More Letters of Charles Darwin Volume Ii Part 20

Notwithstanding the orchids, I have been very glad to see Jamieson's letter; no doubt, as he says, certainty will soon be reached.

With respect to the minor points of Glen Roy, I cannot feel easy with a mere barrier of ice; there is so much sloping, stratified detritus in the valleys. I remember that you somewhere have stated that a running stream soon cuts deeply into a glacier. I have been hunting up all old references and pamphlets, etc., on shelves in Scotland, and will send them off to Mr. J., as they possibly may be of use to him if he continues the subject. The Eildon Hills ought to be specially examined.

Amongst MS. I came across a very old letter from me to you, in which I say: "If a glacialist admitted that the sea, before the formation of the shelves, covered the country (which would account for the land-straits above the level of the shelves), and if he admitted that the land gradually emerged, and if he supposed that his lakes were banked up by ice alone, he would make out, in my opinion, the best case against the marine origin of the shelves." (529/1. See Letter 522.) This seems very much what you and Mr. J. have come to.

The whole glacial theory is really a magnificent subject.

LETTER 530. TO C. LYELL. Down, April 1st [1862].

I am not quite sure that I understand your difficulty, so I must give what seems to me the explanation of the glacial lake theory at some little length. You know that there is a rocky outlet at the level of all the shelves. Please look at my map. (530/1. The map accompanying Mr. Darwin's paper in the "Phil. Trans. R. Soc." 1839.) I suppose whole valley of Glen Spean filled with ice; then water would escape from an outlet at Loch Spey, and the highest shelf would be first formed.

Secondly, ice began to retreat, and water will flow for short time over its surface; but as soon as it retreated from behind the hill marked Craig Dhu, where the outlet on level of second shelf was discovered by Milne (530/2. See note, Letter 521.), the water would flow from it and the second shelf would be formed. This supposes that a vast barrier of ice still remains under Ben Nevis, along all the lower part of the Spean. Lastly, I suppose the ice disappeared everywhere along L. Loggan, L. Treig, and Glen Spean, except close under Ben Nevis, where it still formed a barrier, the water flowing out at level of lowest shelf by the Pa.s.s of Mukkul at head of L. Loggan. This seems to me to account for everything. It presupposes that the shelves were formed towards the close of the Glacial period. I come up to London to read on Thursday a short paper at the Linnean Society. Shall I call on Friday morning at 9.30 and sit half an hour with you? Pray have no scruple to send a line to Queen Anne Street to say "No" if it will take anything out of you. If I do not hear, I will come.

LETTER 531. TO J. PRESTWICH. Down, January 3rd, 1880.

You are perfectly right. (531/1. Prof. Prestwich's paper on Glen Roy was published in the "Phil. Trans. R. Soc." for 1879, page 663.) As soon as I read Mr. Jamieson's article on the parallel roads, I gave up the ghost with more sighs and groans than on almost any other occasion in my life.

2.IX.IV. CORAL REEFS, FOSSIL AND RECENT, 1841-1881.

LETTER 532. TO C. LYELL. Shrewsbury, Tuesday, 6th [July, 1841].

Your letter was forwarded me here. I was the more glad to receive it, as I never dreamed of your being able to find time to write, now that you must be so very busy; and I had nothing to tell you about myself, else I should have written. I am pleased to hear how extensive and successful a trip you appear to have made. You must have worked hard, and got your Silurian subject well in your head, to have profited by so short an excursion. How I should have enjoyed to have followed you about the coral-limestone. I once was close to Wenlock (532/1. The Wenlock limestone (Silurian) contains an abundance of corals. "The rock seems indeed to have been formed in part by ma.s.sive sheets and bunches of coral" (Geikie, "Text-book of Geology," 1882, page 678.), something such as you describe, and made a rough drawing, I remember, of the ma.s.ses of coral. But the degree in which the whole ma.s.s was regularly stratified, and the quant.i.ty of mud, made me think that the reefs could never have been like those in the Pacific, but that they most resembled those on the east coast of Africa, which seem (from charts and descriptions) to confine extensive flats and mangrove swamps with mud, or like some imperfect ones about the West India Islands, within the reefs of which there are large swamps. All the reefs I have myself seen could be a.s.sociated only with nearly pure calcareous rocks. I have received a description of a reef lying some way off the coast near Belize (terra firma), where a thick bed of mud seems to have invaded and covered a coral reef, leaving but very few islets yet free from it. But I can give you no precise information without my notes (even if then) on these heads...

Bermuda differs much from any other island I am acquainted with. At first sight of a chart it resembles an atoll; but it differs from this structure essentially in the gently shelving bottom of the sea all round to some distance; in the absence of the defined circular reefs, and, as a consequence, of the defined central pool or lagoon; and lastly, in the height of the land. Bermuda seems to be an irregular, circular, flat bank, encrusted with knolls and reefs of coral, with land formed on one side. This land seems once to have been more extensive, as on some parts of the bank farthest removed from the island there are little pinnacles of rock of the same nature as that of the high larger islands. I cannot pretend to form any precise notion how the foundation of so anomalous an island has been produced, but its whole history must be very different from that of the atolls of the Indian and Pacific oceans--though, as I have said, at first glance of the charts there is a considerable resemblance.

LETTER 533. TO C. LYELL. [1842.]

Considering the probability of subsidence in the middle of the great oceans being very slow; considering in how many s.p.a.ces, both large ones and small ones (within areas favourable to the growth of corals), reefs are absent, which shows that their presence is determined by peculiar conditions; considering the possible chance of subsidence being more rapid than the upward growth of the reefs; considering that reefs not very rarely perish (as I cannot doubt) on part, or round the whole, of some encircled islands and atolls: considering these things, I admit as very improbable that the polypifers should continue living on and above the same reef during a subsidence of very many thousand feet; and therefore that they should form ma.s.ses of enormous thickness, say at most above 5,000 feet. (533/1. "...As we know that some inorganic causes are highly injurious to the growth of coral, it cannot be expected that during the round of change to which earth, air, and water are exposed, the reef-building polypifers should keep alive for perpetuity in any one place; and still less can this be expected during the progressive subsidences...to which by our theory these reefs and islands have been subjected, and are liable" ("The Structure and Distribution of Coral Reefs," page 107: London, 1842).) This admission, I believe, is in no way fatal to the theory, though it is so to certain few pa.s.sages in my book.

In the areas where the large groups of atolls stand, and where likewise a few scattered atolls stand between such groups, I always imagined that there must have been great tracts of land, and that on such large tracts there must have been mountains of immense alt.i.tudes. But not, it appears to me, that one is only justified in supposing that groups of islands stood there. There are (as I believe) many considerable islands and groups of islands (Galapagos Islands, Great Britain, Falkland Islands, Marianas, and, I believe, Viti groups), and likewise the majority of single scattered islands, all of which a subsidence between 4,000 and 5,000 feet would entirely submerge or would leave only one or two summits above water, and hence they would produce either groups of nothing but atolls, or of atolls with one or two encircled islands. I am far from wishing to say that the islands of the great oceans have not subsided, or may not continue to subside, any number of feet, but if the average duration (from all causes of destruction) of reefs on the same spot is limited, then after this limit has elapsed the reefs would perish, and if the subsidence continued they would be carried down; and if the group consisted only of atolls, only open ocean would be left; if it consisted partly or wholly of encircled islands, these would be left naked and reefless, but should the area again become favourable for growth of reefs, new barrier-reefs might be formed round them. As an ill.u.s.tration of this notion of a certain average duration of reefs on the same spot, compared with the average rate of subsidence, we may take the case of Tahiti, an island of 7,000 feet high. Now here the present barrier-reefs would never be continued upwards into an atoll, although, should the subsidence continue at a period long after the death of the present reefs, new ones might be formed high up round its sides and ultimately over it. The case resolves itself into: what is the ordinary height of groups of islands, of the size of existing groups of atolls (excepting as many of the highest islands as there now ordinarily occur encircling barrier-reefs in the existing groups of atolls)? and likewise what is the height of the single scattered islands standing between such groups of islands? Subsidence sufficient to bury all these islands (with the exception of as many of the highest as there are encircled islands in the present groups of atolls) my theory absolutely requires, but no more. To say what amount of subsidence would be required for this end, one ought to know the height of all existing islands, both single ones and those in groups, on the face of the globe--and, indeed, of half a dozen worlds like ours. The reefs may be of much greater [thickness]

than that just sufficient on an average to bury groups of islands; and the probability of the thickness being greater seems to resolve itself into the average rate of subsidence allowing upward growth, and average duration of reefs on the same spot. Who will say what this rate and what this duration is? but till both are known, we cannot, I think, tell whether we ought to look for upraised coral formations (putting on one side denudation) above the unknown limit, say between 3,000 and 5,000 feet, necessary to submerge groups of common islands. How wretchedly involved do these speculations become.

LETTER 534. TO E. VON MOJSISOVICS. Down, January 29th, 1879.

I thank you cordially for the continuation of your fine work on the Tyrolese Dolomites (534/1. "Dolomitriffe Sudtirols und Venetiens": Wien, 1878.), with its striking engravings and the maps, which are quite wonderful from the amount of labour which they exhibit, and its extreme difficulty. I well remember more than forty years ago examining a section of Silurian limestone containing many corals, and thinking to myself that it would be for ever impossible to discover whether the ancient corals had formed atolls or barrier reefs; so you may well believe that your work will interest me greatly as soon as I can find time to read it. I am much obliged for your photograph, and from its appearance rejoice to see that much more good work may be expected from you.

I enclose my own photograph, in case you should like to possess a copy.

LETTER 535. TO A. AGa.s.sIZ.

(535/1. Part of this letter is published in "Life and Letters," III., pages 183, 184.)

Down, May 5th, 1881.

It was very good of you to write to me from Tortugas, as I always feel much interested in hearing what you are about, and in reading your many discoveries. It is a surprising fact that the peninsula of Florida should have remained at the same level for the immense period requisite for the acc.u.mulation of so vast a pile of debris. (535/2. Alexander Aga.s.siz published a paper on "The Tortugas and Florida Reefs" in the "Mem. Amer. Acad. Arts and Sci." XI., page 107, 1885. See also his "Three Cruises of the 'Blake,'" Volume I., 1888.)

You will have seen Mr. Murray's views on the formation of atolls and barrier reefs. (535/3. "On the Structure and Origin of Coral Reefs and Islands," "Proc. R. Soc. Edin." Volume X., page 505, 1880. Prof. Bonney has given a summary of Sir John Murray's views in Appendix II. of the third edition of Darwin's "Coral Reefs," 1889.) Before publishing my book, I thought long over the same view, but only as far as ordinary marine organisms are concerned, for at that time little was known of the mult.i.tude of minute oceanic organisms. I rejected this view, as from the few dredgings made in the 'Beagle' in the S. Temperate regions, I concluded that sh.e.l.ls, the smaller corals, etc., etc., decayed and were dissolved when not protected by the deposition of sediment; and sediment could not acc.u.mulate in the open ocean. Certainly sh.e.l.ls, etc., were in several cases completely rotten, and crumbled into mud between my fingers; but you will know well whether this is in any degree common. I have expressly said that a bank at the proper depth would give rise to an atoll, which could not be distinguished from one formed during subsidence. I can, however, hardly believe, in the former presence of as many banks (there having been no subsidence) as there are atolls in the great oceans, within a reasonable depth, on which minute oceanic organisms could have acc.u.mulated to the thickness of many hundred feet.

I think that it has been shown that the oscillations from great waves extend down to a considerable depth, and if so the oscillating water would tend to lift up (according to an old doctrine propounded by Playfair) minute particles lying at the bottom, and allow them to be slowly drifted away from the submarine bank by the slightest current.

Lastly, I cannot understand Mr. Murray, who admits that small calcareous organisms are dissolved by the carbonic acid in the water at great depths, and that coral reefs, etc., etc., are likewise dissolved near the surface, but that this does not occur at intermediate depths, where he believes that the minute oceanic calcareous organisms acc.u.mulate until the bank reaches within the reef-building depth. But I suppose that I must have misunderstood him.

Pray forgive me for troubling you at such a length, but it has occurred to me that you might be disposed to give, after your wide experience, your judgment. If I am wrong, the sooner I am knocked on the head and annihilated so much the better. It still seems to me a marvellous thing that there should not have been much and long-continued subsidence in the beds of the great oceans. I wish that some doubly rich millionaire would take it into his head to have borings made in some of the Pacific and Indian atolls, and bring home cores for slicing from a depth of 500 or 600 feet. (535/4. In 1891 a Committee of the British a.s.sociation was formed for the investigation of an atoll by means of boring. The Royal Society took up the scheme, and an expedition was sent to Funafuti, with Prof. Sollas as leader. Another expedition left Sydney in 1897 under the direction of Prof. Edgeworth David, and a deeper boring was made. The Reports will be published in the "Philosophical Transactions," and will contain Prof. David's notes upon the boring and the island generally, Dr. Hinde's description of the microscopic structure of the cores and other examinations of them, carried on at the Royal College of Science, South Kensington. The boring reached a depth of 1114 feet; the cores were found to consist entirely of reef-forming corals in situ and in fragments, with foraminifera and calcareous algae; at the bottom there were no traces of any other kind of rock. It seems, therefore, to us, that unless it can be proved that reef-building corals began their work at depths of at least 180 fathoms--far below that hitherto a.s.signed--the result gives the strongest support to Darwin's theory of subsidence; the test which Darwin wished to be applied has been fairly tried, and the verdict is entirely in his favour.)

2.IX.V. CLEAVAGE AND FOLIATION, 1846-1856.

LETTER 536. TO D. SHARPE.

(536/1. The following eight letters were written at a time when the subjects of cleavage and foliation were already occupying the minds of several geologists, including Sharpe, Sorby, Rogers, Haughton, Phillips, and Tyndall. The paper by Sharpe referred to was published in 1847 ("Quart. Journ. Geol. Soc." Volume III.), and his ideas were amplified in two later papers (ibid., Volume V., 1849, and "Phil. Trans." 1852).

Darwin's own views, based on his observations during the "Beagle"

expedition, had appeared in Chapter XIII. of "South America" (1846) and in the "Manual of Scientific Enquiry" (1849), but are perhaps nowhere so clearly expressed as in this correspondence. His most important contribution to the question was in establishing the fact that foliation is often a part of the same process as cleavage, and is in nowise necessarily connected with planes of stratification. Herein he was opposed to Lyell and the other geologists of the day, but time has made good his position. The postscript to Letter 542 is especially interesting. We are indebted to Mr. Harker, of St. John's College, for this note.)

Down, August 23rd [1846?].

I must just send one line to thank you for your note, and to say how heartily glad I am that you stick to the cleavage and foliation question. Nothing will ever convince me that it is not a n.o.ble subject of investigation, which will lead some day to great views. I think it quite extraordinary how little the subject seems to interest British geologists. You will, I think live to see the importance of your paper recognised. (536/2. Probably the paper "On Slaty Cleavage." "Quart.

Journ. Geol. Soc." Volume III., page 74, 1847.) I had always thought that Studer was one of the few geologists who had taken a correct and enlarged view on the subject.

LETTER 537. TO D. SHARPE. Down [November 1846].

I have been much interested with your letter, and am delighted that you have thought my few remarks worth attention. My observations on foliation are more deserving confidence than those on cleavage; for during my first year in clay-slate countries, I was quite unaware of there being any marked difference between cleavage and stratification; I well remember my astonishment at coming to the conclusion that they were totally different actions, and my delight at subsequently reading Sedgwick's views (537/1. "Remarks on the Structure of Large Mineral Ma.s.ses, and especially on the Chemical Changes produced in the Aggregation of Stratified Rocks during different periods after their Deposition." "Trans. Geol. Soc." Volume III., page 461, 1835. In the section of this paper dealing with cleavage (page 469) Prof. Sedgwick lays stress on the fact that "the cleavage is in no instance parallel to the true beds."); hence at that time I was only just getting out of a mist with respect to cleavage-laminae dipping inwards on mountain flanks. I have certainly often observed it--so often that I thought myself justified in propounding it as usual. I might perhaps have been in some degree prejudiced by Von Buch's remarks, for which in those days I had a somewhat greater deference than I now have. The Mount at M. Video (page 146 of my book (537/2. "Geol. Obs. S. America." page 146.

The mount is described as consisting of hornblendic slate; "the laminae of the slate on the north and south side near the summit dip inwards.")) is certainly an instance of the cleavage-laminae of a hornblendic schist dipping inwards on both sides, for I examined this hill carefully with compa.s.s in hand and notebook. I entirely admit, however, that a conclusion drawn from striking a rough balance in one's mind is worth nothing compared with the evidence drawn from one continuous line of section. I read Studer's paper carefully, and drew the conclusion stated from it; but I may very likely be in an error. I only state that I have frequently seen cleavage-laminae dipping inwards on mountain sides; that I cannot give up, but I daresay a general extension of the rule (as might justly be inferred from the manner of my statement) would be quite erroneous. Von Buch's statement is in his "Travels in Norway" (537/3.

"Travels through Norway and Lapland during the years 1806-8": London, 1813.); I have unfortunately lost the reference, and it is a high crime, I confess, even to refer to an opinion without a precise reference. If you never read these travels they might be worth skimming, chiefly as an amus.e.m.e.nt; and if you like and will send me a line by the general post of Monday or Tuesday, I will either send it up with Hopkins on Wednesday, or bring it myself to the Geological Society. I am very glad you are going to read Hopkins (537/4. "Researches in Physical Geology,"

by W. Hopkins. "Phil. Trans. R. Soc." 1839, page 381; ibid, 1842, page 43, etc.); his views appear to me eminently worth well comprehending; false views and language appear to me to be almost universally held by geologists on the formation of fissures, dikes and mountain chains. If you would have the patience, I should be glad if you would read in my "Volcanic Islands" from page 65, or even pages 54 to 72--viz., on the lamination of volcanic rocks; I may add that I sent the series of specimens there described to Professor Forbes of Edinburgh, and he thought they bore out my views.

There is a short extract from Prof. Rogers (537/5. "On Cleavage of Slate-strata." "Edinburgh New Phil. Journ." Volume XLI., page 422, 1846.) in the last "Edinburgh New Phil. Journal," well worth your attention, on the cleavage of the Appalachian chain, and which seems far more uniform in the direction of dip than in any case which I have met with; the Rogers doctrine of the ridge being thrown up by great waves I believe is monstrous; but the manner in which the ridges have been thrown over (as if by a lateral force acting on one side on a higher level than on the other) is very curious, and he now states that the cleavage is parallel to the axis-planes of these thrown-over ridges.

Your case of the limestone beds to my mind is the greatest difficulty on any mechanical doctrine; though I did not expect ever to find actual displacement, as seems to be proved by your sh.e.l.l evidence. I am extremely glad you have taken up this most interesting subject in such a philosophical spirit; I have no doubt you will do much in it; Sedgwick let a fine opportunity slip away. I hope you will get out another section like that in your letter; these are the real things wanted.

LETTER 538. TO D. SHARPE. Down, [January 1847].

I am very much obliged for the MS., which I return. I do not quite understand from your note whether you have struck out all on this point in your paper: I much hope not; if you have, allow me to urge on you to append a note, briefly stating the facts, and that you omitted them in your paper from the observations not being finished.

I am strongly tempted to suspect that the cleavage planes will be proved by you to have slided a little over each other, and to have been planes of incipient tearing, to use Forbes' expression in ice; it will in that case be beautifully a.n.a.logical with my laminated lavas, and these in composition are intimately connected with the metamorphic schists.

The beds without cleavage between those with cleavage do not weigh quite so heavily on me as on you. You remember, of course, Sedgwick's facts of limestone, and mine of sandstone, breaking in the line of cleavage, transversely to the planes of deposition. If you look at cleavage as I do, as the result of chemical action or crystalline forces, super-induced in certain places by their mechanical state of tension, then it is not surprising that some rocks should yield more or less readily to the crystalline forces.

I think I shall write to Prof. Forbes (538/1. Prof. D. Forbes.) of Edinburgh, with whom I corresponded on my laminated volcanic rocks, to call his early attention to your paper.

LETTER 539. TO D. SHARPE. Down, October 16th [1851].