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.

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 elevation, 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. But I will write no more, and
pray your forgiveness for this long, ill-written outpouring. I am very
glad you keep to your subject of the terraces. I have lately observed that
you have one great authority (C. Prevost), [not] that authority signifies a