THE LAKE-BASINS OF IAECONNAUGHT. 125 



water can remain in them. Examples of these 

 waterless rock-basins occur on the hills of Errisbeg, 

 near Roundstone and Lettermore, north of Bally- 

 nakill Harbour, which respectively are composed of 

 hornblende-rock and quartz-schist, rocks which, on 

 account of their hard nature, are not easily broken 

 small, and consequently are likely to form loose, 

 open "fault-rock." In places on the low ground 

 similar rocks also occur. Here, however, rock-basins 

 are liable to be invaded by peaty matter carried 

 by wind and streams ; consequently, in many cases, 

 basins that doubtless once leaked are now water- 

 tight. Nevertheless, in places there are large tracts 

 solely drained through subterranean passages. 



In these rock-basins, especially those on Errisbeg, 

 the connection between them and breaks or faults 

 is very apparent. Some of the latter are evidently 

 post-glacial, and point out the large vacancies which 

 may be formed solely by the contraction of the rocks, 

 and the small quantity of debris that a subsequent 

 denudant, in such cases, would have to clear out. 



