26() GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



c LIMA TIC FEATURES. 



The entile route fella within the climatic province <>f variable rains. 

 In some localities there is ordinarily greater precipitation at some sea 

 sons than at ot hers, but t here is no (list net where, at any season pre- 

 cipitation is regular or continuous. Rainfall and snowfall are associated 

 with cyclonic or other disturbances of atmospheric equilibrium. The 

 temperatures are those normal to the temperate zone: except on moun- 

 tains and loftly plateaus, wheat and Indian coin are grown and the 

 apple is t hi 1 leading fruit. 



The most conspicuous climatic contrasts are afforded by the local dif- 

 ferences in the annual amount of precipitation and in the annual and 

 diurnal range of temperature. The outward route from Washington 

 to .Minneapolis, and the return route from the Missouri river to Now 

 York traverse a region of essentially the same climatic type. The 

 annual temperature mean ranges from -HP to 55° F. (4° to 13° 0.) the 

 precipitation ranges from 30 to 45 inches (!)() to 135 em.). The surface 

 is normally timbered and deciduous trees prevail. Occasional patches 

 of prairie interrupt the forest throughout, but they increase toward 

 the west and finally predominate. The whole country is fertile without 

 irrigation. The annual range of temperature (the difference between 

 the coldest month and the warmest) is about ."»()' P.(27°0.). The length 

 of the summer suffices for the growth of the leading food staples. 



Westward to the base of the Rocky mountains the rainfall gradually 

 diminishes to less than L6 inches (45 cm.); trees disappear except along 

 the water courses, where the cottonwood ( Populm monilifera) nourishes, 

 and they are replaced by grasses. Farther westward the grasses 

 become scant, and are partially replaced by an open growth of low 

 bushes. Farming without irrigation becomes at first precarious and 

 finally impossible, and grazing supersedes agriculture as the leading 

 industry. At the same time the annual temperature range increases 

 to about 60° F. (33° ('.), and the diurnal range is likewise greater as 

 the excursionists will readily discover. 



Throughout the more westerly portion of the route, in the Rocky 

 mountains, the Great basin, and the Plateau region, the most important 

 loeal climatic condition is altitude, and the native floras and faunas are 

 arranged in belts which follow contours, but these contours run some 

 what lower at the north than at the south. Precipitation varies with 

 altitude, ami temperature inversely with altitude. The plain of west 

 ern Utah, with an altitude of 4,500 feet (1,375 m.), has an annual tem- 

 perature of 75° F. (24° 0.) ami a rainfall of about 7 inches (21 em ) 

 Yellowstone Park, with an altitude of 6,500 feet (1,980 m) has an 

 annual temperature of 40°F. (4° 0.) and a precipitation of 20 inches (no 

 em.). A, Leadville, 10,200 feet (3,100m.) above, he sea, the annual 

 temperature is 35° F. (2° 0.) and the precipitation 12 inches (40 en, ) 



