More Letters of Charles Darwin Volume Ii Part 19

P.S.--Some facts mentioned in my "Geology of S. America," page 24 (521/6. The creeks which penetrate the western sh.o.r.es of Tierra del Fuego are described as "almost invariably much shallower close to the open sea at their mouths than inland...This shoalness of the sea-channels near their entrances probably results from the quant.i.ty of sediment formed by the wear and tear of the outer rocks exposed to the full force of the open sea. I have no doubt that many lakes--for instance, in Scotland--which are very deep within, and are separated from the sea apparently only by a tract of detritus, were originally sea-channels, with banks of this nature near their mouths, which have since been upheaved" ("Geol. Obs. S. America," page 24, footnote.), with regard to the shoaling of the deep fiords of T. del Fuego near their mouths, and which I have remarked would tend, with a little elevation, to convert such fiords into lakes with a great mound-like barrier of detritus at their mouths, might, possibly, have been of use to you with regard to the lakes of Glen Roy.

LETTER 522. TO C. LYELL. Down, Wednesday, 8th.

Many thanks for your paper. (522/1. "On the Ancient Glaciers of Forfars.h.i.+re." "Proc. Geol. Soc." Volume III., page 337, 1840.) 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, and I have endeavoured my best to be honest. Poor Mrs. Lyell will have, I foresee, a long letter to read aloud, but I will try to write better than usual. Imprimis, it is provoking that Mr. Milne (522/2. "On the Parallel Roads of Lochaber, etc." "Trans. R. Soc. Edinb." Volume XVI., page 395, 1849. [Read March 1st and April 5th, 1847.]) has read my paper (522/3. "Observations on the Parallel Roads of Glen Roy, etc." "Phil. Trans. R. Soc." 1839, page 39. [Read February 7th, 1839.].) with little attention, for he makes me say several things which I do not believe--as, that the water sunk suddenly! (page 10), that the Valley of Glen Roy, page 13, and Spean was filled up with detritus to level of the lower shelf, against which there is, I conceive, good evidence, etc., but I suppose it is the consequence of my paper being most tediously written. He gives me a just snub for talking of demonstration, and he fights me in a very pleasant manner.

Now for business. I utterly disbelieve in the barriers (522/4. See note, Letter 521.) for his lakes, and think he has left that point exactly where it was in the time of MacCulloch (522/5. "On the Parallel Roads of Glen Roy." "Geol. Trans." Volume IV., page 314, 1817 (with several maps and sections).) and d.i.c.k. (522/6. "On the Parallel Roads of Lochaber."

"Trans. R. Soc. Edinb." Volume IX., page 1, 1823.) Indeed, in showing that there is a pa.s.sage at Glen Glaster at the level of the intermediate shelf, he makes the difficulty to my mind greater. (522/7. See Letter 521, note.) When I think of the gradual manner in which the two upper terraces die out at Glen Collarig and at the mouth of Glen Roy, the smooth rounded form of the hills there, and the lower shelf retaining its usual width where the immense barrier stood, 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," etc.: but I do not adhere to this remark with such strength when applied to the glacier-lake theory. Oddly, I was never at all staggered by this theory until now, having read Mr. Milne's argument against it. I now can hardly doubt that a great glacier did emerge from Loch Treig, and this by the ice itself (not moraine) might have blocked up the three outlets from Glen Roy. I do not, however, yet believe in the glacier theory, for reasons which I will presently give.

There are three chief hostile considerations in Mr. Milne's paper.

First, the Glen [shelf?], not coinciding in height with the upper one [outlet?], from observations giving 12 feet, 15 feet, 29 feet, 23 feet: if the latter are correct the terrace must be quite independent, and the case is hostile; but Mr. Milne shows that there is one in Glen Roy 14 feet below the upper one, and a second one again (which I observed) beneath this, and then we come to the proper second shelf. Hence there is no great improbability in an independent shelf having been found in Glen Gluoy.

This leads me to Mr. Milne's second cla.s.s of facts (obvious to every one), namely the non-extension of the three shelves beyond Glen Roy; but I abide by what I have written on that point, and repeat that if in Glen Roy, where circ.u.mstances have been so favourable for the preservation or formation of the terraces, a terrace could be formed quite plain for three-quarters of a mile with hardly a trace elsewhere, we cannot argue, from the non-existence of shelves, that water did not stand at the same levels in other valleys. Feeling absolutely convinced that there was no barrier of detritus at the mouth of Glen Roy, and pretty well convinced that there was none of ice, the manner in which the terraces die out when entering Glen Spean, which must have been a tideway, shows on what small circ.u.mstances the formation of these shelves depended. With respect to the non-existence of shelves in other parts of Scotland, Mr.

Milne shows that many others do exist, and their heights above the sea have not yet been carefully measured, nor have even those of Glen Roy, which I suspect are all 100 feet too high. Moreover, according to Bravais (522/8. "On the Lines of Ancient Level of the Sea in Finmark."

By A. Bravais, Member of the Scientific Commission of the North. "Quart.

Journ. Geol. Soc." Volume I., page 534, 1845 (a translation).), we must not feel sure that either the absolute height 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, nearly N. and S. There should be levels taken at right angles to this line and 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 and newest part of the paper. If Sir James Clark would like to attend to any particular points, direct his attention to this: especially to follow Glen Glaster from Glen Roy to L. 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 ma.s.ses 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 Pa.s.s 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 pa.s.s. We know the size of the river at the present day which would flow out through this pa.s.s, 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 acc.u.mulation 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 Pa.s.s 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 b.u.t.tresses, 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 acc.u.mulating reasons: because, 1st, the receding water must have been that of a lake in Glen Spean, and of the sea in the other valleys of Scotland, where I saw similar b.u.t.tresses at many levels; 2nd, because the outlets of the supposed lakes as already stated seem, from Mr.

Milne's statements, too much worn and too large; 3rd, when the lake stood at the three-quarters of a mile shelf the water from it must have flowed over ice itself for a very long time, and kept at the same exact level: certainly this shelf required a long time for its formation; 4th, I cannot believe a glacier would have blocked up the short, very wide valley of Kilfinnin, the Great Glen of Scotland also being very low there; 5th, the country at some places where Mr. Milne has described terraces is not mountainous, and the number of ice-lakes appears to me very improbable; 6th, I do not believe any lake could scoop the rocks so much as they are at the entrance to Loch Treig or cut them off at the head of Upper Glen Roy; 7th, the very gradual dying away of the terraces at the mouth of Glen Roy does not look like a barrier of any kind; 8th, I should have expected great terminal moraines across the mouth of Glen Roy, Glen Collarig, and Glaster, at least at the bottom of the valleys.

Such, I feel pretty sure, do not exist.

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 Mr. Milne's theory of barriers of detritus, 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. I do not think he gives the smallest satisfaction with respect to the successive and 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 and iceberg action, and sea action well. I wish the Queen would command you. I had intended being in London to-morrow, but one of my princ.i.p.al plagues will, I believe, stop me; if I do I will a.s.suredly call on you. I have not yet read Mr. Milne on Elevation (522/11. "On a Remarkable Oscillation of the Sea, observed at Various Places on the Coasts of Great Britain in the First Week of July, 1843." "Trans. R.

Soc. Edinb." Volume XV., page 609, 1844.), so will keep his paper for a day or two.

P.S.--As you cannot want this letter, I wish you would return it to me, as it will serve as a memorandum for me. Possibly I shall write to Mr.

Chambers, though I do not know whether he will care about what I think on the subject. This letter is too long and ill-written for Sir J.

Clark.

LETTER 523. TO LADY LYELL. [October 4th, 1847.]

I enclose a letter from Chambers, which has pleased me very much (which please return), but I cannot feel quite so sure as he does. If the Lochaber and Tweed roads really turn out exactly on a level, the sea theory is proved. What a magnificent proof of equality of elevation, which does not surprise me much; but I fear I see cause of doubt, for as far as I remember there are numerous terraces, near Galas.h.i.+els, with small intervals of height, so that the coincidence of height might be cooked. Chambers does not seem aware of one very striking coincidence, viz., that I made by careful measurement my Kilfinnin terrace 1202 feet above sea, and now Glen Gluoy is 1203 feet, according to the recent more careful measurements. Even Aga.s.siz (523/1. "On the Glacial Theory," by Louis Aga.s.siz, "Edinb. New Phil. Journ." Volume x.x.xIII., page 217, 1842.

The parallel terraces are dealt with by Aga.s.siz, pages 236 et seq.) would be puzzled to block up Glen Gluoy and Kilfinnin by the same glacier, and then, moreover, the lake would have two outlets. With respect to the middle terrace of Glen Roy--seen by Chambers in the Spean (figured by Aga.s.siz, and seen by myself but not noticed, as I thought it might have been a sheep track)--it might yet have been formed on the ice-lake theory by two independent glaciers going across the Spean, but it is very improbable that two such immense ones should not have been united into one. Chambers, unfortunately, does not seem to have visited the head of the Spey, and I have written to propose joining funds and sending some young surveyor there. If my letter is published in the "Scotsman," how Buckland (523/2. Professor Buckland may be described as joint author, with Aga.s.siz, of the Glacier theory.), as I have foreseen, will crow over me: he will tell me he always knew that I was wrong, but now I shall have rather ridiculously to say, "but I am all right again."

I have been a good deal interested in Miller (523/3. Hugh Miller's "First Impressions of England and its People," London, 1847.), but I find it not quick reading, and Emma has hardly begun it yet. I rather wish the scenic descriptions were shorter, and that there was a little less geologic eloquence.

Lyell's picture now hangs over my chimneypiece, and uncommonly glad I am to have it, and thank you for it.

LETTER 524. TO C. LYELL. Down, September 6th [1861].

I think the enclosed is worth your reading. I am smashed to atoms about Glen Roy. My paper was one long gigantic blunder from beginning to end.

Eheu! Eheu! (524/1. See "Life and Letters," I., pages 68, 69, also pages 290, 291.)

LETTER 525. TO C. LYELL. Down, September 22nd [1861].

I have read Mr. Jamieson's last letter, like the former ones, with very great interest. (525/1. Mr. Jamieson visited Glen Roy in August 1861 and in July 1862. His paper "On the Parallel Roads of Glen Roy, and their Place in the History of the Glacial Period," was published in the "Quarterly Journal of the Geological Society" in 1863, Volume XIX., page 235. His latest contribution to this subject was published in the "Quarterly Journal," Volume XLVIII., page 5, 1892.) What a problem you have in hand! It beats manufacturing new species all to bits. It would be a great personal consolation to me if Mr. J. can admit the sloping Spean terrace to be marine, and would remove one of my greatest difficulties--viz. the vast contrast of Welsh and Lochaber valleys. But then, as far as I dare trust my observations, the sloping terraces ran far up the Roy valley, so as to reach not far below the lower shelf. If the sloping fringes are marine and the shelves lacustrine, all I can say is that nature has laid a shameful trap to catch an unwary wretch. I suppose that I have underrated the power of lakes in producing pebbles; this, I think, ought to be well looked to. I was much struck in Wales on carefully comparing the glacial scratches under a lake (formed by a moraine and which must have existed since the Glacial epoch) and above water, and I could perceive NO difference. I believe I saw many such beds of good pebbles on level of lower shelf, which at the time I could not believe could have been found on sh.o.r.es of lake. The land-straits and little cliffs above them, to which I referred, were quite above the highest shelf; they may be of much more ancient date than the shelves.

Some terrace-like fringes at head of the Spey strike me as very suspicious. Mr. J. refers to absence of pebbles at considerable heights: he must remember that every storm, every deer, every hare which runs tends to roll pebbles down hill, and not one ever goes up again. I may mention that I particularly alluded to this on S. Ventanao (525/2.

"Geolog. Obs. on South America," page 79. "On the flanks of the mountains, at a height of 300 or 400 feet above the plain, there were a few small patches of conglomerate and breccia, firmly cemented by ferruginous matter to the abrupt and battered face of the quartz--traces being thus exhibited of ancient sea-action.") in N. Patagonia, a great isolated rugged quartz-mountain 3,000 feet high, and I could find not one pebble except on one very small spot, where a ferruginous spring had firmly cemented a few to the face of mountain. If the Lochaber lakes had been formed by an ice-period posterior to the (marine?) sloping terraces in the Spean, would not Mr. J. have noticed gigantic moraines across the valley opposite the opening of Lake Treig? I go so far as not to like making the elevation of the land in Wales and Scotland considerably different with respect to the ice-period, and still more do I dislike it with respect to E. and W. Scotland. But I may be prejudiced by having been so long accustomed to the plains of Patagonia. But the equality of level (barring denudation) of even the Secondary formations in Britain, after so many ups and downs, always impresses my mind, that, except when the crust-cracks and mountains are formed, movements of elevation and subsidence are generally very equable.

But it is folly my scribbling thus. You have a grand problem, and heaven help you and Mr. Jamieson through it. It is out of my line nowadays, and above and beyond me.

LETTER 526. TO J.D. HOOKER. Down, September 28th [1861].

It is, I believe, true that Glen Roy shelves (I remember your Indian letter) were formed by glacial lakes. I persuaded Mr. Jamieson, an excellent observer, to go and observe them; and this is his result.

There are some great difficulties to be explained, but I presume this will ultimately be proved the truth...

LETTER 527. TO C. LYELL. Down, October 1st [1861].

Thank you for the most interesting correspondence. What a wonderful case that of Bedford. (527/1. No doubt this refers to the discovery of flint implements in the Valley of the Ouse, near Bedford, in 1861 (see Lyell's "Antiquity of Man," pages 163 et seq., 1863.) I thought the problem sufficiently perplexing before, but now it beats anything I ever heard of. Far from being able to give any hypothesis for any part, I cannot get the facts into my mind. What a capital observer and reasoner Mr.

Jamieson is. The only way that I can reconcile my memory of Lochaber with the state of the Welsh valleys is by imagining a great barrier, formed by a terminal moraine, at the mouth of the Spean, which the river had to cut slowly through, as it drained the lowest lake after the Glacial period. This would, I can suppose, account for the sloping terraces along the Spean. I further presume that sharp transverse moraines would not be formed under the waters of the lake, where the glacier came out of L. Treig and ab.u.t.ted against the opposite side of the valley. A nice mess I made of Glen Roy! I have no spare copy of my Welsh paper (527/2. "Notes on the Effects produced by the Ancient Glaciers of Caernarvons.h.i.+re, and on the Boulders transported by Floating Ice," "Edinb. New Phil. Journ." Volume x.x.xIII., page 352, 1842.); it would do you no good to lend it. I suppose I thought that there must have been floating ice on Moel Tryfan. I think it cannot be disputed that the last event in N. Wales was land-glaciers. I could not decide where the action of land-glaciers ceased and marine glacial action commenced at the mouths of the valleys.

What a wonderful case the Bedford case. Does not the N. American view of warmer or more equable period, after great Glacial period, become much more probable in Europe?

But I am very poorly to-day, and very stupid, and hate everybody and everything. One lives only to make blunders. I am going to write a little book for Murray on Orchids (527/3. "On the Various Contrivances by which Orchids are Fertilised by Insects," London, 1862.), and to-day I hate them worse than everything. So farewell, in a sweet frame of mind.

LETTER 528. TO C. LYELL. Down, October 14th [1861].

I return Jamieson's capital letter. I have no comments, except to say that he has removed all my difficulties, and that now and for evermore I give up and abominate Glen Roy and all its belongings. It certainly is a splendid case, and wonderful monument of the old Ice-period. You ought to give a woodcut. How many have blundered over those horrid shelves!

That was a capital paper by Jamieson in the last "Geol. Journal."

(528/1. "On the Drift and Rolled Gravel of the North of Scotland,"

"Quart. Journ. Geol. Soc." Volume XVI., page 347, 1860.) I was never before fully convinced of the land glacialisation of Scotland before, though Chambers tried hard to convince me.

I must say I differ rather about Ramsay's paper; perhaps he pushes it too far. (528/2. "On the Glacial Origin of Certain Lakes, etc." "Quart.

Journ. Geol. Soc." Volume XVIII., page 185. See Letter 503.) It struck me the more from remembering some years ago marvelling what could be the meaning of such a mult.i.tude of lakes in Friesland and other northern districts. Ramsay wrote to me, and I suggested that he ought to compare mountainous tropical regions with northern regions. I could not remember many lakes in any mountainous tropical country. When Tyndall talks of every valley in Switzerland being formed by glaciers, he seems to forget there are valleys in the tropics; and it is monstrous, in my opinion, the accounting for the Glacial period in the Alps by greater height of mountains, and their lessened height, if I understand, by glacial erosion. "Ne sutor ultra crepidam," I think, applies in this case to him. I am hard at work on "Variation under Domestication." (528/3.

Published 1868.)

P.S.--I am rather overwhelmed with letters at present, and it has just occurred to me that perhaps you will forward my note to Mr. Jamieson; as it will show that I entirely yield. I do believe every word in my Glen Roy paper is false.

LETTER 529. TO C. LYELL. Down, October 20th [1861].