Чарльз Дарвин
More Letters of Charles Darwin — Volume 2
LETTER 513. TO D. MACKINTOSH. Down, November 13th, 1880.
Your discovery is a very interesting one, and I congratulate you on it. (513/1. "On the Precise Mode of Accumulation and Derivation of the Moel-Tryfan Shelly Deposits; on the Discovery of Similar High-level Deposits along the Eastern Slopes of the Welsh Mountains; and on the Existence of Drift-Zones, showing probable Variations in the Rate of Submergence." By D. Mackintosh, "Quart. Journ. Geol. Soc." Volume XXXVII., pages 351-69, 1881. {Read April 27th, 1881.}) I failed to find shells on Moel Tryfan, but was interested by finding ("Philosoph. Mag." 3rd series, Volume XXI., page 184) shattered rocks (513/2. In reviewing the work by previous writers on the Moel-Tryfan deposits, Mackintosh refers to Darwin's "very suggestive description of the Moel-Tryfan deposits...Under the drift he saw that the surface of the slate, TO A DEPTH OF SEVERAL FEET, HAD BEEN SHATTERED AND CONTORTED IN A VERY PECULIAR MANNER." The contortion of the slate, which Mackintosh regarded as "the most interesting of the Moel-Tryfan phenomena," had not previously been regarded as "sufficiently striking to arrest attention" by any geologist except Darwin. The Pleistocene gravel and sand containing marine shells on Moel-Tryfan, about five miles south-east of Caernarvon, have been the subject of considerable controversy. By some geologists the drift deposits have been regarded as evidence of a great submergence in post-Pliocene times, while others have explained their occurrence at a height of 1300 feet by assuming that the gravel and sand had been thrust uphill by an advancing ice-sheet. (See H.B. Woodward, "Geology of England and Wales," Edition II., 1887, pages 491, 492.) Darwin attributed the shattering and contorting of the slates below the drift to "icebergs grating over the surface.") and far-distant rounded boulders, which I attributed to the violent impact of icebergs or coast-ice. I can offer no opinion on whether the more recent changes of level in England were or were not accompanied by earthquakes. It does not seem to me a correct expression (which you use probably from haste in your note) to speak of elevations or depressions as caused by earthquakes: I suppose that every one admits that an earthquake is merely the vibration from the fractured crust when it yields to an upward or downward force. I must confess that of late years I have often begun to suspect (especially when I think of the step-like plains of Patagonia, the heights of which were measured by me) that many of the changes of level in the land are due to changes of level in the sea. (513/3. This view is an agreement with the theory recently put forward by Suess in his "Antlitz der Erde" (Prag and Leipzig, 1885). Suess believes that "the local invasions and transgressions of the continental areas by the sea" are due to "secular movements of the hydrosphere itself." (See J. Geikie, F.R.S., Presidential Address before Section E at the Edinburgh Meeting of the British Association, "Annual Report," page 794.) I suppose that there can be no doubt that when there was much ice piled up in the Arctic regions the sea would be attracted to them, and the land on the temperate regions would thus appear to have risen. There would also be some lowering of the sea by evaporation and the fixing of the water as ice near the Pole.
I shall read your paper with much interest when published.
LETTER 514. TO J. GEIKIE. Down, December 13th, 1880.
You must allow me the pleasure of thanking you for the great interest with which I have read your "Prehistoric Europe." (514/1. "Prehistoric Europe: a Geological Sketch," London, 1881.) Nothing has struck me more than the accumulated evidence of interglacial periods, and assuredly the establishment of such periods is of paramount importance for understanding all the later changes of the earth's surface. Reading your book has brought vividly before my mind the state of knowledge, or rather ignorance, half a century ago, when all superficial matter was classed as diluvium, and not considered worthy of the attention of a geologist. If you can spare the time (though I ask out of mere idle curiosity) I should like to hear what you think of Mr. Mackintosh's paper, illustrated by a little map with lines showing the courses or sources of the erratic boulders over the midland counties of England. (514/2. "Results of a Systematic Survey, in 1878, of the Directions and Limits of Dispersion, Mode of Occurrence, and Relation to Drift-Deposits of the Erratic Blocks or Boulders of the West of England and East of Wales, including a Revision of Many Years' Previous Observations," D. Mackintosh, "Quart. Journ. Geol. Soc." Volume XXXV., page 425, 1879.) It is a little suspicious their ending rather abruptly near Wolverhampton, yet I must think that they were transported by floating ice. Fifty years ago I knew Shropshire well, and cannot remember anything like till, but abundance of gravel and sand beds, with recent marine shells. A great boulder (514/3. Mackintosh alludes (loc. cit., page 442) to felstone boulders around Ashley Heath, the highest ground between the Pennine and Welsh Hills north of the Wrekin; also to a boulder on the summit of the eminence (774 feet above sea-level), "probably the same as that noticed many years ago by Mr. Darwin." In a later paper, "On the Correlation of the Drift-Deposits of the North-West of England with those of the Midland and Eastern Counties" ("Quart. Journ. Geol. Soc." Volume XXXVI., page 178, 1880) Mackintosh mentions a letter received from Darwin, "who was the first to elucidate the boulder-transporting agency of floating ice," containing an account of the great Ashley Heath boulder, which he was the first to discover and expose...so as to find that the block rested on fragments of New Red Sandstone, one of which was split into two and deeply scored...The facts mentioned in the letter from Mr. Darwin would seem to show that the boulder must have fallen through water from floating ice with a force sufficient to split the underlying lump of sandstone, but not sufficient to crush it.") which I had undermined on the summit of Ashley Heath, 720 (?) feet above the sea, rested on clean blocks of the underlying red sandstone. I was also greatly interested by your long discussion on the Loss (514/4. For an account of the Loss of German geologists — "a fine-grained, more or less homogeneous, consistent, non-plastic loam, consisting of an intimate admixture of clay and carbonate of lime," see J. Geikie, loc. cit., page 144 et seq.); but I do not feel satisfied that all has been made out about it. I saw much brick-earth near Southampton in some manner connected with the angular gravel, but had not strength enough to make out relations. It might be worth your while to bear in mind the possibility of fine sediment washed over and interstratified with thick beds of frozen snow, and therefore ultimately dropped irrespective of the present contour of the country.
I remember as a boy that it was said that the floods of the Severn were more muddy when the floods were caused by melting snow than from the heaviest rains; but why this should be I cannot see.
Another subject has interested me much — viz. the sliding and travelling of angular debris. Ever since seeing the "streams of stones" at the Falkland Islands (514/5. "Geological Observations on South America" (1846), page 19 et seq.), I have felt uneasy in my mind on this subject. I wish Mr. Kerr's notion could be fully elucidated about frozen snow. Some one ought to observe the movements of the fields of snow which supply the glaciers in Switzerland.
Yours is a grand book, and I thank you heartily for the instruction and pleasure which it has given me.
For heaven's sake forgive the untidiness of this whole note.
LETTER 515. TO JOHN LUBBOCK {Lord Avebury}. Down, November 6th, 1881.
If I had written your Address (515/1. Address delivered by Lord Avebury as President of the British Association at York in 1881. Dr. Hicks is mentioned as having classed the pre-Cambrian strata in "four great groups of immense thickness and implying a great lapse of time" and giving no evidence of life. Hicks' third formation was named by him the Arvonian ("Quart. Journ. Geol. Soc." Volume XXXVII., 1881, Proc., page 55.) (but this requires a fearful stretch of imagination on my part) I should not alter what I had said about Hicks. You have the support of the President {of the} Geological Society (515/2. Robert Etheridge.), and I think that Hicks is more likely to be right than X. The latter seems to me to belong to the class of objectors general. If Hicks should be hereafter proved to be wrong about this third formation, it would signify very little to you.
I forget whether you go as far as to support Ramsay about lakes as large as the Italian ones: if so, I would myself modify the passage a little, for these great lakes have always made me tremble for Ramsay, yet some of the American geologists support him about the still larger N. American lakes. I have always believed in the main in Ramsay's views from the date of publication, and argued the point with Lyell, and am convinced that it is a very interesting step in Geology, and that you were quite right to allude to it. (515/3. "Glacial Origin of Lakes in Switzerland, Black Forest, etc." ("Quart. Journ. Geol. Soc." Volume XVIII., pages 185-204, 1862). Sir John Lubbock (Lord Avebury) gives a brief statement of Ramsay's views concerning the origin of lakes (Presidential Address, Brit. Assoc. 1881, page 22): "Prof. Ramsay divides lakes into three classes: (1) Those which are due to irregular accumulations of drift, and which are generally quite shallow; (2) those which are formed by moraines; and (3) those which occupy true basins scooped by glaciers out of the solid rocks. To the latter class belong, in his opinion, most of the great Swiss and Italian lakes...Professor Ramsay's theory seems, therefore, to account for a large number of interesting facts." Sir Archibald Geikie has given a good summary of Ramsay's theory in his "Memoir of Sir Andrew Crombie Ramsay," page 361, London, 1895.)
LETTER 516. TO D. MACKINTOSH. Down, February 28th, 1882.
I have read professor Geikie's essay, and it certainly appears to me that he underrated the importance of floating ice. (516/1. "The Intercrossing of Erratics in Glacial Deposits," by James Geikie, "Scottish Naturalist," 1881.) Memory extending back for half a century is worth a little, but I can remember nothing in Shropshire like till or ground moraine, yet I can distinctly remember the appearance of many sand and gravel beds — in some of which I found marine shells. I think it would be well worth your while to insist (but perhaps you have done so) on the absence of till, if absent in the Western Counties, where you find many erratic boulders.
I was pleased to read the last sentence in Geikie's essay about the value of your work. (516/2. The concluding paragraph reads as follows: "I cannot conclude this paper without expressing my admiration for the long-continued and successful labours of the well-known geologist whose views I have been controverting. Although I entered my protest against his iceberg hypothesis, and have freely criticised his theoretical opinions, I most willingly admit that the results of his unwearied devotion to the study of those interesting phenomena with which he is so familiar have laid all his fellow-workers under a debt of gratitude." Mr. Darwin used to speak with admiration of Mackintosh's work, carried on as it was under considerable difficulties.)
With respect to the main purport of your note, I hardly know what to say. Though no evidence worth anything has as yet, in my opinion, been advanced in favour of a living being, being developed from inorganic matter, yet I cannot avoid believing the possibility of this will be proved some day in accordance with the law of continuity. I remember the time, above fifty years ago, when it was said that no substance found in a living plant or animal could be produced without the aid of vital forces. As far as external form is concerned, Eozoon shows how difficult it is to distinguish between organised and inorganised bodies. If it is ever found that life can originate on this world, the vital phenomena will come under some general law of nature. Whether the existence of a conscious God can be proved from the existence of the so-called laws of nature (i.e., fixed sequence of events) is a perplexing subject, on which I have often thought, but cannot see my way clearly. If you have not read W. Graham's "Creed of Science," (516/3. "The Creed of Science: Religious, Moral, and Social," London, 1881.), it would, I think, interest you, and he supports the view which you are inclined to uphold.
2. IX.III. THE PARALLEL ROADS OF GLEN ROY, 1841-1880.
(517/1. In the bare hilly country of Lochaber, in the Scotch Highlands, the slopes of the mountains overlooking the vale of Glen Roy are marked by narrow terraces or parallel roads, which sweep round the shoulders of the hills with "undeviating horizontality." These roads are described by Sir Archibald Geikie as having long been "a subject of wonderment and legendary story among the Highlanders, and for so many years a source of sore perplexity among men of science." (517/2. "The Scenery of Scotland," 1887, page 266.) In Glen Roy itself there are three distinct shelves or terraces, and the mountain sides of the valley of the Spean and other glens bear traces of these horizontal "roads."
The first important papers dealing with the origin of this striking physical feature were those of MacCulloch (517/3. "Trans. Geol. Soc." Volume IV., page 314, 1817.) and Sir Thomas Lauder Dick (517/4. "Trans. R. Soc. Edinb." Volume IX., page 1, 1823.), in which the writers concluded that the roads were the shore-lines of lakes which once filled the Lochaber valleys. Towards the end of June 1838 Mr. Darwin devoted "eight good days" (517/5. "Life and Letters," I., page 290.) to the examination of the Lochaber district, and in the following year he communicated a paper to the Royal Society of London, in which he attributed their origin to the action of the sea, and regarded them as old sea beaches which had been raised to their present level by a gradual elevation of the Lochaber district.
In 1840 Louis Agassiz and Buckland (517/6. "Edinb. New Phil. Journal," Volume XXXIII., page 236, 1842.) proposed the glacier-ice theory; they described the valleys as having been filled with lakes dammed back by glaciers which formed bars across the valleys of Glen Roy, Glen Spean, and the other glens in which the hill-sides bear traces of old lake-margins. Agassiz wrote in 1842: "When I visited the parallel roads of Glen Roy with Dr. Buckland we were convinced that the glacial theory alone satisfied all the exigencies of the phenomenon." (517/7. Ibid., page 236.)
Mr. David Milne (afterwards Milne-Home) (517/8. "Trans. R. Soc. Edinb." Volume XVI., page 395, 1847.) in 1847 upheld the view that the ledges represent the shore-lines of lakes which were imprisoned in the valleys by dams of detrital material left in the glens during a submergence of 3,000 feet, at the close of the Glacial period. Chambers, in his "Ancient Sea Margins" (1848), expressed himself in agreement with Mr. Darwin's marine theory. The Agassiz-Buckland theory was supported by Mr. Jamieson (517/9. "Quart. Journ. Geol. Soc." Volume XIX., page 235, 1863.), who brought forward additional evidence in favour of the glacial barriers. Sir Charles Lyell at first (517/10. "Elements of Geology," Edition II., 1841.) accepted the explanation given by Mr. Darwin, but afterwards (517/11. "Antiquity of Man," 1863, pages 252 et seq.) came to the conclusion that the terrace-lines represent the beaches of glacial lakes. In a paper published in 1878 (517/12. "Phil. Trans. R. Soc." 1879, page 663.), Prof. Prestwich stated his acceptance of the lake theory of MacCulloch and Sir T. Lauder Dick and of the glacial theory of Agassiz, but differed from these authors in respect of the age of the lakes and the manner of formation of the roads.
The view that has now gained general acceptance is that the parallel roads of Glen Roy represent the shores of a lake "that came into being with the growth of the glaciers and vanished as these melted away." (517/13. Sir Archibald Geikie, loc. cit., page 269.)
Mr. Darwin became a convert to the glacier theory after the publication of Mr. Jamieson's paper. He speaks of his own paper as "a great failure"; he argued in favour of sea action as the cause of the terraces "because no other explanation was possible under our then state of knowledge." Convinced of his mistake, Darwin looked upon his error as "a good lesson never to trust in science to the principle of exclusion." (517/14. "Life and Letters," I., page 69.)
LETTER 517. TO C. LYELL. {March 9th, 1841.}
I have just received your note. It is the greatest pleasure to me to write or talk Geology with you...
I think I have thought over the whole case without prejudice, and remain firmly convinced they {the parallel roads} are marine beaches. My principal reason for doing so is what I have urged in my paper (517/15. "Observations on the Parallel Roads of Glen Roy, and of other parts of Lochaber in Scotland, with an attempt to prove that they are of Marine Origin." "Phil. Trans. R. Soc." 1839, page 39.), the buttress-like accumulations of stratified shingle on sides of valley, especially those just below the lowest shelf in Spean Valley.
2nd. I can hardly conceive the extension of the glaciers in front of the valley of Kilfinnin, where I found a new road — where the sides of Great Glen are not very lofty.
3rd. The flat watersheds which I describe in places where there are no roads, as well as those connected with "roads." These remain unexplained.
I might continue to add many other such reasons, all of which, however, I daresay would appear trifling to any one who had not visited the district. With respect to equable elevation, it cannot be a valid objection to any one who thinks of Scandinavia or the Pampas. With respect to the glacier theory, the greatest objection appears to me the following, though possibly not a sound one. The water has beyond doubt remained very long at the levels of each shelf — this is unequivocally shown by the depth of the notch or beach formed in many places in the hard mica-slate, and the large accumulations or buttresses of well-rounded pebbles at certain spots on the level of old beaches. (The time must have been immense, if formed by lakes without tides.) During the existence of the lakes their drainage must have been at the head of the valleys, and has given the flat appearance of the watersheds. All this is very clear for four of the shelves (viz., upper and lower in Glen Roy, the 800-foot one in Glen Spean, and the one in Kilfinnin), and explains the coincidence of "roads" with the watersheds more simply than my view, and as simply as the common lake theory. But how was the Glen Roy lake drained when the water stood at level of the middle "road"? It must (for there is no other exit whatever) have been drained over the glacier. Now this shelf is full as narrow in a vertical line and as deeply worn horizontally into the mountain side and with a large accumulation of shingle (I can give cases) as the other shelves. We must, therefore, on the glacier theory, suppose that the surface of the ice remained at exactly the same level, not being worn down by the running water, or the glacier moved by its own movement during the very long period absolutely necessary for a quiet lake to form such a beach as this shelf presents in its whole course. I do not know whether I have explained myself clearly. I should like to know what you think of this difficulty. I shall much like to talk over the Jura case with you. I am tired, so goodbye.
LETTER 518. TO L. HORNER. Down {1846}.
(518/1. It was agreed at the British Association meeting held at Southampton in 1846 "That application be made to Her Majesty's Government to direct that during the progress of the Ordnance Trigonometrical Surveys in the North of Scotland, the so-called Parallel Roads of Glen Roy and the adjoining country be accurately surveyed, with the view of determining whether they are truly parallel and horizontal, the intervening distances, and their elevations above the present sea-level" ("British Association Report," 1846, page xix). The survey was undertaken by the Government Ordnance Survey Office under Col. Sir Henry James, who published the results in 1874 ("Notes on the Parallel Roads of Glen Roy"); the map on which the details are given is sheet 63 (one-inch scale).)
In following your suggestion in drawing out something about Glen Roy for the Geological Committee, I have been completely puzzled how to do it. I have written down what I should say if I had to meet the head of the Survey and wished to persuade him to undertake the task; but as I have written it, it is too long, ill expressed, seems as if it came from nobody and was going to nobody, and therefore I send it to you in despair, and beg you to turn the subject in your mind. I feel a conviction if it goes through the Geological part of Ordnance Survey it will be swamped, and as it is a case for mere accurate measurements it might, I think without offence, go to the head of the real Surveyors.
If Agassiz or Buckland are on the Committee they will sneer at the whole thing and declare the beaches are those of a glacier-lake, than which I am sure I could convince you that there never was a more futile theory.
I look forward to Southampton (518/2. The British Association meeting (1846).) with much interest, and hope to hear to-morrow that the lodgings are secured to us. You cannot think how thoroughly I enjoyed our geological talks, and the pleasure of seeing Mrs. Horner and yourself here. (518/3. This letter is published in the privately printed "Memoir of Leonard Horner," II., page 103.)
{Here follows Darwin's Memorandum.}
The Parallel Roads of Glen Roy, in Scotland, have been the object of repeated examination, but they have never hitherto been levelled with sufficient accuracy. Sir T. Lauder Dick (518/4. "On the Parallel Roads of Lochaber" (with map and plates), by Sir Thomas Lauder Dick, "Trans. R. Soc. Edinb." Volume IX., page 1, 1823.) procured the assistance of an engineer for this purpose, but owing to the want of a true ground-plan it was impossible to ascertain their exact curvature, which, as far as could be estimated, appeared equal to that of the surface of the sea. Considering how very rarely the sea has left narrow and well-defined marks of its action at any considerable height on the land, and more especially considering the remarkable observations by M. Bravais (518/5. "On the Lines of Ancient Level of the Sea in Finmark," by M. A. Bravais, translated from "Voyages de la Commission Scientifique du Nord, etc."; "Quart. Journ. Geol. Soc." Volume I., page 534, 1845.) on the ancient sea-beaches of Scandinavia, showing the they are not strictly parallel to each other, and that the movement has been greater nearer the mountains than on the coast, it appears highly desirable that the roads of Glen Roy should be examined with the utmost care during the execution of the Ordnance Survey of Scotland. The best instruments and the most accurate measurements being necessary for this end almost precludes the hope of its being ever undertaken by private individuals; but by the means at the disposal of the Ordnance, measurements would be easily made even more accurate than those of M. Bravais. It would be desirable to take two lines of the greatest possible length in the district, and at nearly right angles to each other, and to level from the beach at one extremity to that at the other, so that it might be ascertained whether the curvature does exactly correspond with that of the globe, or, if not, what is the direction of the line of greatest elevation. Much attention would be requisite in fixing on either the upper or lower edge of the ancient beaches as the standard of measurement, and in rendering this line conspicuous. The heights of the three roads, one above the other and above the level of the sea, ought to be accurately ascertained. Mr. Darwin observed one short beach-line north of Glen Roy, and he has indicated, on the authority of Sir David Brewster, others in the valley of the Spey. If these could be accurately connected, by careful measurements of their absolute heights or by levelling, with those of Glen Roy, it would make a most valuable addition to our knowledge on this subject. Although the observations here specified would probably be laborious, yet, considering how rarely such evidence is afforded in any quarter of the world, it cannot be doubted that one of the most important problems in Geology — namely, the exact manner in which the crust of the earth rises in mass — would be much elucidated, and a great service done to geological science.
LETTER 519. R. CHAMBERS TO D. MILNE-HOME. St. Andrews, September 7th, 1847.
I have had a letter to-day from Mr. Charles Darwin, beseeching me to obtain for him a copy of your paper on Glen Roy. (519/1. No doubt Mr. Milne's paper "On the Parallel Roads of Lochaber," "Trans. R. Soc. Edinb." Volume XVI., page 395, 1849. {Read March 1st and April 5th, 1847.}) I am sure you will have pleasure in sending him one; his address is "Down, Farnborough, Kent." I have again read over your paper carefully, and feel assured that the careful collection and statement of facts which are found in it must redound to your credit with all candid persons. The suspicions, however, which I obtained some time ago as to land-straits and heights of country being connected with sea-margins and their ordinary memorials still possesses me, and I am looking forward to some means of further testing the Glen Roy mystery. If my suspicion turn out true, I shall at once be regretful on your account, and shall feel it as a great check and admonition to myself not to be too confident about anything in science till it has been proved over and over again. The ground hereabouts is now getting clear of the crops; perhaps when I am in town a few days hence we may be able to make some appointment for an examination of the beaches of the district, my list of which has been greatly enlarged during the last two months.
LETTER 520. TO R. CHAMBERS. September 11th, 1847.
I hope you will read the first part of my paper before you go {to Glen Roy}, and attend to the manner in which the lines end in Glen Collarig. I wish Mr. Milne had read it more carefully. He misunderstands me in several respects, but {I} suppose it is my own fault, for my paper is most tediously written. Mr. Milne fights me very pleasantly, and I plead guilty to his rebuke about "demonstration." (520/1. See Letter 521, note.) I do not know what you think; but Mr. Milne will think me as obstinate as a pig when I say that I think any barriers of detritus at the mouth of Glen Roy, Collarig and Glaster more utterly impossible than words can express. I abide by all that I have written on that head. Conceive such a mass of detritus having been removed, without great projections being left on each side, in the very close proximity to every little delta preserved on the lines of the shelves, even on the shelf 4, which now crosses with uniform breadth the spot where the barrier stood, with the shelves dying gradually out, etc. To my mind it is monstrous. Oddly enough, Mr. Milne's description of the mouth of Loch Treig (I do not believe that valley has been well examined in its upper end) leaves hardly a doubt that a glacier descended from it, and, if the roads were formed by a lake of any kind, I believe it must have been an ice-lake. I have given in detail to Lyell my several reasons for not thinking ice-lakes probable (520/2. Mr. Darwin gives some arguments against the glacier theory in the letter (517) to Sir Charles Lyell; but the letter alluded to is no doubt the one written to Lyell on "Wednesday, 8th" (Letter 522), in which the reasons are fully stated.); but to my mind they are incomparably more probable than detritus of rock-barriers. Have you ever attended to glacier action? After having seen N. Wales, I can no more doubt the former existence of gigantic glaciers than I can the sun in the heaven. I could distinguish in N. Wales to a certain extent icebergs from glacier action (Lyell has shown that icebergs at the present day score rocks), and I suspect that in Lochaber the two actions are united, and that the scored rock on the watersheds, when tideways, were rubbed and bumped by half-stranded icebergs. You will, no doubt, attend to Glen Glaster. Mr. Milne, I think, does not mention whether shelf 4 enters it, which I should like to know, and especially he does not state whether rocks worn on their upper faces are found on the whole 212 {feet} vertical course of this Glen down to near L. Loggan, or whether only in the upper part; nor does he state whether these rocks are scored, or polished, or moutonnees, or whether there are any "perched" boulders there or elsewhere. I suspect it would be difficult to distinguish between a river-bed and tidal channel. Mr. Milne's description of the Pass of Mukkul, expanding to a width of several hundred yards 21 feet deep in the shoalest part, and with a worn islet in the middle, sounds to me much more like a tidal channel than a river-bed. There must have been, on the latter view, plenty of fresh water in those days. With respect to the coincidence of the shelves with the now watersheds, Mr. Milne only gives half of my explanation. Please read page 65 of my paper. (520/3. "Observations on the Parallel Roads of Glen Roy, and of other Parts of Lochaber in Scotland, with an Attempt to Prove that they are of Marine Origin." "Phil. Trans. R. Soc." 1839, page 39. {Read February 7th, 1839.}) I allude only to the head of Glen Roy and Kilfinnin as silted up. I did not know Mukkul Pass; and Glen Roy was so much covered up that I did not search it well, as I was not able to walk very well. It has been an old conjectural belief of mine that a rising surface becomes stationary, not suddenly, but by the movement becoming very slow. Now, this would greatly aid the tidal currents cutting down the passes between the mountains just before, and to the level of, the stationary periods. The currents in the fiords in T. del Fuego in a narrow crooked part are often most violent; in other parts they seem to silt up.
