THE VALLEYS OF SOME OF THE IRISH LAKES. 161 



If the deeps in the basin of Lough Derg below 

 the 50-feet line of soundings were mapped, they 

 would make a S3 r stem of lakes somewhat similar to 

 those of this part of Clare, provided that each deep 

 were connected with the next by a subterranean 

 passage, while the great deep in the east and west 

 reach had a passage opening somewhere below the 

 present sea-level. 



Before leaving these lakes, we may refer again 

 to Lough Corrib, as the river flowing from it has 

 suddenly become dry four times during the his- 

 torical period, namely, in the years A.D. 1178, 1190, 

 1647, and 1683. In the paper previously referred 

 to, 1 I suggested that these failures of water might 

 be accounted for by subterranean passages, as dur- 

 ing very low tides the water from the lake would 

 force out the sand, or whatever else choked these up, 

 while the return of the tide would fill them up again ; 

 and if such passages do exist, they must be connected 

 with the former drainage of the basin. 



1 Page 141. 



