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 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 Roy from Roy Bridge, we
pass the mouth of a lateral glen, called Glen Glaster, running
eastward 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 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 watershed, 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 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-riveted by this new and unexpected 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 subsequently upheaved; that there
were pauses in the process of upheaval, during which th