SURFACE OF THE GLOBE. 51 



the Ural flow, and the Dead sea : and, in all of them, 

 it must be supposed that the waste by evaporation is 

 equivalent to the average supply of water. 



Some lakes of less importance are found to emit 

 rivers without previously receiving any. The Seliger 

 lake is the source of the Wolga ; and the Hoanho and 

 Kiam of China, are said to arise from similar sources. 

 But it is more probable that many of the examples 

 quoted by geographical writers, are the receptacles of 

 insignificant alpine rills, like Loch Spey in our own 

 island. 



The last modification of lakes consists of those which 

 neither receive nor emit streams, such as those of 

 Agnano and Averno in Italy, which, with some others 

 described by authors, appear to have been the craters 

 of volcanoes. In such cases, we must conceive that 

 they contain internal springs adequate to the supply 

 of the mean evaporation. 



Subterranean pools and lakes have been described 

 by authors ; and, of these, Pen-park hole, in Glou- 

 cestershire, is an example in our own country. These" 

 are sometimes evidently formed by irregularities, or 

 occasional depressions, in the eourses of the subter- 

 ranean rivers so often found in limestone countries. 

 Where they are found to exist in the volcanic regions 

 of America, supplying, as is supposed, the fish some- 

 times ejected by these volcanoes, they probably occupy 

 caverns of volcanic origin. 



E 2 



