THE PA HA LL EL HO A DS F GL EN ROT. 161 



cupied by a continuous lake, the level of which would 

 obviously be determined by the col at the head of Loch 

 Laggan. The water in Glen Roy would sink from the level 

 it had previously maintained, to the level of its new place 

 of escape. This new hike-surface would correspond exactly 

 with the lowest parallel road, and it would form that road 

 by its action upon the drift of the adjacent mountains. 



In presence of the observed facts, this solution commends 

 itself strongly to the scientific mind. The question next 

 occurs, What was the character of the assumed barrier 

 which stopped the glens? There are at the present 

 moment vast masses of detritus in certain portions of Glen 

 Spean, and of such detritus Sir Thomas Dick-Lauder 

 imagined his barriers to have been formed. By some un- 

 known convulsion, this detritus 'had been heaped up. 

 But, once given, and once granted that it was subsequently 

 removed in the manner indicated, the single road of Glen 

 Giuoy and the highest and lowest roads of Glen Hoy, would 

 be explained in a satisfactory manner. 



To account for the second or middle road of Glen Roy, 

 Sir Thomas Dick-Lander invoked a new agency. He sup- 

 posed that at a certain point in the breaking down or 

 waste of his dam, a halt occurred, the barrier holding its 

 ground at a particular level sufficiently long to dam a lake 

 rising to the height of, and forming the second road. This 

 point of weakness was at once detected by Mr. Darwin, and 

 adduced by him as proving that the levels of the cols did 

 not constitute an essential feature in the phenomena of the 

 parallel roads. Though not destroyed, Sir Thomas Dick- 

 Lauder's theory was seriously shaken by this argument, 

 and it became a point of capital importance, if the facts 

 permitted, to remove such source of weakness This was 

 done in 1817 by Mr. David Milne, now Mr. Miine-Honre. 

 On walking up Glen Roy from Roy Bridge, we pass the 

 mouth of a lateral glen, called Glen Glaster, running east- 

 ward from Gleu Roy. There is nothing in this lateral glen 

 to attract attention, or to suggest that it could have any 

 conspicuous influence in the production of the parallel 

 roads. Hence, probably, the failure of Sir Thomas Dick- 

 Lauder to notice it. But Mr. Milne-Home entered this 

 glen, on the northern side of which the middle and lowest 

 roads are fairly shown. The principal stream running 

 through the glen turns at a certain point northward and 



