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13. The Prairie Region. We 
_ The prairies, the steppes of North America, are treeless 
plains, in which the severe cold of winter is succeeded by a. ff, 
short period of active growth ushered in by a transitory ee 
rainy season, and a dry rainless summer. ‘The cause of the 
summer drought is the dryness of the prevalent western aa 
Pacific winds ; and this is the result of the moisture of the ve i x 
air being previously withdrawn by the Rocky Mountains — 
and the chain of mountains along the Californian coast. 
The heavy rains which fall on these mountains are, it is 4 
true, the feeders of great rivers which cross the prairies ; but 
these water-courses, which are often of great size, the | © 
affluents of the Mississippi, the Colorado, and the Rio — 
Grande del Norte, are of but little service for the irrigation cee 
of the soil, because they have worn their way down ~ § 
into the soil, often to such a depth that no arable land 
is left in the deep chasms or river-beds known as ‘ cahons’ ; 
and sometimes there is not even a belt of trees by the side © 
of the stream. The south-western portion of the region. “ t 
is covered by an inhospitable salt-desert, where the soil is - 
often completely bare, or produces a vegetation consisting 
almost entirely of a few sparse Chenopodiacez and social 
Artemisias. There are, however, here and there in the Pree 
vailing desert, a few scattered oases, among which that of the — 
Great Salt Lake is the most important. A contrast to the - 
salt-desert is afforded by the northern portion of the 
Tegion, a true grass-steppe, the home of the bison; while oe 
in the south agaves, aloes, and yuccas abound, and — the a 
Cactacez attain their maximum development. The few a 
trees and shrubs which are found here and there on the a 
banks of the rivers and slopes of the mountains, have mostly . 
migrated from the forest-region. Among the characherleal Oe 
- forms of vegetation are the Mimosez, especially the genus — 
Prosopis, which forms by itself the feature in the. landscape a 
