column or cone (see my elegant drawing) of lava [Fig. 4]. I do Letter 486 not doubt that the dikes are thus indirectly connected with eruptive vents. E. de B. seems to have observed many of his T; now without he supposes the whole line of fissure or dike to have poured out lava (which implies, as above remarked, craters of an elliptic or almost linear shape) on both sides, how extraordinarily improbable it is, that there should have been in a single line of section so many intersections of points of eruption; he must, I think, make his orifices of eruption almost linear or, if not so, astonishingly numerous. Fig. 3. One must refer to what one has seen oneself: do pray, when you go home, look at the section of a minute cone of eruption at the Galapagos, p. 109,1 which is the most perfect natural dissection of a crater which I have ever heard of, and the drawing of which you may, I assure you, trust; here the arching over of the streams as they were poured out over the lip of the crater was evident, and are now thus seen united to the central irregular column. Again, at St. Jago I saw some horizontal sections of the bases of small craters, and the sources or feeders were circular. I really cannot 1 Geological Observations on Volcanic Islands. London, 1890, p. 238. entertain a doubt that E. de B. is grossly wrong, and that evidence I will never admit that a dike joins on rectangularly Letter 486 to a stream of lava. Your argument about the perpendicularity of the dike strikes me as good. The map of Etna, which I have been just looking at, looks like a sudden falling in, does it not? I am not much surprised at the linear vent in Santorin (this linear tendency ought to be difficult to a circular-crater-of-elevation-believer), I think Abich describes having seen the same actual thing forming within the crater of Vesuvius. In such cases what outline do you give to the upper surface of the lava in the dike connecting them? Surely it would be very irregular and would send up irregular cones or columns as in my above splendid drawing. At the Royal on Friday, after more doubt and misgiving than I almost ever felt, I voted to recommend Forbes for Royal Medal, and that view was carried, Sedgwick taking the lead. 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.2 I saw Dan Sharpe3 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. 1 Geologische Beobachtungen über die vulkanischen Erscheinungen und Bildungen in Unter- und Mittel-Italien. Braunschweig, 1841. This seems to refer to the death of Sir Charles Lyell's father, which occurred on Nov. 8th, 1849. 3 Daniel Sharpe (1806-56) left school at the age or sixteen, and became a clerk in the service of a Portuguese merchant. At the age of twenty-four he went for a year to Portugal, and afterwards spent a considerable amount of time in that country. The results of his geological work, carried out in the intervals of business, were published in the Journal of the Geological Society of London (Quart. Journ. Geol. Soc., Vol. V., p. 142; Vol. VI., p. 135). Although actively engaged in business all his life, Sharpe communicated several papers to the Geological Society, his researches into the origin of slaty cleavage being among the ablest and most important of his contributions to geology (Quart. Journ. Geol. Soc., Vol. III., p. 74; Vol. V., p. 111). A full account of Sharpe's work is given in an obituary notice published in the Quart. Journ. Geol. Soc., Vol. XIII., p. xlv. Letter 487 To C. H. L. Woodd. Down, March 4th [1850]. I have read over your paper1 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 the heat plays quite a 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. subordinate part in the upward and downward movements of Letter 487 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 overcautious; 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 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 (p. 605, Vol. V., Geolog. Trans.). I can throw no light on the subject. I presume you remember that Hopkins in some one (I forget which) "On the Connexion 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., Vol. V., p. 601, 1840). On p. 605 Darwin records instances of the simultaneous activity after an earthquake of several volcanoes in the Cordillera. * See "Report on the Geological Theories of Elevation and Earthquakes," by W. Hopkins, Brit. Assoc. Rep., 1847, p. 34. Letter 488 |