164 PHYSICAL GEOGRAPHY. 



The eastern slope towards the Atlantic commences by 

 a belt, formed mostly of sandstone, 150 miles in width, 

 which rests on the shoulder of the chain, with an inclination 

 of about 37 feet in the mile, in its descent from 8,000 feet 

 above the sea, to 2,500. The more gradual slope of the 

 great prairies, beginning at the last-named elevation, has a 

 breadth of 700 or 800 miles, and retains in its descent the 

 prairie character of a treeless, sandy, and moderately un- 

 dulated, or, as it is locally named, " rolling " plain. 



Most of the streams which cross the prairie flow through 

 deep furrows, sunk abruptly below the general level; never- 

 theless the Mississippi, Missouri, and some of their larger 

 tributaries have wider valleys, skirted by successive terraces 

 and alluvial deposits. On the banks of all the rivers there 

 are belts of woodland, and clumps of trees that encroach on 

 the prairie, intercepting grassy lawns, and producing re- 

 markably fine park scenery, which is often enlivened by 

 small lakes. In the interior of the prairie, however, water 

 is scarce, and there is such a total want of wood, that for 

 days together the traveller can find no other fuel than the 

 dung of the bison. Near the mountains the soil is coarsely 

 sandy, strewn with boulders, and sterile ; farther eastward 

 the sand is finer, and the boulders disappear, but they 

 recur in numbers on the lower border of the prairie : they 

 are also scarce or wholly absent over very extensive tracts 

 of the rich alluvial deposits of the valley of the Mississippi, 

 south of the Ohio. 



The Mississippi drains the entire space between the 

 Rocky Mountains and the Alleghanies or Apalachian 

 chain, embracing thirty degrees of longitude. The whole 

 of this vast water- shed may be considered as one valley, 

 whose bottom, indicated by the channel of the river, has a 

 southerly course, inclining slightly to the eastward. The 

 length of the river, from its source in Itasca Lake, at an 



