22 INTRODUCTION. 



regions present great diversity of form, extent, and 

 direction, and often exhibit basins or hollows, which 

 are occasionally filled with water. 



Descendin ^ into the plains, we find that they are 

 seldom perfectly levels but are formed into slopes of 

 small inclination and of various extent. The pam- 

 pas of South America, for example, stretch from the 

 base of the Andes to Buenos Ayres, over a space of 

 900 miles ; i;nd in Africa are vast expanses of nearly 

 level land, where the traveller, day after day, sees 

 the horizon preserving the same distance as he pro- 

 ceeds, and bounding an ocean of arid sand. Large 

 flats are also found at great elevations above the 

 sea, such as those of Tartary, Thibet, and Mexico. 



Of the other inequalities of the land, the more re- 

 markable are the cavities forming lakes, and the 

 grooves occupied by the beds of rivers. The former 

 are of all sizes, from several hundred miles in cir- 

 cumference down to very small dimensions, and 

 occur in all situations, — between mountain-chains, 

 like the Caspian, — in plains, like Onega, — and along 

 the course of rivers, like those of Canada. The 

 streams necessarily flow in the line which marks 

 the greatest depression of the valleys ; although, in 

 some instances, towards their mouths, they occupy 

 a higher level, their beds having been raised by the 

 deposition of the debris carried down by the torrent. 



The bottom of the ocean, being merely the conti- 

 nuation of the surface of the land, may be supposed 

 to present inequalities of a similar nature, although, 

 owing to the action of currents, they are probably 

 not so distinctly marked. The transition from what 

 is above to that which is under the water is not in 

 general denoted by any striking phenomenon, ex- 



