From Charles Lyell 2 November 1869
2. Nov— 1869—
I have read the article, & cannot attach the importance—to it which you seem to do.1 To me the most interesting parts of it are the quotations from Thomson, & what is said about changes in the acceleration of the earth’s rotation, or that of the moon by the friction of the tides.2 I do not yet comprehend the argument but am quite willing to believe that physicists will prove something important on this question; but it was surely a good hit of Huxley’s to point out that whatever calculations they have been making about the effects of the tides in the course of geological ages, must be very loose & incomplete, if they took no account of the quantity of water locked up in the shape of ice at different periods, when for example the Miocene forests were flourishing at or near the pole with scarcely any ice, & when at the height of the Glacial Period, Wales & Yorkshire were covered by a sheet of ice.3 I am astounded at the coolness with which they talk of the theory of Helmholtz as to solar heat being the only one left to us, viz, that it is a hot body cooling & that 10 or 15 millions is all that can be allowed to the geologist (p 438);4 & how they can speak of there being no access of energy into the sun than meteors, as if they knew so much of the universe as to warrant such assumptions.5
I am said (p 422) to talk about tens of thousands of millions of years as the very least period I will accept, but he takes care not to refer to any passage.6 I suppose you have seen Geikie’s calculations that denudation like that of the Mississippi would reduce a mass of rock equalling in volume all the land now above the sea to sand, pebbles & mud in 6.000.000 years—7 Have you any where in reference to speculations of this kind spoken of the supposed fact that the average depth of the sea is 15 times as great as the average height of the land, the latter being 1000 & the sea 15000.
You have spoken somewhere of the permanence of continents,8 to which in a general way I have no objection, but I should like you to consider 〈if〉 the sea 〈be less〉 deep (suppose even the mean depth to be 10.000 ft) would it not have a great bearing on the question of the rate of denudation & permanence of continents. Nothing can be clearer to me than that the continent from which the sedimentary strata of the Alleghanies were derived was placed where the Atlantic is now situated.
The sedimentary strata of the Alleghanies being all Paleozoic are 42.000 ft. thick. Do think of this & give me any references to yourself or others which may bear upon it. I am sure it will have much to do with estimates of geological time. The rate of denudation must be balanced by an equal amount of volcanic movement & the changes in species will have reference to these two forces & will require probably as much time as you may at any time have thought.9
The permanence of continents depends greatly on the formation of deltas near the coast, or in other words, on rivers clearing themselves of mud, sand & pebbles in the first receptacle they meet with—just as they leave behind all their sediment when they pass through a lake. If therefore the height of the land be to the average depth of the sea as 1 to 15—a moderate quantity of movement in the areas fringing the continents will cause the deltas to emerge & the same matter will do duty several times over in 〈the cour〉se of the 6.000.000 years— But the same amount of movement will cause but little of the deep ocean to emerge if the average depth be 3 miles— The movements however are sufficient at length to cause the submergence & emergence of the bed of the deep sea, & the changes of level seem to be going on so slowly that I cannot but think the conversion of great oceanic basins into continents & of continents into oceanic basins may have been a work of as much time as your speculations have ever demanded— I do not think that the subaërial denudationists allow sufficiently for the constant repair of the loss of land by the up-lifting of comparatively modern strata thrown down near the coast,10 the Eocene strata are 12,000 feet high on the Alps & the Miocene from 3000 to 7000 (?)ft— At the end of a great number of periods that chain may be as high as ever.—although the quantity of matter which has passed through the denudation mill may equal in volume all the land now above the sea yet this has been supplied by grinding the same stuff over & over again. I still think that the quantity of movement shown by Moel Tryfaen & by the submergence & re-emergence of a large part of the British Isles while the Mollusca have been scarcely in the least changed implies that an enormous quantity of movement & denudation corresponds to an insignificant amount of organic fluctuation.11
Comments on Huxley’s address ["Geological reform", Q. J. Geol. Soc. Lond. 25 (1869): xxxviii–liii].
Physicists have ignored variation in sea-level in calculating effects.
Doubts if sun only source of heat.
Notes average depth of sea is 15 times height of land.
Criticises CD’s concept of permanent continents.
Sedimentary strata of Alleghenies must have derived from continent located where Atlantic is. Thinks enormous amount of denudation, submergence, and elevation may have accompanied relatively insignificant organic changes.
- Letter no.
- Lyell, Charles
- Darwin, C. R.
- Source of text
- Edinburgh University Library, Special Collections Department, (Lyell 1) [Gen.113.ff.3734–3737]
- Physical description
Please cite as
Darwin Correspondence Project, “Letter no. 6967,” accessed on 23 January 2017, http://www.darwinproject.ac.uk/DCP-LETT-6967