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 respect to the coincidence of the shelves with the now watersheds, Mr. Milne only gives half of my explanation. Please read page 65 of my paper. (520/3. "Observations on the Parallel Roads of Glen Roy, and of other Parts of Lochaber in Scotland, with an Attempt to Prove that they are of Marine Origin." "Phil. Trans. R. Soc." 1839, page 39. [Read February 7th, 1839.]) I allude only to the head of Glen Roy and Kilfinnin as silted up. I did not know Mukkul Pass; and Glen Roy was so much covered up that I did not search it well, as I was not able to walk very well. It has been an old conjectural belief of mine that a rising surface becomes stationary, not suddenly, but by the movement becoming very slow. Now, this would greatly aid the tidal currents cutting down the passes between the mountains just before, and to the level of, the stationary periods. The currents in the fiords in T. del Fuego in a narrow crooked part are often most violent; in other parts they seem to silt up.