THE LAKE-BASINS OF IARCONN AUGHT. 115 



Limerick, Kerry, Cork, &c., places miles apart, 

 terraces that mark the margin of this ancient sea. 

 These gravelly accumulations are generally associated 

 with rocks to form the lake-basins ; and in many 

 valleys after their elevation, rain and rivulets have 

 cut channels through the bars and banks, thereby 

 eventually draining the hollows in which lakes at 

 the first existed; so that now only in a few, where the 

 cut was not deep enough to drain all the hollows, 

 do lakes exist. If we examine the Kylemore valley, 

 we find that its lake once extended considerably fur- 

 ther up and down the valley, and that the small 

 lakes there at present were only deeps in the ancient 

 lake, which the present outfall cannot drain. 



The third class of bars are those due to glacial 

 action. Hooker, Forbes, and other Alpine or Arctic 

 explorers, have described the moraines which form 

 across valleys and hollows, also the avalanches and 

 other debacles of rocks, stones, clay, and mud which 

 slide down into valleys during thaws, damming up 

 the drainage, and stopping the original water-courses, 

 often forming lakes of greater or less dimensions. 

 Snow drifts may act similarly, as the up-side may 

 become a sheet of ice, and prior to its melting away 

 a river or stream may cut a new channel, and desert 

 its old one. In the mountainous parts of Great 

 Britain and Ireland, bars of drift, apparently of 

 moraine origin, or accumulations from successive 



