My dear Sir.

I have read over your paper with attention;¹ but first let me thank you for your very kind expressions towards myself. I really feel hardly competent to discuss the questions raised by your paper; I feel the want of mathematical mechanics— All such problems strike me as awfully complicated; we do not even know what effect great pressure has on retarding liquefaction by heat, nor, I apprehend on expansion— The chief objection which strikes me is a doubt whether a mass of strata, when heated, & therefore in some slight degree at least softened, would bow outwards like a bar of metal.— Consider of how many subordinate layers each great mass would be composed & of the mineralogical changes in any length of any one stratum: I should have thought that the strata would in every case have crumpled up, & we know how commonly in metamorphic strata, which have undergone heat the subordinate layers are wavy & sinuous which has always been attributed to their expansion whilst heated;

Before rocks are dryed & quarried, manifold facts show how extremely flexible they are even when not at all heated. Without the bowing out & subsequent filling in of the roof of cavity, if I understand you, there would be no subsidence— of course the crumpling up of the Strata would thicken them, & I see with you that this might compress the underlying fluidified rock, which in its turn might escape by a volcano or raise a weaker part of earth’s crust but I am too ignorant to have any opinion whether force would be easily propagated through a viscid mass like molten rock; or whether such viscid mass would not act in some degree like sand and refuse to transmit pressure: as in the old experiment of trying to burst a piece of paper tyed over end of a tube with a stick an inch or two of sand being only interposed. I have always myself felt the greatest difficulty in believing in waves of heat coming first to this & then to that quarter of the world; I suspect the heat plays quite a subordinate part in the upward & downward movements of the earths crust; though of course it must swell the Strata where first affected— I can understand Sir J. Herschell’s manner of bringing heat to unheated Strata, namely by covering them up by a mile or so of new strata—& then the heat would travel into the lower ones, But who can tell what effect this mile or two of new sedimentary strata, would have from mere gravity on the level of the supporting surface? of course such considerations do not render less true, that the expansion of the strata by heat would have some effect on the level of the surface; but they show us how awfully complicated the Phenomenon is— All young geologists have a great turn for speculation; I have burned my fingers pretty sharply in that way, & am now perhaps become over cautious;² & feel inclined to cavil at speculation when the direct & immediate effect of a cause in question cannot be shown— How neatly you draw your diagrams; I wish you would turn your attention to real sections of the earths crust, & then speculate to your hearts content on them; I can have no doubt that speculative men, with a curb on make far the best observers.— I sincerely wish I could have made any remarks of more interest to you, & more directly bearing on your Paper, but the subject strikes me as too difficult & complicated.

with every good wish that you may go on with your Geological studies, speculations & especially observations | pray believe me. | My dear Sir. | Yours sincerely | C. Darwin.