xv The Continental Area 277 



356. Continental Slopes. The simplest conceivable 

 continent would consist of two land-slopes meeting, like the 

 roof of a house, along a central line or axis, so that a 

 section across it would resemble A, Fig. 56. The axis of 

 a continent is usually formed by a mountain range of eleva- 



FIG. 56. Typical Section of a Continent. In BCD the short slope is shown 

 to the left, the long slope to the right. 



tion ( 303), which most frequently occurs near the edge of 

 the slope of the world ridge, and consequently near one 

 side of the continent, so as to produce a short slope on one 

 side and a long slope on the other, giving a section like B. 

 A mountain chain is rarely single, and is about equally steep 

 on both sides. It occupies a narrow strip of a continent ; so 

 while the short slope of the continent is nearly uniform to the 

 sea, the long slope is broken into a steep and a gentle portion, 

 giving the section C. But since both sides of a continent have 

 been ridged up, a lower and broken mountain range usually 

 intervenes between the long slope and the sea, converting the 

 central part of the continent into a wide valley, and forming 

 a second short slope to the seaward side, as shown in section 

 D. The various slopes form parts of river-basins ( 319), 

 and the course of rivers in an ordinary map serves to mark 

 out the direction of the slopes. Where there are no rivers, 

 or when rivers flow into a salt lake, a region of internal 

 drainage results. Such regions occur in every continent 

 wherever the arrangement of the heights cuts off rainfall 

 and allows full scope to the action of evaporation. One- 

 quarter of the Earth's land surface is thus situated. 

 The long slopes of all the continents are directed toward 

 the Atlantic Ocean and its seas, which thus receive the 

 drainage of more than half the land (Plate XIII.) All the 

 continents turn their backs, so to speak, on the Indian and 

 Pacific Oceans. The following table is calculated by Dr. 

 John Murray. 3 The small area draining into the Southern 

 Ocean is added, in the table, to those of the Atlantic and 

 Pacific Oceans. 



