



















223 
to the point of saturation as they mingle with the cooler elevated atmos- 
phere of the mountains, and precipitating their moisture in rain or snow 
upon the mountain sides, they pass on ina dry and highly electrical 
condition. Mountains also influence a climate by the shelter they afford 
from severe winds. They also inerease the local showers by gathering 
and precipitating the moisture accumulated in the atmosphere by evapo- 
ration from the earth’s surface. This effect of mountains is also shown 
_ by the opposite influence of level, barren plains, where not éven a tree is 
found to intercept the moisture-laden currents. 
The absorptive power of the earth has a climatic influence of greater 
or less degree according to the nature of the soil. A very dry atmos. 
phere will be rarely found where the soilis composed of closely-packed 
clay, with a tendency to form swamps, morasses, and sloughs. On the 
contrary, a porous, dry sand and light, friable, loose earth absorb, to a 
great extent, the moisture from the air. Sand also loses its moisture 
more easily than clay ; if heats more quickly, and also cools more rapidly. 
Such are, in general, the influences that control a climate, and with 
a knowledge of these and of the physical nature of this selected see- 
tion of Colorado we may form a good idea of what to expect here. 
The altitude of this region, outside of the mountains, is from 5,000 to 
7,000 feet above the sea. The latitude is between the thirty-eighth 
and forty-first parallels, about the same as the cities of Washington, © 
Philadelphia, and New York. The distance from the Pacific Ocean is 
abont 1,440 miles, and from the Atlantic about 2,100 miles. 
The principal portion is a rolling prairie, bare and brown, except 
along the few water-courses, where sparse belts of cottonwood trees re- 
lieve the monotonous and desert-like appearance. The earthy billows 
of this ocean-like plain rise and break against the rocky slopes of a vast 
mountain range, running nearly north and south, and stretching west- 
ward to the Pacific. Its lofty peaks rear their summits 14,000 feet 
above the level of the sea, and promontories or spurs jut forth here and 
here into the plain. T'rom these rocky slopes the decomposed sand- 
one has been washed down upon the plain below, and formed sandy, 
avelly deposits of wonderful porosity and depth, capable of absorb- 
ing any quantity of rain and moisture. 
_ And now, with this bird’s-eye view of the climate, let us examine it 
nore closely and in detail and see how it affects tree-growth. In this 
we shall be aided by the constancy and regularity of most of the phe- 
omena, 

