Page:Popular Science Monthly Volume 23.djvu/538

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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 extending 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 underlaid 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 tertiary has a flora of its own, generally known as the sage-brush (Artemisia) 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