GEOLOGY. 243 



The densely wooded region. 3. Alternate woods and prairie. 

 4. Vast grassy plains, where the trees are restrieled to tlie im- 

 mediate banks of the streams. 5. Vast arid plains, often bare 

 of vejretation, and covered to some extent with saline efflores- 

 cences. 



From latitude 60° on Hudson Bay, and thence extending north- 

 westerly as far as the Arctic Ocean, lie the '• barren grounds" so 

 well described by Richardson. They are treeless, and the simpler 

 kinds of vegetation abound, such as lichens, mosses, and fungi. 

 Below the " barren grounds" we enter upon a forest belt, which 

 stretches continuously to the Gulf of Mexico. The prairie slope has 

 its greatest transverse expansion on the Missouri, and narrows as 

 it goes north. In the temperate zone, the western side of the forest 

 belt would bear south-east, passing west of the head of Lake Su- 

 perior and striking the west shore of Lake ^lichigan, whence it is 

 protracted south-west into eastern Texas. Clumps of spruce fir 

 form its outlines to the north, while its southern extension em- 

 braces the magnolia and the palmetto. With reference to the 

 forest range, as determined by lines of latitude, and therefore by 

 the vicissitudes of summer and winter temperature rather than the 

 varying supplies of moisture, it may be stated that many of the 

 Canadian t3'pes, following the course of the AUeghanies, reach as 

 far south as Virginia, and even Georgia, where they intermingle 

 with forms purely subtropical. While the geographical range of 

 arborescent forms is limited in their northern and southern range 

 by the conditions of temperature, their eastern and western range, 

 taking the AUeghanies as the axis, is limited by the conditions of 

 moisture, and their limits are more circumscribed by the latter 

 cause than by the former. The eastern rim of the Mississippi 

 valley contains many characteristic trees which are but feebly rep- 

 resented where the prairies commence, and disappear altogethei* 

 beyond the Missouri. On the other haud, vegetable forms are 

 not represented in the eastern margin, which attain their full 

 develooment as we approach the base of the Rocky Mountain^?. 

 These changes are wholly independent of isothermal lines, but 

 dependent on the varying supply of moisture. 



In the zone of alternate wood and prairie, we include the region 

 between the eastern shore of Lake Michigan and the eastern slope 

 of the Mississippi basin in Iowa, latitude 42° north, longitude 9.3° 

 west, and thence the western boundary is protracted a little west 

 of south toward the mouth of the Rio Grande. Tliis line is far 

 from being well defined, since the trees follow all of the great val- 

 leys of the Mississippi and Missouri to within 500 or 600 miles of 

 the Rocky Mountains. It is in tiiis region that the grasses become 

 predominant over the forest. That the limits of the forest were 

 not more extended in former times is evident from the fact that 

 the sloughs yield no entombed trunks of trees, which in other re- 

 gions are preserved for an indefinite period of time. The differ- 

 ences in the retentive power of moisture in the soil give to the 

 eastern line of the prairie region an irregular outline, which may 

 be likened to a deeply indented coast, far-entering bays, project- 

 ing headlands, and an archipidago of islands. What are known 



