50 ON THE GENERAL DISPOSITION OF THE 



Wales; and this has been imagined to form also the 

 termination of the Niger. In a few instances, small 

 rivers are found to sink into the ground and disappear; 

 while, in others, after a subterranean course, they rise 

 again. The action of rivers in changing or in- 

 fluencing the forms of the land, will be considered in 

 another chapter. 



The description of valleys has already explained the 

 origin of lakes, which must necessarily exist where- 

 ever the middle of such a valley is considerably lower 

 than its lowest extremity. Thus they are necessarily 

 most frequent in mountainous countries ; although 

 very extensive collections of water of this nature are 

 also found in plains. They may of course exist at 

 any elevation above the sea ; and, in some rare cases, 

 of which Loch Ness is an example, their beds are even 

 depressed below its level. 



In general, lakes both receive and emit a river, 

 being depressions in the course of its stream ; or else, 

 receiving many streams, they emit only one. The 

 most conspicuous lakes of this nature in Europe are 

 that of Geneva, traversed by the Rhone at an elevation 

 of 1134 feet above the sea, that of Lucern, at 1392, 

 traversed by the Reuss, that of Constance, at 1089, 

 giving passage to the Rhine, those of Brientz, Thun, 

 Zurich, and the lakes Como, Garda, and Maggiore in 

 Italy. In America, a whole chain, consisting of the 

 Lakes Superior, Erie, Ontario, and Huron, is traversed 

 by the Saint Lawrence; and, in our island, this is the 

 character of all which are in any way remarkable for 

 their extent. 



In a few instances, lakes receive rivers without 

 emitting any; and, of these the most remarkable is 

 the Caspian, just noticed, into which the Wolga and 



