Laggan. Mr. Milne describes this as an old and great river-course with a fall of 212 feet. He states that the rocks are smooth on upper face and rough on lower, but he does not mention whether this character prevails throughout the whole 212 vertical feet--a most important consideration; nor does he state whether these rocks are polished or scratched, as might have happened even to a considerable depth beneath the water (Mem. great icebergs in narrow fiords of T. del Fuego (522/9. In the "Voyage of the 'Beagle'" a description is given of the falling of great masses of ice from the icy cliffs of the glaciers with a crash that "reverberates like the broadside of a man-of-war, through the lonely channels" 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. 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.

Charles Darwin

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