18411880] GLEN ROY 179 



course of this Glen down to near L. Loggan, or whether only Letter 520 

 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 ft. 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 p. 65 of my paper. 1 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. 



Shall you do any levelling ? I believe all the levelling 

 has been [done] in Glen Roy, nearly parallel to the Great 

 Glen of Scotland. For inequalities of elevation, the valley 

 of the Spean, at right angles to the apparent axes of eleva- 

 tion, would be the one to examine. If you go to the head of 

 Glen Roy, attend to the apparent shelf above the highest one 

 in Glen Roy, lying on the south side of Loch Spey, and 

 therefore beyond the watershed of Glen Roy. It would be 

 a crucial case. I was too unwell on that day to examine it 

 carefully, and I had no levelling instruments. Do these 

 fragments coincide in level with Glen Gluoy shelf? 



MacCulloch talks of one in Glen Turret above the shelf, 

 I could not see it. These would be important discoveries. 



1 " 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, p. 39. [Read Feb. 



7 th, 1839.] 



