THE PARALLEL ROADS OF GLEN ROY. 265 



1 

 detritus as he assumed could have existed without 



leaving traces behind them ; but there is no trace left. 

 There, is detritus enough in Glen Spean, but not where 

 it is 'wanted. The two highest parallel roads stop 

 abruptly at different points near the mouth of Glen 

 Roy, but no remnant of the barrier against which they 

 abutted is to be seen. It might be urged that the sub- 

 sequent invasion of the valley by glaciers has swept 

 the detritus away ; but there have been no glaciers in 

 these valleys since the disappearance of the lakes. 

 Professor Geikie has favoured me with a drawing of 

 the Glen Spean ' road ' near the entrance to Glen 

 Trieg. The road forms a shelf round a great mound 

 of detritus which, had a glacier followed the formation 

 of the shelf, must have been cleared away. Taking all 

 the circumstances into account, you may, I think, with 

 safety dismiss the detrital barrier as incompetent to 

 account for the present condition of Glen Gluoy and 

 Glen Koy. 



Hypotheses in science, though apparently trans- 

 cending experience, are in reality experience modified 

 by scientific thought and pushed into an ultra experien- 

 tial region. At the time that he wrote, Sir Thomas 

 Dick-Lauder could not possibly have discerned the 

 cause subsequently assigned for the blockage of these 

 glens. A knowledge of the action of ancient glaciers 

 was the necessary antecedent to the new explanation, 

 and experience of this nature was not possessed by the 

 distinguished writer just mentioned. The extension of 

 Swiss glaciers far beyond their present limits, was first 

 made known by a Swiss engineer named Venetz, who 

 established, by the marks they had left behind them, 

 their former existence in places which they had long 

 forsaken. The subject of glacier extension was subse- 

 quently followed up with distinguished success by 



