THE SURFACE OF THE EARTH. 



sive tablelands and plains having little declivity, are compara- 

 tively long and sluggish. 



127. Example in South America. South America pre- 

 sents a similar example. The chain of the Andes traversing the 

 country north and soth, and much nearer to the western than 

 the eastern coast, gives a similar character to the rivers, those 

 which now to the west being short and rapid, and those which 

 flow to the east being longer and slower. 



In the northern part of South America, the principal moun- 

 tain chain, diverging into several distinct ridges of the Cordilleras, 

 produces a complicated system of ravines and valleys, which divert 

 the course of the waters in various directions, so that many of 

 the rivers flow northwards and north-westwards into the Carib- 

 bean Sea. 



128. Effect of Parallel ridges. When a tract of coun- 

 try is traversed by two ridges in nearly parallel directions, their 

 declivities, which look towards each other, form a valley of greater 

 or less width. The rain precipitated upon these slopes, collecting 

 in streams, descends from either side to the lowest point of the 

 valley where they coalesce, and settling into a bed or channel, 

 flow along the lowest level of the valley, forming a river whose 

 course is parallel to the general direction of the bounding ridges, 

 and which continues its course until it discharges its waters into 

 the sea. 



In ascending a river, it is found, as may be expected, that the 

 quantity of water which flows in it becomes less and less as the 

 distance from its mouth increases. Since the total collection of 

 water must be proportionate to the number and magnitude of the 

 tributaries above the point of observation, the higher that point 

 is, the less will the number of such tributaries be, and consequently 

 the less the quantity of water in the main stream. 



129. Chief tributaries considerable rivers. In the case 

 of all the great rivers, the principal tributaries themselves are 

 rivers of considerable magnitude and importance, and some 

 which have been classed as tributaries might witk greater pro- 

 priety have been considered as the main stream. 



130. Example of Missouri. Thus for example the Mississippi 

 receives as tributaries, streams so important as the lied River, the 

 Arkansas, the Ohio, the Missouri, and the Illinois. Now the 

 Missouri is itself a river of much greater length, width, and 

 depth than that which above their confluence has been denomi- 

 nated the Upper Mississippi, and if, of two confluent streams, the 

 greater be entitled to be regarded as the continuation of the 

 main stream, the river which is now called the Missouri ought to 

 be denominated the Upper Mississippi. 



158 



