194 Proceedings of the Royal Irish Academy. 



that melting gh'es rise to considerable slips. The groove is then intensified by 

 snow-slide action on a large scale. The avalanches consist partly of snow 

 and partly of muddy detritus, and represent a marked removal of the rock 

 that has been broken up by frost. The heaps formed by snow-slides are 

 conspicuous on the slopes of Triassic strata on the south shore of Sassen Bay 

 (PI. X., fig. 3). They also make mounds of angular blocks at the foot of the 

 Carboniferous crags in the Smaland Eidge west of Green Baj', where they 

 resemble dissected morames along the mountain-side. 



This action of penetrating water and repeated frosts along the margins of 

 melting snow-patches leads to a rapid destruction of the rock. The process 

 would naturally not be so effective in granite, with comparatively few joints to 

 the square metre, as it is in schist or shale ; but the burrowing down- 

 ward of snow-patches into the ground, in a climate where alternate melting 

 and freezing can take place, is a factor very clearly to be reckoned with. 



The head of the snow-filled groove in which this action is taking place is 

 naturally cirque-like. (PI. X., fig. 4.) As water trickles into it from various 

 sides, it becomes enlarged into a basin, open towards the descending moimtain- 

 side. The snow in the floor of the basin receives infiltering water from above ; 

 it passes from the neve state, through neve-ice, into true glacier-ice. A 

 miniature glacier then occupies the basin that was primarily' worked out by 

 frost-action and by snow-sUdes. The opening of the normal cirque-stage has 

 now been reached ; and glacial action assists in carrying off the detritus that is 

 shoM'ered down from the growing cliffs above. Snow-slide action remains, 

 however, an important factor, and works back the upper part of the basin in 

 the mountain-wall. The cirque-cliff increases in height as it recedes towards 

 the higher ground behind, while the floor at its foot is becoming lowered 

 down the moimtain-side through the denuding effect of the glacier and the 

 running water under it. Comparatively small cirques can generate considerable 

 glaciers, and these, by removing the detritus and allowing new surfaces to be 

 exposed, lead to the rapid enlargement of the hollow. 



Illustrations of the Origin of Cirques. 



A large part of the literature of cirques has been so well discussed by 

 W. H. Hobbs' that it is unnecessary to review it here. Many persons, 

 however, must have realized that the steep bounding wall in a cirque was 

 formed by subaerial erosion, and mainly by fi-ost-action, before the pubUeation 



i"The Cycle of Mountain Glaciation," Geograph. Joum., 1910, p. 149. See also W. M. 

 Daris, " Glacial Erosion in N. Wales," and printed discussion, Quart. Joum. Geol. Soc. London, 

 vol. Ixv (1909), pp. 281-350. 



