448 SECTION H. 



affluents. The plateau lies from four to ten thousand feet 

 above sea level, but there are great contrasts in elevation 

 from 14,000 feet above to 300 below the datum. In this 

 region the north and south . ranges of the Rockies break up 

 and form a complex of mountains running east and west,, 

 plateaus, plains, basins, buttes, broad valleys and narrow 

 canyons giving great diversity of the most remarkable natural 

 features to be found in the world. 



The geology is that of later rocks, principally the easily eroded 

 Cretaceous and other mesozoic formations. Tremendous 

 volcanic activity in former times has poured out vast floods 

 of lava which together with tuff form the most noticeable 

 physiographic features of the region. 



Latitude, elevation, and natural barriers here conspire to 

 produce modifications in climate. This is seen in the convolu- 

 tions of the isotherms of 50 , 59 and 68° crossing the region,, 

 the scanty rainfall occurring in the winter and summer months,, 

 the excessive insolation, the extremes of day and night tem- 

 perature, the high winds and rarefied air, which characterize 

 the arid environments. 



The fitness of the southwest to sustain biotic forms depends 

 mainly upon rainfall, which itself is regulated by cosmic and 

 geographic conditions. Thus the uprush of heated air from 

 the sun-baked plateaus during the summer draws in moisture- 

 laden air from the oceans, producing rains which are un- 

 equally distributed; the higher mountains acting as condens- 

 ing centers receive the most, while the plains are scantily 

 watered. The receptivity of the land must also be considered, 

 the mountains covered with vegetation storing water and the 

 bare land shedding it into the rivers which must carry at 

 times vast floods and during long periods remain perfectly dry. 

 Everywhere is evidence of the colossal agencies which are at 

 work reducing the land to sea-level. This workshop is littered 

 with the bones of the mountains, and the dust that is sorted 

 by water and wind moves freely to lower levels or is blown 

 higher to again resume its gravitational course. 



Despite the generally adverse conditions set forth, there 

 flourishes here a flora of a peculiar character which forms 



