More Letters of Charles Darwin Volume Ii Part 17

P.S.--There is a capital paper in the September number of "Annals and Magazine," translated from Pictet and Humbert, on Fossil Fish of Lebanon, but you will, I daresay, have received the original. (507/2.

"Recent Researches on the Fossil Fishes of Mount Lebanon," "Ann. Mag.

Nat. Hist." Volume XVIII., page 237, 1866.) It is capital in relation to modification of species; I would not wish for more confirmatory facts, though there is no direct allusion to the modification of species.

Hooker, by the way, gave an admirable lecture at Nottingham; I read it in MS., or rather, heard it. I am glad it will be published, for it was capital. (507/3. Sir Joseph Hooker delivered a lecture at the Nottingham meeting of the British a.s.sociation (1866) on "Insular Floras," published in the "Gardeners' Chronicle," 1867. See Letters 366-377, etc.)

Sunday morning.

P.S.--I have just received a letter from Asa Gray with the following pa.s.sage, so that, according to this, I am the chief cause of Aga.s.siz's absurd views:--

"Aga.s.siz is back (I have not seen him), and he went at once down to the National Academy of Sciences, from which I sedulously keep away, and, I hear, proved to them that the Glacial period covered the whole continent of America with unbroken ice, and closed with a significant gesture and the remark: 'So here is the end of the Darwin theory.' How do you like that?

"I said last winter that Aga.s.siz was bent on covering the whole continent with ice, and that the motive of the discovery he was sure to make was to make sure that there should be no coming down of any terrestrial life from Tertiary or post-Tertiary period to ours. You cannot deny that he has done his work effectually in a truly imperial way."

LETTER 508. TO C. LYELL. Down, July 14th, 1868.

Mr. Aga.s.siz's book has been read aloud to me, and I am wonderfully perplexed what to think about his precise statements of the existence of glaciers in the Ceara Mountains, and about the drift formation near Rio.

(508/1. "Sur la Geologie de l'Amazone," by MM. Aga.s.siz and Continho, "Bull. Soc. Geol. France," Volume XXV., page 685, 1868. See also "A Journey in Brazil," by Professor and Mrs. Louis Aga.s.siz, Boston, 1868.) There is a sad want of details. Thus he never mentions whether any of the blocks are angular, nor whether the embedded rounded boulders, which cannot all be disintegrated, are scored. Yet how can so experienced an observer as A. be deceived about lateral and terminal moraines? If there really were glaciers in the Ceara Mountains, it seems to me one of the most important facts in the history of the inorganic and organic world ever observed. Whether true or not, it will be widely believed, and until finally decided will greatly interfere with future progress on many points. I have made these remarks in the hope that you will coincide. If so, do you think it would be possible to persuade some known man, such as Ramsay, or, what would be far better, some two men, to go out for a summer trip, which would be in many respects delightful, for the sole object of observing these phenomena in the Ceara Mountains, and if possible also near Rio? I would gladly put my name down for 50 pounds in aid of the expense of travelling. Do turn this over in your mind. I am so very sorry not to have seen you this summer, but for the last three weeks I have been good for nothing, and have had to stop almost all work. I hope we may meet in the autumn.

LETTER 509. TO JAMES CROLL. Down, November 24th, 1868.

I have read with the greatest interest the last paper which you have kindly sent me. (509/1. Croll discussed the power of icebergs as grinding and striating agents in the latter part of a paper ("On Geological Time, and the probable Dates of the Glacial and the Upper Miocene Period") published in the "Philosophical Magazine," Volume x.x.xV., page 363, 1868, Volume x.x.xVI., pages 141, 362, 1868. His conclusion was that the advocates of the Iceberg theory had formed "too extravagant notions regarding the potency of floating ice as a striating agent.") If we are to admit that all the scored rocks throughout the more level parts of the United States result from true glacier action, it is a most wonderful conclusion, and you certainly make out a very strong case; so I suppose I must give up one more cherished belief. But my object in writing is to trespa.s.s on your kindness and ask a question, which I daresay I could answer for myself by reading more carefully, as I hope hereafter to do, all your papers; but I shall feel much more confidence in a brief reply from you. Am I right in supposing that you believe that the glacial periods have always occurred alternately in the northern and southern hemispheres, so that the erratic deposits which I have described in the southern parts of America, and the glacial work in New Zealand, could not have been simultaneous with our Glacial period?

From the glacial deposits occurring all round the northern hemisphere, and from such deposits appearing in S. America to be as recent as in the north, and lastly, from there being some evidence of the former lower descent of glaciers all along the Cordilleras, I inferred that the whole world was at this period cooler. It did not appear to me justifiable without distinct evidence to suppose that the N. and S. glacial deposits belonged to distinct epochs, though it would have been an immense relief to my mind if I could have a.s.sumed that this had been the case. Secondly, do you believe that during the Glacial period in one hemisphere the opposite hemisphere actually becomes warmer, or does it merely retain the same temperature as before? I do not ask these questions out of mere curiosity; but I have to prepare a new edition of my "Origin of Species," and am anxious to say a few words on this subject on your authority. I hope that you will excuse my troubling you.

LETTER 510. TO J. CROLL. Down, January 31st, 1869.

To-morrow I will return registered your book, which I have kept so long.

I am most sincerely obliged for its loan, and especially for the MS., without which I should have been afraid of making mistakes. If you require it, the MS. shall be returned. Your results have been of more use to me than, I think, any other set of papers which I can remember.

Sir C. Lyell, who is staying here, is very unwilling to admit the greater warmth of the S. hemisphere during the Glacial period in the N.; but, as I have told him, this conclusion which you have arrived at from physical considerations, explains so well whole cla.s.ses of facts in distribution, that I must joyfully accept it; indeed, I go so far as to think that your conclusion is strengthened by the facts in distribution.

Your discussion on the flowing of the great ice-cap southward is most interesting. I suppose that you have read Mr. Moseley's recent discussion on the force of gravity being quite insufficient to account for the downward movement of glaciers (510/1. Canon Henry Moseley, "On the Mechanical Impossibility of the Descent of Glaciers by their Weight only." "Proc. R. Soc." Volume XVII., page 202, 1869; "Phil. Mag." Volume x.x.xVII., page 229, 1869.): if he is right, do you not think that the unknown force may make more intelligible the extension of the great northern ice-cap? Notwithstanding your excellent remarks on the work which can be effected within the million years (510/2. In his paper "On Geological Time, and the probable Date of the Glacial and the Upper Miocene Period" ("Phil. Mag." Volume x.x.xV., page 363, 1868), Croll endeavours to convey to the mind some idea of what a million years really is: "Take a narrow strip of paper, an inch broad or more, and 83 feet 4 inches in length, and stretch it along the wall of a large hall, or round the walls of an apartment somewhat over 20 feet square.

Recall to memory the days of your boyhood, so as to get some adequate conception of what a period of a hundred years is. Then mark off from one of the ends of the strip one-tenth of an inch. The one-tenth of an inch will then represent a hundred years, and the entire length of the strip a million of years" (loc. cit., page 375).), I am greatly troubled at the short duration of the world according to Sir W. Thomson (510/3.

In a paper communicated to the Royal Society of Edinburgh, Lord Kelvin (then Sir William Thomson) stated his belief that the age of our planet must be more than twenty millions of years, but not more than four hundred millions of years ("Trans. R. Soc. Edinb." Volume XXIII., page 157, 1861, "On the Secular Cooling of the Earth."). This subject has been recently dealt with by Sir Archibald Geikie in his address as President of the Geological Section of the British a.s.sociation, 1899 ("Brit. a.s.soc. Report," Dover Meeting, 1899, page 718).), for I require for my theoretical views a very long period BEFORE the Cambrian formation. If it would not trouble you, I should like to hear what you think of Lyell's remark on the magnetic force which comes from the sun to the earth: might not this penetrate the crust of the earth and then be converted into heat? This would give a somewhat longer time during which the crust might have been solid; and this is the argument on which Sir W. Thomson seems chiefly to rest. You seem to argue chiefly on the expenditure of energy of all kinds by the sun, and in this respect Lyell's remark would have no bearing.

My new edition of the "Origin" (510/4. Fifth edition, May, 1869.) will be published, I suppose, in about two months, and for the chance of your liking to have a copy I will send one.

P.S.--I wish that you would turn your astronomical knowledge to the consideration whether the form of the globe does not become periodically slightly changed, so as to account for the many repeated ups and downs of the surface in all parts of the world. I have always thought that some cosmical cause would some day be discovered.

LETTER 511. TO C. LYELL. Down, July 12th [1872].

I have been glad to see the enclosed and return it. It seems to me very cool in Aga.s.siz to doubt the recent upheaval of Patagonia, without having visited any part; and he entirely misrepresents me in saying that I infer upheaval from the form of the land, as I trusted entirely to sh.e.l.ls embedded and on the surface. It is simply monstrous to suppose that the terraces stretching on a dead level for leagues along the coast, and miles in breadth, and covered with beds of stratified gravel, 10 to 30 feet in thickness, are due to subaerial denudation.

As for the pond of salt-water twice or thrice the density of sea-water, and nearly dry, containing sea-sh.e.l.ls in the same relative proportions as on the adjoining coast, it almost pa.s.ses my belief. Could there have been a lively midshipman on board, who in the morning stocked the pool from the adjoining coast?

As for glaciation, I will not venture to express any opinion, for when in S. America I knew nothing about glaciers, and perhaps attributed much to icebergs which ought to be attributed to glaciers. On the other hand, Aga.s.siz seems to me mad about glaciers, and apparently never thinks of drift ice.

I did see one clear case of former great extension of a glacier in T.

del Fuego.

LETTER 512. TO J. GEIKIE.

(512/1. The following letter was in reply to a request from Prof. James Geikie for permission to publish Mr. Darwin's views, communicated in a previous letter (November 1876), on the vertical position of stones in gravelly drift near Southampton. Prof. Geikie wrote (July 15th, 1880): "You may remember that you attributed the peculiar position of those stones to differential movements in the drift itself arising from the slow melting of beds of frozen snow interstratified into the gravels...I have found this explanation of great service even in Scotland, and from what I have seen of the drift-gravels in various parts of southern England and northern France, I am inclined to think that it has a wide application.")

Down, July 19th, 1880.

Your letter has pleased me very much, and I truly feel it an honour that anything which I wrote on the drift, etc., should have been of the least use or interest to you. Pray make any use of my letter (512/2. Professor James Geikie quotes the letter in "Prehistoric Europe," London, 1881 (page 141). Practically the whole of it is given in the "Life and Letters," III., page 213.): I forget whether it was written carefully or clearly, so pray touch up any pa.s.sages that you may think fit to quote.

All that I have seen since near Southampton and elsewhere has strengthened my notion. Here I live on a chalk platform gently sloping down from the edge of the escarptment to the south (512/3. Id est, sloping down from the escarpment which is to the south.) (which is about 800 feet in height) to beneath the Tertiary beds to the north. The (512/4. From here to the end of the paragraph is quoted by Prof. Geikie, loc. cit., page 142.) beds of the large and broad valleys (and only of these) are covered with an immense ma.s.s of closely packed broken and angular flints; in which ma.s.s the skull of the musk-ox [musk-sheep]

and woolly elephant have been found. This great acc.u.mulation of unworn flints must therefore have been made when the climate was cold, and I believe it can be accounted for by the larger valleys having been filled up to a great depth during a large part of the year with drifted frozen snow, over which rubbish from the upper parts of the platforms was washed by the summer rains, sometimes along one line and sometimes along another, or in channels cut through the snow all along the main course of the broad valleys.

I suppose that I formerly mentioned to you the frequent upright position of elongated flints in the red clayey residue over the chalk, which residue gradually subsides into the troughs and pipes corroded in the solid chalk. This letter is very untidy, but I am tired.

P.S. Several palaeolithic celts have recently been found in the great angular gravel-bed near Southampton in several places.

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 Acc.u.mulation and Derivation of the Moel-Tryfan Sh.e.l.ly 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 x.x.xVII., pages 351-69, 1881. [Read April 27th, 1881.]) I failed to find sh.e.l.ls 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 sh.e.l.ls 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 a.s.suming 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 a.s.sociation, "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 acc.u.mulated evidence of interglacial periods, and a.s.suredly 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 cla.s.sed 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, ill.u.s.trated 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 x.x.xV., 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 sh.e.l.ls. 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 x.x.xVI., 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 h.o.m.ogeneous, 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.