Before rocks are dried and quarried, manifold facts show how extremely flexible they are even when not at all heated. Without the bowing out and subsequent filling in of the roof of the cavity, if I understand you, there would be no subsidence. Of course the crumpling up of the strata would thicken them, and 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 the 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 tied over the 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 and then to that quarter of the world: I suspect that heat plays quite a subordinate part in the upward and downward movements of the earth's crust; though of course it must swell the strata where first affected. I can understand Sir J. Herschel's manner of bringing heat to unheated strata--namely, by covering them up by a mile or so of new strata, and 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, and am now perhaps become over-cautious; and feel inclined to cavil at speculation when the direct and 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 earth's crust, and then speculate to your heart's 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, and more directly bearing on your paper; but the subject strikes me as too difficult and complicated. With every good wish that you may go on with your geological studies, speculations, and especially observations... LETTER 488. TO C. LYELL. Down, March 24th [1853].