ere 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, Letter 521.), the water would flow from it and
the second shelf would be formed. This supposes that a vast barrier
of ice still remains under Ben Nevis, along all the lower part of the
Spean. Lastly, I suppose the ice disappeared everywhere along L. Loggan,
L. Treig, and Glen Spean, except close under Ben Nevis, where it still
formed a barrier, the water flowing out at level of lowest shelf by the
Pass of Mukkul at head of L. Loggan. This seems to me to account for
everything. It presupposes that the shelves were formed towards the
close of the Glacial period. I come up to London to read on Thursday a
short paper at the Linnean Society. Shall I call on Friday morning at