THE VALLEYS OF SOME OF THE IRISH LAKES. 159 



glacier, most, if not all the loose portions of the 

 rocks should have been removed out of it, leaving 

 the ice, prior to its final disappearance, compara- 

 tively speaking free from rock detritus contained 

 in it, to encumber the lake-basin. At the present, 

 the deepest spot in the lake is only eleven feet 

 below the low-water of ordinary spring tides at 

 Limerick, the soundings on the chart having been 

 reduced to the level of 108 feet above that height ; 

 but as the level of the water of the lake is so much 

 higher, there would be an underground drainage if 

 these holes were connected with subterranean 

 passages. During the glacial period, the water 

 under the ice would have found vent through these 

 passages, denuding away their walls, and enlarging 

 the deeps from which it flowed. After the glacial 

 period, water may still have flowed through these 

 holes, until eventually the land sank so low that 

 they were incapable of acting, or they were choked 

 by the deposition of mud or marl. But even now 

 there is reason for supposing that such passages 

 may be partially open, as in the summer less water 

 leaves the lake-basin at Killaloe than flows into it 

 by the main feeder at Portumna and from the 

 numerous smaller tributaries. Part of this deficit is 

 certainly due to evaporation, but all of it can not 

 thus be accounted for. 



Many of the Irish lake-basins which now have 



