Volume Ii Part 19 (1/2)

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.