GENERAL FEATURES OF THE EARTH. 23 



which also flow northeastward, deriving some waters from the 

 Rocky Mountains through the Saskatchewan, and reaching the 

 ocean through Hudson's Bay. Winnipeg Lake is here included. 

 These belong • with the St. Lawrence, the whole together consti- 

 tuting 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 

 existing streams, nor kept dry by the heat and drought of the 

 climate. They occur (1) over the interior of table-lands, as about 

 the head-waters of the Mississippi ; (2) along the depressions 

 between the great slopes of a continent, as the line of lakes in 

 British North America running northwest from Lake Superior ; 

 (3) in confined areas among the ridges of mountains. . The natural 

 forms of continents — that is, their having high borders — tend to 

 occasion the existence of lakes in their interior. 



If a lake has no outlet to the ocean, its water is usually salt ; 

 and any plain or plateau whose streams dry up without communi- 

 cating with the sea contains salt basins and efflorescences. The 

 Caspian, Aral, and Dead Sea are some of the salt lakes of Asia ; 

 and the Great Salt Lake of the Rocky Mountains is a noted one 

 on this continent. Many parts of the Rocky Mountains, the 

 Great Basin of the West, the Pampas of South America, and all 

 the desert regions of the globe, afford saline efflorescences. 



2. SYSTEM IN THE RELIEFS OR SURFACE-FORMS OF THE 

 CONTINENTS. 



28. Law of the system. — The mountains, plateaus, low lands, 

 and river-regions are the elements in the arrangement of which 

 the system in the surface-form of the continents is exhibited. The 

 law at the basis of the system depends on a relation between the 

 continents and their bordering oceans, and is as follows : — 



First. The continents have in general elevated mountain-borders 

 and a low or basin-like interior. 



Secondly. The highest border faces the larger ocean. 



A survey of the continents in succession with reference to this 

 law will exhibit both the unity of system among them and the 



