breadth the spot where the barrier stood, with the shelves dying gradually
out, etc. To my mind it is monstrous. Oddly enough, Mr. Milne's
description of the mouth of Loch Treig (I do not believe that valley has
been well examined in its upper end) leaves hardly a doubt that a glacier
descended from it, and, if the roads were formed by a lake of any kind, I
believe it must have been an ice-lake. I have given in detail to Lyell my
several reasons for not thinking ice-lakes probable (520/2. Mr. Darwin
gives some arguments against the glacier theory in the letter (517) to Sir
Charles Lyell; but the letter alluded to is no doubt the one written to
Lyell on "Wednesday, 8th" (Letter 522), in which the reasons are fully
stated.); but to my mind they are incomparably more probable than detritus
of rock-barriers. Have you ever attended to glacier action? After having
seen N. Wales, I can no more doubt the former existence of gigantic
glaciers than I can the sun in the heaven. I could distinguish in N. Wales
to a certain extent icebergs from glacier action (Lyell has shown that
icebergs at the present day score rocks), and I suspect that in Lochaber
the two actions are united, and that the scored rock on the watersheds,
when tideways, were rubbed and bumped by half-stranded icebergs. You will,
no doubt, attend to Glen Glaster. Mr. Milne, I think, does not mention
whether shelf 4 enters it, which I should like to know, and especially he
does not state whether rocks worn on their upper faces are found on the
whole 212 [feet] vertical course of this Glen down to near L. Loggan, or
whether only in the upper part; nor does he state whether these rocks are
scored, or polished, or moutonnees, or whether there are any "perched"
boulders there or elsewhere. I suspect it would be difficult to
distinguish between a river-bed and tidal channel. Mr. Milne's description
of the Pass of Mukkul, expanding to a width of several hundred yards 21
feet deep in the shoalest part, and with a worn islet in the middle, sounds
to me much more like a tidal channel than a river-bed. There must have
been, on the latter view, plenty of fresh water in those days. With