islands, we shall have the currents endeavouring to cut down and deepen
some shallow parts in the channels as they are successively brought near
the surface, but tending from the opposition of tides to choke up others
with littoral deposits. During a long interval of rest, from the length of
time allowed to the above processes, the tendency would often prove
effective, both in forming, by accumulation of matter, isthmuses, and in
keeping open channels. Hence such isthmuses and channels just kept open
would oftener be formed at the level which the waters held at the interval
of rest, than at any other (page 65). I look at the Pass of Mukkul (21
feet deep, Milne) as a channel just kept open, and the head of Glen Roy
(where there is a great bay silted up) and of Kilfinnin (at both which
places there are level-topped mounds of detritus above the level of the
terraces) as instances of channels filled up at the stationary levels. I
have long thought it a probable conjecture that when a rising surface
becomes stationary it becomes so, not at once, but by the movements first
becoming very slow; this would greatly favour the cutting down many gaps in
the mountains to the level of the stationary periods.

GLACIER THEORY.

If a glacialist admitted that the sea, before the formation of the
terraces, covered the country (which would account for land-straits above
level of terraces), and that the land gradually emerged, and if he supposed
his lakes were banked by ice alone, he would make out, in my opinion, the
best case against the marine origin of the terraces. From the scattered
boulders and till, you and I must look at it as certain that the sea did
cover the whole country, and I abide quite by my arguments from the
buttresses, etc., that water of some kind receded slowly from the valleys
of Lochaber (I presume Mr. Milne admits this). Now, I do not believe in
the ice-lake theory, from the following weak but accumulating reasons: