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 Agassiz 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 shells 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-shells in the same relative proportions as on the adjoining coast, it almost passes 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, Agassiz 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.