IXTEODUCTORY : THE OCCUREEXCE OF LAKES. 13 



over fifteen degrees north and south, and a nearly equal breadth east and 

 west, within the tropics, there are many lakes of great size, at an elevation 

 of 3,000 feet and over above the sea-level. The region is one of great pre- 

 cipitation, and the orographic structure of the continental mass is eminently 

 favorable to the formation of lakes, it being an elevated and comparatively 

 level plateau surrounded by a rim of mountains. But there is still another 

 reason for the existence of these great bodies of water. The rock is largely 

 granitic, or crystalline in texture, not taking up a large percentage of the 

 rainfall, but being almost impermeable. Most of these lakes are, in all prob- 

 ability, shallow ; and their area is known to be quite variable, their dimen- 

 sions changing with the season and with the succession of seasons. 



Turning to equatorial South America, we find a complete contrast to the 

 conditions just noticed as occurring in Central Africa with regard to the 

 existence of lakes. Equatorial South America is drained by a large river, — 

 the larsrest one in the world, — and .so thoroughlv drained that there are no 

 important lakes at all in the whole of its basin, and but few small ones, ex- 

 cept such as have a "bayou" character, and are evidently formed l)y the con- 

 stant shifting of the channel naturally connected with the flow of a mighty 

 river, with immense tributaries, subject to periodical inundations, through an 

 almost level region. Of such lakes there are doubtless an abundance, but 

 they are too small to be exhibited on our ordinary geographical maps, and 

 their origin cannot be for a moment misunderstood. The orographic cause 

 of this condition of things, offering such a contrast with the lake features of 

 Central Africa, becomes evident on an examination of the structure of the 

 reo-ion. While in Africa there is a rim all around the table-land, or elevated 

 basin forming the interior of that continent, there is in South America no 

 eastern raised border ; but, on the other hand, a gradual inclination of the 

 surface from the crest of the Andes to the Atlantic. Besides, the rocks over 

 by far the larger part of the basin of the Amazons are of a kind easily retain- 

 ing large quantities of water, and not granitic or crystalline. 



There is probably hardl_v a single geologist among those most inclined to 

 magnify the Avork of ice who would ascribe a glacial origin to the lakes of 

 Central Africa, lying as they do within the tropics ; but there are other 

 lacustrine districts in the temperate zones in regard to which there would be 

 less unanimity of opinion. 



There are two especially interesting and important lake regions, besides 

 that of Central Africa, to whicli allusion has already been made : one of 



