522 THE POPULAR SCIENCE MONTHLY. 



the loess-beds are not very thick, and, therefore easily cut through 

 by streams, wherever they are thus eroded, we find dense groves of 

 oak, walnut, hickory, and other trees characteristic of the Ohio region. 

 "With these exceptions however, the whole region is prairie ; hence it 

 would seem that the loess is not capable of sustaining forest-growths 

 for any length of time, for it evidently was timbered during the time 

 that part of it was covered by lakes and marshes. But when the 

 great rivers cut their beds down to nearly their present level, the 

 timber gradually died out ; not being burned, as some suppose, but 

 disappearing because the geological formation will not retain moisture 

 enough to sustain forest-growth. 



The Texan region, lying south of the Illinois region, and extend- 

 ing west to 101 of longitude on the Rio Grande, is a continuation of 

 the Mississippi region, and is underlaid with cretaceous and tertiary 

 deposits. It is covered in many places with loess. It, therefore, has 

 the characteristic trees of the Mississippi region wherever trees grow, 

 and the characteristic loess flora on the prairies. 



The Comanche region, lying south, and the Dakota region, north 

 of latitude 38, are nearly destitute of timber. The former is under- 

 laid with triassic and the latter with cretaceous and tertiary beds ; but 

 they are covered with loess from ten to one hundred feet or more, and 

 hence the loess flora predominates. 



The mountain-region of the Rocky Mountain province is composed 

 of granite, but has enough trachyte and other volcanic rocks to 

 modify its flora to some extent. It also has some beds of Silurian on 

 the eastern border, and here, strongly corroborating our views, we find 

 some of the Eastern flora mixed with the Western and Southwestern 

 that lie next to it. Its valleys and parks are covered with loess, and 

 are treeless. 



The Saline region, comprising the remainder of this province, is 

 underlaid with tertiary of a different epoch from that of the Atlantic 

 coast, but is covered with heavy beds of basalt in many places. This 

 basalt is covered with a deposit analogous to the loess, and is treeless, 

 but has a flora very similar to that of the Dakota region. The ter- 

 tiary has a flora of its own, generally known as the sage-brush (Arte- 

 misia) flora, being composed of a number of shrubs peculiar to this 

 region. 



The Caurine province is composed of basaltic rocks principally, 

 but has some tertiary beds, and the higher mountains are granitic at 

 their tops. As the rocks are of an entirely different character from 

 those of the Atlantic side of the continent, we should not be surprised 

 at finding an entirely different flora. In fact, none of the Eastern trees 

 reach this province, nor do any of its trees appear farther east than 

 the Rocky Mountains. The geological formation of this province 

 being mostly basaltic, the trees are characteristic of that formation, 

 for the tertiary beds, wherever they are of sufficient size to make an 



