ls" which
intersect the coast-line of Tierra del Fuego. Loc. cit., page 246.))
by the action of icebergs, for that icebergs transported boulders on to
terraces, I have no doubt. Mr. Milne's description of the outlets of
his lake sound to me more like tidal channels, nor does he give any
arguments how such are to be distinguished from old river-courses.
I cannot believe in the body of fresh water which must, on the lake
theory, have flowed out of them. At the Pass of Mukkul he states that
the outlet is 70 feet wide and the rocky bottom 21 feet below the level
of the shelf, and that the gorge expands to the eastwards into a broad
channel of several hundred yards in width, divided in the middle by what
has formerly been a rocky islet, against which the waters of this large
river had chafed in issuing from the pass. We know the size of the river
at the present day which would flow out through this pass, and it seems
to me (and in the other given cases) to be as inadequate; the whole
seems to me far easier explained by a tideway than by a formerly more
humid climate.

With respect to the very remarkable coincidence between the shelves and
the outlets (rendered more remarkable by Mr. Milne's discovery of the
outlet to the intermediate shelf at Glen Glaster (522/10. See Letter
521, note.)), Mr. Milne gives only half of my explanation; he alludes
to (and disputes) the smoothing and silting-up action, which I still
believe in. I state: If we consider what must take place during
the gradual rise of a group of 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.