REMARKS ON GLEN ROY. 27 



to meet this difficulty Prestwich is obliged to imagine a barrier of 

 some kind at Makoul, 100 or 200 feet high, of whose existence we 

 have absolutely no evidence whatever. Similar hypothetical barriers 

 are wanted at the other cols^ all bursting suddenly in order to allow 

 of the lake subsiding rapidly from a greater height. 



Granting, however, all the conditions assumed to be necessary, it 

 is not at all evident why there should have been any such regular 

 and universal slide of debris when the water subsided ; and even if 

 a slide did take place I have a difficulty in believing that it could 

 have stopped with such precision about the same level all along for 

 so many miles. It seems to me that it must have descended to 

 much more variable depths, according to the degree of slope and 

 downward momentum of the sinking mass. Moreover, seeing that 

 it is admitted there luas a lake in Glen Roy, there appears to be no 

 good reason why it should not leave behind it a set of beach-lines 

 without needing those slides of detritus which Prof. Prestwich 

 invokes. The existence of the lake even on his own showing could 

 not have been so very transient, for the great barrier of ice he re- 

 quires at the mouth of Glen Spean could not have melted away ver}^ 

 soon. The theory therefore labours under this further objection, 

 that, a lake being admitted, it is not allowed to act along its margin 

 as every such lake must have done. 



Discussion. 



Prof. Bonnet stated that, notwithstanding the skill with which 

 the case in favour of a glacier- barrier had been stated, he considered 

 that the hypothesis of a marine origin for the ' roads ' presented less 

 difficulties. He pointed out that the absence of sea-shells might be 

 made a difficulty in terraces in j^orway which undoubtedly were of 

 marine origin, and the rarity and alleged horizontality of the ' roads ' 

 were not a serious objection. It must be remembered that great 

 earth-movements were admitted to have taken place at a very late 

 date in the earth's history : over 500 feet in Canada and Labrador, 

 1000 in the Arctic regions, up to 1295 in S. America, not less than 

 600 and probably much more in Norway ; and a considerable uprising 

 was not denied in Scotland and England — certainly some 300 feet 

 in Shropshire, and, as he fully believed, some 1200 at Moel Tryfaen. 

 But he thought the following the most serious difficulty in the glacier- 

 lake hypothesis. The district about Glen Poy was not materially 

 lower than the hill-region to the west, and the rainfall, though less 

 heavy than in it, was still considerable. To form large glaciers in 

 the region of Ben l^evis would require a temperature at the coast 

 not higher than 30° (really, this would hardly be low enough) ; if 

 so, the temperature on the hills above Glen Eoy would not rise above 

 26° (annual). If then the cold sufficed to form glaciers large enough 

 to choke Glen Eoy or Glen Spean, there would be glaciers also in Glen 

 Eoy itself, and it would be filled with ice, not water. In fact, with 

 a temperature of 30° at the coast this western district would be 

 something like the Oberland above the contour-line of 8000 feet, 



