THE PAKALLEL ROADS OF GLEN EOY. 211 



the highest and lowest roads of Glen Roy would be ex- 

 plained in a satisfactory manner. 



To account for the second or middle road of Glen 

 Roy, Sir Thomas Dick-Lauder invoked a new agency. 

 He supposed 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 de- 

 tected 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 

 1847 by Mr. David Milne, now Mr. Milne-Home. On 

 walking up Glen Roy from Roy Bridge, we pass the 

 mouth of a lateral glen, called Grlen Glaster, running 

 eastward from Grlen 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 northwards and loses itself among hills 

 too high to offer any outlet. But another branch of 

 the glen turns to the south-east ; and, following up 

 this branch, Mr. Milne- Home reached a col, or water- 

 shed, of the precise level of the second Glen Roy road. 

 When the barrier blocking the glens had been so far 

 removed as to open this col, the water in Glen Roy 

 would sink to the level of the second road. A new 



