SURVEY OF COLOEADO AND NEW MEXICO. 139 



SOIL. 



The soil thronghout this district presents great uniformity in quality, 

 as is clearly shown by the striking similarity of the plants of its differ- 

 ent parts. It is chiefly a light loam, in which the silicious and micaceous 

 ingredients predominate. Yet there is a considerable difference between 

 the two sections in one respect : while in the northern the particles are 

 coarse and sharp, in the southern they are fine and rounded, and the 

 arenaceous portion bears a larger ratio to the whole. 



But to form a correct idea of this soil, especially in the northern sec- 

 tion of the district, it must be remembered that the streams, in passing 

 from the snow-clad crests of the vast range of mountains to the broad 

 prairies of the plains, sweep over the upheaved margins of the entire 

 geological series represented in this region. And as they rush down the 

 mountain gorges, and along the rocky canons, they bear away with 

 them the debris from all the strata they touch, from the primary granite 

 to the most recent tertiaxy representative, mingling it together and scat- 

 tering it over the plains they cross ; not only the confined streams of 

 the present era, but all the waters which have swept the mountain 

 slope since it was lifted up by the vast subterranean force by which they 

 were formed. The atmospheric currents driving to and fro the lighter 

 and dry particles on the surface, have assisted in the mingling process. 

 This combination of the various mineral elements gives to the soil an 

 adaptability to a large variety of plants. The predominance of silicious 

 matter renders it peculiarly adapted to the growth of wheat and oats, and 

 the addition of decayed vegetable materials causes it to produce heavy 

 crops of succulent and bulbous vegetables. 



It varies considerably in depth ; near the foot of the mountains, where 

 the water traveled more rapidly, it has covered the boulders and gravels 

 with a thin crust, while farther down on the plains it grows thicker as 

 we recede from the mountains. Although the bottoms along the creeks 

 contain a greater proportion of decayed vegetable matter than the ter- 

 races and ridges, yet the latter are equally rich in the primary elements, 

 and by a sufficient supply of water, will produce the cereals as heavily 

 as the former. And, as on these terraces vegetation ripens some eight 

 or ten days earlier than on the bottoms, they possess this advantage. 



In the southern section the case is somewhat different, the Divide 

 being largely composed of loose conglomerate of well-worn particles ; 

 when these are carried down by the more slowly running water, they 

 become more finely comminuted and worn, forming heavier beds of sand 

 nearer the base. In consequence of this fact the water sinks much 

 sooner than in the northern section. This deeper deposit of sand is 

 often very apparent along the margins of the streams where they have 

 cut away the banks. 



CLIMATE. 



This strip of country lying longitudinally north and south along the 

 east flanks of the mountain, the temperature necessarily varies as the 

 points recede from each other. And as we descend from the higher por- 

 tions along the base of the mountain to the valleys of the plains the 

 isothermal lines bend considerably northward. But the average tem- 

 perature of the northern section may be compared with that along the 

 eastern slope of the Alleghenies, in Pennsylvania, with which it very 

 nearly corresponds. The altitude varying from three to seven thousand 

 feet above the level of the sea, and the line of perpetual snow not far 



