THE VALLEYS OF SOME OF THE IRISH LAKES. 153 



and the latter 124 feet, which would seem to suggest 

 that these holes may have been the connection 

 between the subterranean passage and the drainage 

 of the valleys when the land was at a higher level than 

 at present. When this basin, and also that of Lough 

 Corrib, were filled with ice, the water associated with 

 the latter must have found a vent somewhere, and 

 probably it was through such deep places ; while after 

 the ice disappeared these may also have acted until the 

 land sank so low that there was no fall from them, or 

 until they were silted up by matter carried into them 

 by rain and rivers. Lakes in a limestone country are 

 usually very irregular in outline, and at first it might 

 be supposed that there is no connection between their 

 shape and the structure of the subjacent rocks. On 

 an examination, however, it generally appears evident 

 that the bays and all wide stretches across the lake- 

 basins conform with lines of breaks or displacements, 

 while the minor features of the coast-lines are due to 

 the weathering along joint systems or lines of bed- 

 ding, generally the former. 



The basin of Lough Derg, through which the 

 Shannon flows, well illustrates the effect of breaks on 

 a lake-basin. On looking at the chart of this lake, 1 

 it will be seen that by a few displacements the basin 

 might be made to occupy a N.N.E. and S.S.W. 

 valley, and that each of these displacements nearly 



1 Admiralty Chart, No. 1552, Lough Derg, Ireland. 



