178 GEOLOGY [Chap. IX 



Letter 520 lines end in Glen Collarig. I wish Mr. Milne had read 

 it more carefully. He misunderstands me in several re- 

 spects, but [I] suppose it is my own fault, for my paper 

 is most tediously written. Mr. Milne fights me very 

 pleasantly, and I plead guilty to his rebuke about " demon- 

 stration." * I do not know what you think ; but Mr. Milne 

 will think me as obstinate as a pig when I say that I think 

 any barriers of detritus at the mouth of Glen Roy, Collarig 

 and Glaster more utterly impossible than words can express. 

 I abide by all that I have written on that head. Conceive 

 such a mass of detritus having been removed, without great 

 projections being left on each side, in the very close proximity 

 to every little delta preserved on the lines of the shelves, 

 even on the shelf 4, which now crosses with uniform 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 2 my several reasons for not thinking ice-lakes 

 probable ; but to my mind they are incomparably more pro- 

 bable 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 



1 See p. 180, note 3. 



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. 



