theory, the greatest objection appears to me the following, though possibly
not a sound one. The water has beyond doubt remained very long at the
levels of each shelf--this is unequivocally shown by the depth of the notch
or beach formed in many places in the hard mica-slate, and the large
accumulations or buttresses of well-rounded pebbles at certain spots on the
level of old beaches. (The time must have been immense, if formed by lakes
without tides.) During the existence of the lakes their drainage must have
been at the head of the valleys, and has given the flat appearance of the
watersheds. All this is very clear for four of the shelves (viz., upper
and lower in Glen Roy, the 800-foot one in Glen Spean, and the one in
Kilfinnin), and explains the coincidence of "roads" with the watersheds
more simply than my view, and as simply as the common lake theory. But how
was the Glen Roy lake drained when the water stood at level of the middle
"road"? It must (for there is no other exit whatever) have been drained
over the glacier. Now this shelf is full as narrow in a vertical line and
as deeply worn horizontally into the mountain side and with a large
accumulation of shingle (I can give cases) as the other shelves. We must,
therefore, on the glacier theory, suppose that the surface of the ice
remained at exactly the same level, not being worn down by the running
water, or the glacier moved by its own movement during the very long period
absolutely necessary for a quiet lake to form such a beach as this shelf
presents in its whole course. I do not know whether I have explained
myself clearly. I should like to know what you think of this difficulty.
I shall much like to talk over the Jura case with you. I am tired, so
goodbye.


LETTER 518. TO L. HORNER.
Down [1846].