowing backward towards the northeast, gives a prevalence of westerly winds in the 
north temperate zone, which tends to spread the moisture from the Pacific over the western 
portion of our continent. These winds, however, ascending the western slope of the mountain 
ridges, are deprived of their*moisture by the diminished temperature of the increased elevation ; 
and hence it is that the plains and valleys on the eastern side of the ridges are generally 
parched and barren, and that the mountain system, as a whole, presenting, as it were, a 
screen against the moisture with which the winds from the west come laden, has for its eastern 
margin a sterile belt, which probably extends along the whole range, with a width varying 
from 250 to 300 and 400 miles. 
From the foregoing sketch it will be perceived that the lines of exploration must traverse 
three different divisions or regions of country lying parallel to each other, and extending north 
and south through the whole of the western possessions of the United States. The first is that 
of the country between the Mississippi and the eastern edge of the sterile belt, having a varying 
width of from 500 to 600 miles. The second is the sterile region, varying in width from 200 
to 400 miles ; and the third, the mountain region, having a breadth of from 500 to 900 miles. 
Explorations show that the surface of the first division, with few exceptions, rises in gentle 
slopes from the Mississippi to its western boundary, at the rate of about six feet to the mile, 
and that it offers no material obstacle to the construction of a railroad. It is, therefore, west 
of this that the difficulties are to be overcome. 
The concurring testimony uf reliable observers had indicated that the second division, or 
that called the sterile region, was so inferior in vegetation and character of soil, and so deficient 
in moisture, that it had received, and probably deserved, the name of the desert. This opinion 
is confirmed by the results of the recent explorations, which prove that the soil of the greater 
part of this region is, from its constituent parts, necessarily sterile; and that of the remaining 
part, although well constituted for fertility, is, from the absence of rains at. certain seasons, 
except where capable of irrigation, as uncultivable and unproductive as the other. 
This general character of extreme sterility likewise belongs to the country embraced in the 
mountain region, From the western slopes of the Rocky mountains to the 112th meridian, or 
the western limit of the basin of the Colorado, the soil generally is of the same formation as 
at lying east of that mountain crest, mixed, in the latitudes of 35° and 32°, with igneous 
rocks; and the region being one of great aridity, especially in the summer, the areas of 
cultivable land are limited. The western slopes of the highest mountain chains and spurs 
within this region being of a constitution favorable to fertility, and receiving much larger 
depositions of rain than the plains, have frequently in their small valleys a luxuriant growth 
of grasses, which sometimes clothes the mountain-sides; and where the wash is deposited on 
mountain stream or river-bottom the soil is fertile, and can be cultivated, if the elevations are 
not too great, and the means of irrigation available. Such mountain-valleys and river-bottoms 
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