THE PAEALLEL EOADS OF GLEN EOT. 
379 
detritus in certain portions of Glen Spean, and of such detritus 
Sir Thomas Dick-Lauder imagined his harriers to have been 
formed. By some unknown convulsion, this detritus had been 
heaped up. But, once given, and once granted that it was 
subsequently removed, the single read of Glen Gluoy and the 
highest and lowest roads of Glen Boy would be explained in a 
satisfactory manner. 
To account for the second or middle road of Glen Boy, 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 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 1847 by Mr. David Milne, now Mr. Milne-Home. On 
walking up Glen Boy from Boy Bridge, we pass the mouth of a 
lateral glen, called Glen Glaster, running eastward from Glen 
Boy. 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, I think, 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 north- 
wards 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 watershed, of the precise level of the second Glen Boy Boad. 
When the barrier blocking the glens had been so far removed 
as to open this col, the water in Glen Boy would sink to the- 
level of the second road. A new lake of diminished depth would 
be thus formed, the surplus water of which would escape over 
the Glen Glaster col into Glen Spean. The margin of this new 
lake, acting upon the detrital matter, would form the second 
road. The theory of Sir Thomas Dick-Lauder, as regards the- 
part played by the cols, was re-rivetted by this new and un- 
expected discovery. 
I have referred to Mr. Darwin, whose powerful mind swayed 
for a time the convictions of the scientific world in relation to 
this question. His notion was — and it is a notion which very 
naturally presents itself — that the parallel roads were formed 
by the sea ; that this whole region was once submerged and 
