THE VALLEYS OF SOME OF THE IRISH LAKES. 141 



along which lie the different deeps. In a paper 

 written some years ago on this lake-basin/ I ignored 

 the open joints, faults, and the like, and tried to 

 prove that the excavation of the lake-basin was due 

 nearly solely to ice action. Since then, however, it 

 has been found that lines of faults or breaks traverse 

 it, while every bay or arm in the N.W. part is con- 

 nected with one of these lines, and each deep lies 

 along one of them, or at the crossing of two or 

 more ; but that ice was the great carrier would still 

 appear correct, as its traces are quite conspicuous 

 everywhere in and about the basin, while the forms 

 of the lake coincide with the courses of the different 

 ice-streams. This lake is partly situated in a car- 

 boniferous limestone country, and partly in a country 

 occupied by Silurian and metamorphic rocks ; and as 

 the latter rocks are much more broken up and faulted 

 than the former, the most of the part situated in the 

 limestone, is much shallower than the rest. There 

 are two places in which there are deep abrupt holes, 

 that could not have been scooped out by ice, and 

 probably were formed by water that had a sub- 

 terranean passage, assisted by a fissure due to a 

 fault or shrinkage. These deeps will again be men- 

 tioned when we are describing the Lough Mask basin, 

 but prior to entering into that subject we should say 

 a few words on Sluggys, Swallow-holes , Turlougks, 



1 Geological Magazine, Nov. ] 866. 



