I am glad to hear that all your party are pretty well. I know from experience what you must have gone through. From old age with suffering death must be to all a happy release. (486/3. This seems to refer to the death of Sir Charles Lyell's father, which occurred on November 8th, 1849.)
I saw Dan Sharpe the other day, and he told me he had been working at the mica schist (i.e. not gneiss) in Scotland, and that he was quite convinced my view was right. You are wrong and a heretic on this point, I know well.
LETTER 487. TO C.H.L. WOODD. Down, March 4th {1850}.
(487/1. The paper was sent in MS., and seems not to have been published. Mr. Woodd was connected by marriage with Mr. Darwin's cousin, the late Rev. W. Darwin Fox. It was perhaps in consequence of this that Mr. Darwin proposed Mr. Woodd for the Geological Society.)
I have read over your paper with attention; 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, and 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, and the mineralogical changes in any length of any one stratum: I should have thought that the strata would in every case have crumpled up, and we know how commonly in metamorphic strata, which have undergone heat, the subordinate layers are wavy and sinuous, which has always been attributed to their expansion whilst heated.
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}.
I have often puzzled over Dana's case, in itself and in relation to the trains of S. American volcanoes of different heights in action at the same time (page 605, Volume V. "Geological Transactions." (488/1. "On the Connection of certain Volcanic Phenomena in South America, and on the Formation of Mountain Chains and Volcanoes, as the Effect of the same Power by which Continents are Elevated" ("Trans. Geol. Soc." Volume V., page 601, 1840). On page 605 Darwin records instances of the simultaneous activity after an earthquake of several volcanoes in the Cordillera.)) I can throw no light on the subject. I presume you remember that Hopkins (488/2. See "Report on the Geological Theories of Elevation and Earthquakes," by W. Hopkins, "Brit. Assoc. Rep." 1847, page 34.) in some one (I forget which) of his papers discusses such cases, and urgently wishes the height of the fluid lava was known in adjoining volcanoes when in contemporaneous action; he argues vehemently against (as far as I remember) volcanoes in action of different heights being connected with one common source of liquefied rock. If lava was as fluid as water, the case would indeed be hopeless; and I fancy we should be led to look at the deep-seated rock as solid though intensely hot, and becoming fluid as soon as a crack lessened the tension of the super-incumbent strata. But don't you think that viscid lava might be very slow in communicating its pressure equally in all directions? I remember thinking strongly that Dana's case within the one crater of Kilauea proved too much; it really seems monstrous to suppose that the lava within the same crater is not connected at no very great depth.
When one reflects on (and still better sees) the enormous masses of lava apparently shot miles high up, like cannon-balls, the force seems out of all proportion to the mere gravity of the liquefied lava; I should think that a channel a little straightly or more open would determine the line of explosion, like the mouth of a cannon compared to the touch-hole. If a high-pressure boiler was cracked across, no one would think for a moment that the quantity of water and steam expelled at different points depended on the less or greater height of the water within the boiler above these points, but on the size of the crack at these points; and steam and water might be driven out both at top and bottom. May not a volcano be likened to a protruding and cracked portion on a vast natural high-pressure boiler, formed by the surrounding area of country? In fact, I think my simile would be truer if the difference consisted only in the cracked case of the boiler being much thicker in some parts than in others, and therefore having to expel a greater thickness or depth of water in the thicker cracks or parts—a difference of course absolutely as nothing.
I have seen an old boiler in action, with steam and drops of water spurting out of some of the rivet-holes. No one would think whether the rivet-holes passed through a greater or less thickness of iron, or were connected with the water higher or lower within the boiler, so small would the gravity be compared with the force of the steam. If the boiler had been not heated, then of course there would be a great difference whether the rivet-holes entered the water high or low, so that there was greater or less pressure of gravity. How to close my volcanic rivet-holes I don't know.