240 THE SMALL GRAINS 



black soils, with 10 per cent of humus in the best cher- 

 nozem and a soil thickness of 12-20 inches. 



Outside the strictly chernozem soils, but adjacent to them, and 

 bordering the great inland seas, there are (1) near the Black and 

 Azov seas, dark brown soils and gray soils, forming a transition from 

 typical chernozem to the brown or light brown soils of extreme 

 south and southeast Russia, having as subsoils tertiary sandy-clay 

 sediments, usually marl and clay, sometimes impregnated with 

 gypsum and common salt. Also (2) in Astrakhan and adjacent 

 territory, there are light brown soils, sometimes dark brown or light 

 yellow, with humus 2.3 per cent, soil thickness 10 inches and having 

 a subsoil usually of Caspian sediments very often full of sea salts. 



252. The Great Plains of North America. The 

 general features of our Great Plains are known to almost 

 every reader. It is chiefly important here to call atten- 

 tion to the great similarity of this region to that just 

 described. The principal difference is that the relative 

 positions of different portions of the two are reversed. 

 The encroachment of forest and prairie upon each other 

 is, in this case, on the east instead of the west and 

 north, while the approach to aridity of climate and 

 alkalinity of soil is toward the west instead of the east 

 and south. 



The fact that our black prairie soils have been so little 

 studied makes it difficult to give the extent of their area. 

 A painstaking but necessarily rough estimate shows that 

 there are about 275,000,000 acres in the United States 

 alone, with a large area to be added in Canada. In the 

 United States the prairie region includes, in general, al- 

 most the whole of the States of the Great Plains proper, 

 as well as parts of Missouri, Iowa, and Minnesota, and 

 apparently an eastward extension as far as western Ohio, 

 and on the west a large part of Montana and small por- 



