223 



to the point of satilratiou as tlicy 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 in a dry and highly electrical 

 condition. Mountains also influence a climate by the shelter they afibrd 

 from severe winds. They also increase 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 even 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 soil is 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 ; it 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 x>hysical nature of this selected sec- 

 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 ^Tew York. The distance from the Pacific Ocean is 

 about 1,440 miles, and from the Atlantic about 2,100 miles. 



The x^rincipal iiortion is a rolling iiralrie, bare and brown, excei^t 

 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 

 there into the plain. From these rocky slopes the decomposed sand- 

 stone has been washed down upon the plain below, and formed sandy, 

 gravelly 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 

 more 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 i)he- 

 nomena. 



