THE earth's CONTOUE AND SURFACE SUBDIVISIONS. 29 



extent of such a region will vary with the position of the mountains 

 and ocean. 



Over a continent there are the interior and the border river systems 

 or drainage areas ; the former very large and few, the latter many and 

 relatively small. 



In North America, having the Rocky Mountains on the west and the 

 Appalachian on the east, the great interior slopes are three : southward, 

 along the Mississippi ; eastward, along the St. Lawrence ; and northward, 

 along the Mackenzie and other streams. 



The tributary streams of the Mississippi rise on the west, among the 

 heights of the Eocky Mountains, the region in and near the Yellowstone 

 Park supplying waters to the Missouri through a number of tributaries 

 including the Yellowstone and the Front Eange of Wyoming, Colorado, and 

 Xew Mexico, giving origin to the Platte, Arkansas, and Canadian rivers ; 

 on the north, in the central plateau of the continent, in northern Minnesota, 

 west of Lake Superior, near lat. 47°-48°, long. 93°-96°, 1680 feet in elevation 

 — a region of lakes which is the source of the Mississippi of the maps ; 

 and on the east, in the Appalachians, from western New York to Alabama. 

 There are also other rivers flowing from the west into the Gulf of Mexico ; 

 but, in a comprehensive view of the continent, these belong to the same 

 great river system. 



The St. Lawrence commences in the head waters of Lake Superior, about 

 the same central plateau, embraces the Great Lakes with their tributaries, 

 and flows finally northeastward, following 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 northeastward, 

 deriving some waters from the Rocky Mountains through the Saskatchewan, 

 and reaching the ocean through Hudson Bay. Winnipeg Lake is here in- 

 cluded. These belong with the St. Lawrence, the whole together constituting 

 a second continental river system. 



The Mackenzie is the central trunk of the northern river system. Start- 

 ing 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, Great Slave, and Great Bear lakes lie in this district. 



The border river systems depend for their extent on the height and 

 slope of the mountains, the distance from the coast, and the structure of the 

 mountain region. The Appalachian range, mostly below 5000 feet in height, 

 is 150 to .300 miles from the coast. But the mountains are a succession of 

 overlapping parallel ridges, and the rivers in their higher parts go back and 

 forth between the ridges, thus deriving a more gradual slope, a much greater 

 length, and producing a longer range of watered country. The Rock}^ Moun- 

 tains, 10,000 to over 14,000 feet high, are 600 to 1000 miles from the coast. 

 But a second chain of equal height — that of the Sierra and Cascade ranges, 

 with the range of the California peninsula, which is probably a southern 

 continuation of the line — stands as a barrier to the more eastern drainage 



