Distribution of Rivers. 61 



sparingly deposited at elevations of 16,000 and 18,000 

 feet.* 



In North America we find the great northern watershed 

 of the old world extending to the new in about the same 

 parallel of 50°. The elevation of the eastern part of this 

 watershed is only about 600 feet, but it rises on the west 

 into the range of the Rocky Mountains. To the north and 

 west of this watershed lie the numerous lakes and rivers of 

 New Britain, or Hudson Bay territory, the surplus waters of 

 which are carried chiefly by the Mackenzie and the Churchill 

 rivers into the Arctic Ocean ; while on the east, the St Law- 

 rence carries off the surplus of the five large Canadian lakes. 

 On the south slope of this watershed the Mississippi rises, 

 as well as some of its tributaries, and so low is the elevation, 

 and so contiguous are the sources of the southern and north- 

 ern systems of those rivers, that in great floods, from exces- 

 sive rains, the waters of both divisions intermingle. The 

 great watershed of South America, the Andes, assumes 

 a south and north direction, in conformity with the general 

 bearing of the continent. And here, too, there may be ob- 

 served a singular propriety in the arrangement of the surface 

 in relation to the deposition of moisture. The lofty ridges 

 of the Andes run across the western edge of the continent, 

 and thus form a screen by which the moisture of the south- 

 east and north-east trade winds, blowing over the surface of 

 the Atlantic, is completely condensed, and which, flowing 

 down their eastern slopes, waters the wide and extensive 

 plains, and again returns the surplus into the ocean source 

 from which it was originally derived. The tropical region to 

 the west of these high mountains is almost destitute of mois- 

 ture and of rivers. 



We are as yet but imperfectly acquainted with the structure 

 of Africa, yet the Nile evidently derives its principal source 

 from some elevated and snow-clad mountains near the equa- 

 tor, and then flowing northward, refreshes the arid deserts of 

 the centre and north with its cooling waters. Like the rivers 

 of North Asia, the Nile carries almost its whole supplies 

 from its two original sources, for it is joined by only one tri- 



* Dr Thomson's Western Himalaya. 



