mere barrier of ice; there is so much sloping, stratified detritus in the
valleys. I remember that you somewhere have stated that a running stream
soon cuts deeply into a glacier. I have been hunting up all old references
and pamphlets, etc., on shelves in Scotland, and will send them off to Mr.
J., as they possibly may be of use to him if he continues the subject. The
Eildon Hills ought to be specially examined. Amongst MS. I came across a
very old letter from me to you, in which I say: "If a glacialist admitted
that the sea, before the formation of the shelves, covered the country
(which would account for the land-straits above the level of the shelves),
and if he admitted that the land gradually emerged, and if he supposed that
his lakes were banked up by ice alone, he would make out, in my opinion,
the best case against the marine origin of the shelves." (529/1. See
Letter 522.) This seems very much what you and Mr. J. have come to.

The whole glacial theory is really a magnificent subject.


LETTER 530. TO C. LYELL.
Down, April 1st [1862].

I am not quite sure that I understand your difficulty, so I must give what
seems to me the explanation of the glacial lake theory at some little
length. You know that there is a rocky outlet at the level of all the
shelves. Please look at my map. (530/1. The map accompanying Mr.
Darwin's paper in the "Phil. Trans. R. Soc." 1839.) I suppose whole valley
of Glen Spean filled with ice; then water would escape from an outlet at
Loch Spey, and the highest shelf would be first formed. Secondly, ice
began to retreat, and water will flow for short time over its surface; but
as soon as it retreated from behind the hill marked Craig Dhu, where the
outlet on level of second shelf was discovered by Milne (530/2. See note,