My dear Lyell

Many thanks for the Paper.² I do admire your zeal on a subject on which you are not immediately at work. I will give my opinion as briefly as I can & I have endeavoured my best to be honest. Poor M^rs Lyell will have, I foresee, a long letter to read aloud, but I will try to write better than usual.—³

Imprimis, it is provoking that M^r Milne has read my paper with little attention, for he makes me say several things, which I do not believe,—as that the water sunk suddenly p. 10!! that valley of Glen p. 13. & Spean⁴ was filled with detritus up to level of the lower shelf, against which I conceive there is good evidence &c,—but I suppose it is the consequence of my paper being most tediously written. He gives me a just snub for talking of demonstration;⁵ & he fights me in a very pleasant manner.

Now for business,—I utterly disbelieve in the barriers for his lakes & think he has left that point exactly where it was in the time of Macculloch & Dick.⁶ Indeed in showing that there is a passage at Glen Glaster at the level of the intermediate shelf he makes the difficulty to my mind greater. When I think of the gradual manner in which the 2 upper terraces die out at Glen Collarig, & at mouth of Glen Roy—the smooth rounded form of the hills there,—the lower shelf retaining its usual width where the immense barrier stood;⁷ I can deliberately repeat “that more convincing proofs of the non existence of the imaginary Loch Roy could scarcely have been invented with full play given to the imagination.” &c⁸ But I do not adhere to this remark with such strength, when applied to the Glacier lake theory:⁹ oddly I never was at all staggered by this theory, until now having read M^r Milne’s arguments against it; I now can hardly doubt that a great glacier did emerge from L. Treig, & this by the ice itself (not moraine) might have blocked up the 3 outlets from Glen Roy. I do not, however, yet believe in the glacier theory, for reasons which I will presently give.—

There are three chiefly hostile considerations in M^r Milne’s paper.— First the Glen luy shelf not coinciding in height with the upper one: four observations giving 12^ft, 15^ft, 29^ft, 23^ft;¹⁰ if the latter are correct, this terrace must be quite independent & the case is hostile,¹¹ but M^r Milne shows that there is one in Glen Roy 14 ft below the upper one, & a second one again (which I observed) beneath this, & then we come to the proper second shelf. Hence there is no great improbability in an independent shelf having been formed in Glen Gluy.

This leads me to M^r. Milnes second class of facts, (obvious to everyone) namely the non-extension of the three shelves beyond Glen Roy; but I abide by what I have written on that point,¹² & repeat, that if in Glen Roy,, where circumstances have been so favourable for the preservation or formation of the terraces, a terrace could be formed quite plain for $\frac{3}{4}$ of a mile with hardly a trace elsewhere, we cannot argue from the non-existence of shelves, that water did not stand at these same levels in other valleys. Feeling absolutely convinced that there was no barrier of detritus at the mouth of Glen Roy, & pretty well convinced there was none of ice, the manner in which the terraces die out when entering Glen Spean, which must have been a tide way, shows on what small circumstances the formation of these shelves depended. With respect to the non-existence of shelves in other parts of Scotland, M^r Milne shows that many others do exist, & their heights above the sea have not yet been carefully measured, nor have even those of Glen Roy, which I suspect are all 100 ft too high.¹³ Moreover according to Bravais,¹⁴ we must not feel sure that either the absolute heights, or the intermediate heights between the terraces, would be at all the same at distant points. In levelling the terraces in Lochaber, all I believe have been taken in Glen Roy neary N. & S; there sh^d be levels taken at right angles to this line, & to the Great Glen of Scotland, or chief line of elevation.

Thirdly: the nature of the outlets from the supposed lakes: this appears to me the best & newest part of the paper. If Sir James Clark¹⁵ would like to attend to any particular points, direct his attention to this; especially to follow Glen Glastig from Glen Roy to L. Laggan. M^r Milne describes this as an old & great river course with a fall of 212 ft. He states that the rocks are smooth on upper face & 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)¹⁶ by the action of icebergs, for that icebergs transported boulders on to the terraces, I have no doubt. M^r Milne’s description of the outlets of his lakes, 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 Muckul, he states that the outlet is 70 feet wide & the rocky bottom 21 feet below the level of the shelf, & 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.”¹⁷ We know the size of the river at the present day which would flow out through this pass, & it seems to me (& 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 & the outlets (rendered more remarkable by M^r Milne’s discovery of the outlet to the intermediate shelf at Glen Glastig),¹⁸ M^r Milne gives only half of my explanation; he alludes to (& disputes) the smoothing & 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 & 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, & in keeping open channels. Hence such isthmuses & channels just kept open, would oftener be formed at the level, which the waters held at the interval of rest, than at any one other p. 65”¹⁹

I look at the pass of Muckul (21 ft deep, Milne) as a channel just kept open, & the head of Glen Roy (where there is a great Bay silted up) & of Kilfinnin,²⁰ (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.—

Glacial 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) & that the land gradually emerged; & 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 & till, you & I must look at it as certain that the sea did cover the whole country; & I abide quite by my arguments from the buttresses &c &c²¹ that water of some kind receded slowly from the valleys of Lochaber (I presume M^r Milne admits this). Now I do not believe in the ice-lake theory, from the following weak, but accumulated reasons because 1^st the receding water must have been that of a lake in Glen Spean & of the sea in the other valleys of Scotland, where I saw similar buttresses at many levels. 2^d. because the outlets of the supposed lakes, as already stated, seem from M^r Milnes statements too much worn & too large. 3^d. When the lake stood at the $\frac{3}{4}$ of mile shelf, the water from it must have flowed over ice itself for a very long time & kept at the same exact level: certainly this shelf required a long time for its formation 4^th. I cannot believe a glacier c^d. have blocked up the short, very wide, valley of Kilfinnin, the Great Glen of Scotland, also, being very low there. 5^th The country at some places, where M^r Milne has described terraces, is not mountainous, & the number of ice-lakes appears to me very improbable. 6^th I do not believe any lake could scoop the rocks²² so much as they are at the entrance of Loch Treig, or cut them off at the head of upper Glen Roy 7^th. the very gradual dying away of the terraces at the mouth of Glen Roy, does not look like a barrier of any kind.— 7^th I sh^d. have expected great terminal moraines across the mouth of Glen Roy, Glen Collarig & Glastig, at least at the bottoms of the valleys; such I feel pretty sure do not exist.²³

I fear I must have wearied you with the length of this letter, which I have not had time to arrange properly.— I could argue at great length against M^r Milnes theory of barriers of detritus,²⁴ though I could help him in one way, viz by the soundings which occur at the entrances of the deepest fiords in T. del Fuego.²⁵ I do not think he gives the smallest satisfaction with respect to the successive & comparatively sudden breakage of his many lakes.

Well, I enjoyed my trip to Glen Roy very much, but it was time thrown away: I heartily wish you would go there: it should be some one who knows glacier & iceberg action & sea-action well: I wish the Queen would command you.— I had intended being in London tomorrow, but one of my painful plagues will, I believe, stop me; if I do I will assuredly call on you. I have not yet read M^r Milne on Elevation, so will keep his paper for a day or two.²⁶

Farewell, my dear Lyell, ever most truly yours C. Darwin

P.S. As you cannot want this letter, I wish you would return it me, as it will serve as a memorandum for me—possibly I shall write to M^r Chambers,²⁷ though I do not know whether he will care what I think on the subject. This letter is too long & ill-written for Sir J. Clark