10 



OUTLINES OF GEOLOGY. 



IV. 



OF THE INEQUALITIES OF THE LAND. 



17. Considerable portions of the earth's surface have 

 little more slope than is necessary to prevent the surface 

 water from becoming stagnant. Such level surfaces are 

 called plains, or, when elevated, table lands. 



18. Abrupt elevations above the level surface are called 

 hills, and, when of great height, mountains. 



19. Valleys are cavities, or depressions beneath the 

 general level of the surface. They may be formed by 

 surrounding hills, or hollowed out of table lands. A valley 

 is distinguished from the surrounding high lands by the na- 

 ture of their respective curvatures ; that of a valley being 

 concave, and that of the hill or mountain, convex ; and the 

 limit is where the curve changes its character from one of 

 these forms to the other, 



20. Every valley contains the bed of a stream or river. 

 Such stream occupies the line of greatest slope which can 

 be drawn in the valley. 



21. It sometimes happens that a valley is of small extent, 

 and terminates speedily at the shores of the ocean ; more 

 frequently the more elevated valleys communicate with 

 others at a lower level ; these again merge in others. The 

 streams which drain the several valleys unite their waters, 

 until they finally form a single great river. The courses of 

 such streams, if drawn upon a level surface, present the ap- 

 pearance of the trunk of a tree and its branches. 



22. The whole system of valleys which thus unite their 

 waters in a single stream is called the basin, or the valley of 

 that stream. It may be separated from the neighbouring 



