22 PHYSIOGRAPHIC GEOLOGY. 



France, another, at about 1,100 feet; Bavaria another, at 1,660 feet. Persia is a plateau 

 varying in elevation between 3,800 and 4,500 feet, with high ridges in many parts. 

 The Abyssinian plateau, in Africa, has an average elevation of more than 7,000 feet; 

 the region of Sahara, about 1,500; that of the interior of Africa south of the equator, 

 about 2,500 feet. 



River-Systems. — Plateaus and mountains are the sources of rivers. 

 They pour the waters along many channels into the basin or low 

 country toward which they slope ; and the channels, as they continue 

 on, unite into larger channels, and finally into one or more trunks 

 which bear the waters to the sea. The basin and its surrounding 

 slopes make up a river-system. The extent of such a region will 

 vary with the position of the mountains and ocean. It may cover but 

 a few hundred square miles, like the river-regions on a mountainous 

 coast, or it may stretch over the larger part of a continent. 



The interior of the United States belongs to one river-system, — 

 that of the Mississippi ; its tributary streams rise on the west among 

 the snows of the Rocky Mountains, on the north in the central plateau 

 of the continent, west of Lake Superior, near lat. 47° and beyond, 

 long. 93°- 96°, 1,680 feet in elevation, and on the east in the Ap- 

 palachians, from western New York to Alabama. Besides the Mis- 

 sissippi, there are other rivers rising in the Rocky Mountains and 

 flowing into the Gulf of Mexico ; and, in a comprehensive view of 

 the continent, these belong to the same great river-system. 



The St. Lawrence represents another great river-system in North 

 America, — a region which commences in the head- waters of Lake Su- 

 perior, about the same central plateau of the continent that gives rise 

 to the Mississippi, and embraces the great lakes with their tributaries 

 and the rivers of Canada, — and flows finally northeastward into the 

 Atlantic, following thus a northeast slope of the continent. North of 

 Lake Superior and the head-waters of the Mississippi, as far as the 

 parallel of 55°, there are other streams, which also flow northeast- 

 ward, deriving some waters from the Rocky Mountains through the 

 Saskatchewan, and reaching the ocean through Hudson's Bay. Win- 

 nipeg Lake is here included. These belong with the St. Lawrence, 

 the whole together constituting a second continental river-system. 



The Mackenzie is the central trunk of still another river-system, — 

 the northern. Starting from near the parallel of 55°, it takes in the 

 slopes of the Rocky Mountains adjoining, and much of the northern 

 portion of the continent. Athabasca, Slave, and Bear Lakes lie in 

 this district. 



These are examples from among the river-systems of the world. . 



Lakes. — Lakes occupy depressions in the earth's surface which, 

 from their depths or positions, are not completely drained by the exist- 

 ing streams, nor kept dry by the heat and drought of the climate. 



