THE PARALLEL ROADS OF GLEX ROY. 213 



bear examination in detail. No such barriers of de- 

 tritus 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 subsequent 

 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 pres- 

 ent condition of Glen Gluoy and Glen Roy. 



Hypotheses in science, though apparently trans- 

 cending experience, are in reality experience modified 

 by scientific thought and pushed into an ultra-experi- 

 ential 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 



