GENERAL FEATURES OF THE EARTH. 23 



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 communicating 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 Pam- 

 pas of South America, and all the desert regions of the globe, afford 

 saline efflorescences. 



The heights of some American lakes are as follows: Superior, 600 feet; Huron and 

 Michigan, 574; Erie, 570; Ontario, 232; Winnipeg, 1,100; Lake of the Woods, 1,640; 

 Great Salt Lake, 4,285; Yellowstone Lake, 7,788; Shoshone Lake, 7,870; Bear Lake, 

 5,931 feet. 



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



CONTINENTS. 



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 peculiarities 

 of each dependent on their different relations to the oceans. 



(1.) America. — The two Americas are alike in lying between the 

 Atlantic and the Pacific : moreover, South America is set so far to the 

 east of North America (being east of the meridian of Niagara Falls), 

 that each has an almost entire oceanic contour. Moreover, each is tri- 

 angular in outline, with the widest part, or head, to the north. 



North America, in accordance with the law, has on the Pacific side 

 — the side of the great ocean — the Rocky Mountains, on the Atlan- 

 tic side the low Appalachians, and between the two there is the great 

 plain of the interior. This is seen in the annexed section (Fig. 18) 

 from west to east : on the west, the Rocky Mountains, with the double 



