28 PHYSIOGRAPHIC GEOLOGY. 



Fig. 21 is a section from south to north (the heights necessarily much 

 exaggerated in proportion to the length) ; a, the southern mountains ; 

 b, the southern plateau ; c, Lake Tchad depression ; d, Sahara plateau ; 

 e, oases depression ; f, mountains on the Mediterranean, of which 

 there are two or three parallel ranges. Fig. 22 represents the surface* 



Fig. 22. 



W E 



outline from west to east through the southern half of the continent 

 In all these sections, all minor details are omitted, in order to bring 

 out clearly the system, or continental model. 



Africa has, therefore, a basin-like form, but is a double basin ; and its 

 highest mountains are on the side of the largest ocean, the Indian. 

 The height of the mountains adjoining the Mediterranean is the only 

 exception to the relation to the oceans ; and this is small. Moreover, 

 the position of the head of the continent against the continent of 

 Europe with only the Mediterranean between, instead of an ocean, is 

 a sufficient reason for the exception. Africa has some resemblance 

 to America, but America turned about, with the most elevated border 

 on the east instead of the west. 



(4.) Australia. — Australia conforms also to the continental model. 

 The highest mountains are on the side of the Pacific, — the larger of 

 its border-oceans. The Australian Alps, in New South Wales, facing 

 the southeast shores, have peaks 5,000 to 6,500 feet in height. The 

 range is continued northward in the Blue Mountains, which are 3,000 

 to 4,000 feet high, with some more elevated summits, and, beyond 

 these, in ridges under other names, the whole range being mostly be- 

 tween 2,000 and 6,000 feet in elevation. On the side of the Indian 

 Ocean the heights are 1,500 to 2,000 feet. The interior is a low, arid 

 region. The centre about 200 feet above the sea. 



The continents thus exemplify the law laid down, and not merely as 

 to high borders around a depressed interior, — a principle stated by 

 many geographers, — but also as to the highest border being on the 

 side of the greatest ocean. 1 The continents, then, are all built on one 

 model, and, in their structures and origin, have a relation to the oceans 

 that is of fundamental importance. 



It is owing to this law that America and Europe literally stand 

 facing one another, and pouring their waters and the treasures of the 

 soil into a common channel, the Atlantic. America has her loftier 

 mountains, not on the east, as a barrier to intercourse with Europe, 



1 First announced American Jour. Sci., II., vols. iii. 398, iv. 92, 1847, and xxii. 335, 

 1856. 



