THE VALLEYS OF SOME OF THE IRISH LAKES. 143 



variety being excavated from the top downwards, and 

 the others mostly from the bottom upwards. The 

 growth of the first may be studied on the shores of 

 any lake situated in a limestone country such as 

 Lough Corrib. Here, when the drift covering has 

 been recently removed from the rocks by the water 

 of the lake, beautifully polished, etched, and scratched 

 surfaces are exposed. Those that now, since the lake 

 was lowered, 1 are exposed only during the summer 

 months, are full of minute round holes ; those that 

 were formerly exposed only during the summer months 

 are full of larger holes ; while those that are exposed 

 to all weathers are pierced through and through with 

 circular holes. This circular weathering is peculiar, 

 as it does not seem to occur except in the vicinity of 

 lakes. It usually begins on the ice-polished surfaces 

 by forming lines of minute holes along the strise ; 

 but if the rock is affected by joints, the holes will 

 more rapidly increase along the joint lines, so that 

 in a very short time all appearance of the stride 

 disappears. Moreover, even if the rock is not jointy, 

 all appearance of the striee will shortly disappear, as 

 these scratches run regularly alike over the chemically 

 hard and soft portions of the rock, while the latter 

 will weather much more freely than the former. 

 Professor Melville of Galway seems to be of opinion 



1 About twenty years ago, during the works for the Lough Corrib 

 Navigation, the summer level of the lake was lowered three feet. 



