234 F. R. Cowper Reed — Carries of Comeragh Mountains. 



screes against their cliffs and of the debris on their floor. Ice-falls 

 from the edge of tlie ice-cap would assist the radial recession of 

 the cliffs at the head, and tend to develop a horse-shoe shape to the 

 corrie. A concentration of the erosive energy of the glacier must 

 be produced at some central point in the floor, and, in the opinion of 

 many geologists, its effects would be apparent by the excavation 

 of a rock-basin. Even Kiuahan (op. cit., p. 131) admitted that ice 

 could thus enlarge and deepen a corrie. But the controversy as to 

 the power of a glacier to scoop out a basin in solid rock is still raging, 

 though no other agent capable of producing hollows, such as those ia 

 which the Snowdon lakes lie,^ has been discovered, and apart from 

 ice-erosion we have to imagine special local subsidence. The fact 

 that infra-glacial corrosion does take place under certain conditions 

 in modern glaciers during their advance seems now indisputably 

 established. But in the case of the Comeragh lakes it has not been 

 definitely proved that any of them lie in true rock-basins, and in all 

 which 1 have examined the morainic dams which stretch across the 

 mouths of the corries seem sufficient to account for their existence. 

 Detailed soundings are, however, not available at present, and we 

 must not forget Marr's - warning as to the incouclusiveness of the 

 evidence furnislied by the nature of the surface over which the 

 stream flows as it issues from the lake. The glacier occupying the 

 corrie could scarcely fail to scour out and remove all the abundant 

 debris in the shape of screes and loose material strewn on the floor, 

 and would deposit them outside in successive heaps and dams of 

 moraine. Stages and lialts in the dwindling awaj' of the glaciers 

 and in the retreat of the snow-line while glacial conditions passed 

 away are marked by the remains of such dams ; and their present 

 preservation is due to the weakness of the erosive power of the 

 streams issuing from the corries or draining off" the overflow of the 

 lakes held up by them. The frost and snow of every winter splits 

 off fragments from the unprotected face of the cliffs, and the jointing 

 of the rocks tends to preserve their vertical character, while the 

 growth of scree material at their foot is an index of their waste sine© 

 the glacier departed. 



EXPLANATION OF PLATE XIII.' 



Fig. 1. — Mouth of Coumsliingaun (Comeragh Mountains) from surface of lowest 

 moraines, showing steep face of middle morainic dam. 



Fig. 2. — Coumsliingaun (Comeragh Mountains) from Croghaim Hill, sho'n'ing the 

 lower morainic slope strewn with boulders, and the steep-faced midille 

 dam bearing irregular morainic mounds near edge of lake. Crotty's 

 Eock on skyline near right margin of view. 



Fig. 3. — Coumgorra (Comeragh Slountains), showing mouth of inner corrie, from 

 top of lowest shelf of moraine. Outer amphitheatre on right of yiew. 



' Jehu: Trans. Roy. Soc. Edinb., vol. xl, pt. 2, No. 20 (1902), pp. 419-467 

 and references. 



- Marr: Q.J.G.S., vol. li (1895), p. 35 ; ibid., vol. lii (1896), p. 12. 



' [This Plate appeared in the April Number with the first part of Mr. Cowper 

 Reed's paper. — Edit. Geol. Mag.] 



