THE PARALLEL ROADS OF GLEN 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 lake-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 -r-v^ 
moment vast masses of detritus in certain portions of Glen 
Spean, and of such deTrTtus 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 
Gluoy and the highest and lowest roads of Glen Roy, would 
be explained in a satisfactory manner. 
To account for the second or middle road of Glen Roy, ~ 
Sir Thomas Dick-Lauder 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 '-<-j^ ; 
parallel roads. Though not destroyed, Sir Thomas Dick- 
Lauder's theory was seriously shaken by this argument, '^4^ 
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 Glen Glaster, running east- 
ward from Glen 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 
