Volume Ii Part 14 (2/2)
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 ma.s.s like molten rock; or whether such viscid ma.s.s 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. a.s.soc. 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-inc.u.mbent 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 ma.s.ses of lava apparently shot miles high up, like cannon-b.a.l.l.s, 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 quant.i.ty 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 pa.s.sed 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.
I do not know whether you will understand what I am driving at, and it will not signify much whether you do or not. I remember in old days (I may mention the subject as we are on it) often wis.h.i.+ng I could get you to look at continental elevations as THE phenomenon, and volcanic outbursts and tilting up of mountain chains as connected, but quite secondary, phenomena. I became deeply impressed with the truth of this view in S. America, and I do not think you hold it, or if so make it clear: the same explanation, whatever it may be, which will account for the whole coast of Chili rising, will and must apply to the volcanic action of the Cordillera, though modified no doubt by the liquefied rock coming to the surface and reaching water, and so [being] rendered explosive. To me it appears that this ought to be borne in mind in your present subject of discussion. I have written at too great length; and have amused myself if I have done you no good--so farewell.
LETTER 489. TO C. LYELL. Down, July 5th [1856].
I am very much obliged for your long letter, which has interested me much; but before coming to the volcanic cosmogony I must say that I cannot gather your verdict as judge and jury (and not as advocate) on the continental extensions of late authors (489/1. See ”Life and Letters,” II., page 74; Letter to Lyell, June 25th, 1856: also letters in the sections of the present work devoted to Evolution and Geographical Distribution.), which I must grapple with, and which as yet strikes me as quite unphilosophical, inasmuch as such extensions must be applied to every oceanic island, if to any one, as to Madeira; and this I cannot admit, seeing that the skeletons, at least, of our continents are ancient, and seeing the geological nature of the oceanic islands themselves. Do aid me with your judgment: if I could honestly admit these great [extensions], they would do me good service.
With respect to active volcanic areas being rising areas, which looks so pretty on the coral maps, I have formerly felt ”uncomfortable” on exactly the same grounds with you, viz. maritime position of volcanoes; and still more from the immense thicknesses of Silurian, etc., volcanic strata, which thicknesses at first impress the mind with the idea of subsidence. If this could be proved, the theory would be smashed; but in deep oceans, though the bottom were rising, great thicknesses of submarine lava might acc.u.mulate. But I found, after writing Coral Book, cases in my notes of submarine vesicular lava-streams in the upper ma.s.ses of the Cordillera, formed, as I believe, during subsidence, which staggered me greatly. With respect to the maritime position of volcanoes, I have long been coming to the conclusion that there must be some law causing areas of elevation (consequently of land) and of subsidence to be parallel (as if balancing each other) and closely approximate; I think this from the form of continents with a deep ocean on one side, from coral map, and especially from conversations with you on immense subsidences of the Carboniferous and [other] periods, and yet with continued great supply of sediment. If this be so, such areas, with opposite movements, would probably be separated by sets of parallel cracks, and would be the seat of volcanoes and tilts, and consequently volcanoes and mountains would be apt to be maritime; but why volcanoes should cling to the rising edge of the cracks I cannot conjecture. That areas with extinct volcanic archipelagoes may subside to any extent I do not doubt.
Your view of the bottom of Atlantic long sinking with continued volcanic outbursts and local elevations at Madeira, Canaries, etc., grates (but of course I do not know how complex the phenomena are which are thus explained) against my judgment; my general ideas strongly lead me to believe in elevatory movements being widely extended. One ought, I think, never to forget that when a volcano is in action we have distinct proof of an action from within outwards. Nor should we forget, as I believe follows from Hopkins (489/2. ”Researches in Physical Geology,”
W. Hopkins, ”Trans. Phil. Soc. Cambridge,” Volume VI., 1838. See also ”Report on the Geological Theories of Elevation and Earthquakes,” W.
Hopkins, ”Brit. a.s.soc. Rep.” page 33, 1847 (Oxford meeting).), and as I have insisted in my Earthquake paper, that volcanoes and mountain chains are mere accidents resulting from the elevation of an area, and as mountain chains are generally long, so should I view areas of elevation as generally large. (489/3. ”On the Connexion of certain Volcanic Phenomena in S. 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. ”Bearing in mind Mr. Hopkins' demonstration, if there be considerable elevation there must be fissures, and, if fissures, almost certainly unequal upheaval, or subsequent sinking down, the argument may be finally thus put: mountain chains are the effects of continental elevations; continental elevations and the eruptive force of volcanoes are due to one great motive, now in progressive action...” (loc. cit., page 629).)
Your old original view that great oceans must be sinking areas, from there being causes making land and yet there being little land, has always struck me till lately as very good. But in some degree this starts from the a.s.sumption that within periods of which we know anything there was either a continent in such areas, or at least a sea-bottom of not extreme depth.
LETTER 490. TO C. LYELL. King's Head Hotel, Sandown, Isle of Wight, July 18th [1858].
I write merely to thank you for the abstract of the Etna paper. (490/1.
”On the Structure of Lavas which have Consolidated on Steep Slopes, with Remarks on the Mode of Origin of Mount Etna and on the Theory of 'Craters of Elevation,'” by C. Lyell, ”Phil. Trans. R. Soc.” Volume CXLVIII., page 703, 1859.) It seems to me a very grand contribution to our volcanic knowledge. Certainly I never expected to see E. de B.'s [Elie de Beaumont] theory of slopes so completely upset. He must have picked out favourable cases for measurement. And such an array of facts he gives! You have scotched, and will see die, I now think, the Crater of Elevation theory. But what vitality there is in a plausible theory!
(490/2. The rest of this letter is published in ”Life and Letters,” II., page 129.)
LETTER 491. TO C. LYELL. Down, November 25th [1860].
I have endeavoured to think over your discussion, but not with much success. You will have to lay down, I think, very clearly, what foundation you argue from--four parts (which seems to me exceedingly moderate on your part) of Europe being now at rest, with one part undergoing movement. How it is, that from this you can argue that the one part which is now moving will have rested since the commencement of the Glacial period in the proportion of four to one, I do not pretend to see with any clearness; but does not your argument rest on the a.s.sumption that within a given period, say two or three million years, the whole of Europe necessarily has to undergo movement? This may be probable or not so, but it seems to me that you must explain the foundation of your argument from s.p.a.ce to time, which at first, to me was very far from obvious. I can, of course, see that if you can make out your argument satisfactorily to yourself and others it would be most valuable. I can imagine some one saying that it is not fair to argue that the great plains of Europe and the mountainous districts of Scotland and Wales have been at all subjected to the same laws of movement. Looking to the whole world, it has been my opinion, from the very size of the continents and oceans, and especially from the enormous ranges of so many mountain-chains (resulting from cracks which follow from vast areas of elevation, as Hopkins argues (491/1. See ”Report on the Geological Theories of Elevation and Earthquakes.” by William Hopkins. ”Brit. a.s.soc. Rep.” 1847, pages 33-92; also the Anniversary Address to the Geological Society by W. Hopkins in 1852 (”Quart. Journ.
Geol. Soc.” Volume VIII.); in this Address, pages lxviii et seq.) reference is made to the theory of elevation which rests on the supposition ”of the simultaneous action of an upheaving force at every point of the area over which the phenomena of elevation preserve a certain character of continuity...The elevated ma.s.s...becomes stretched, and is ultimately torn and fissured in those directions in which the tendency thus to tear is greatest...It is thus that the complex phenomena of elevation become referable to a general and simple mechanical cause...”)) and from other reasons, it has been my opinion that, as a general rule, very large portions of the world have been simultaneously affected by elevation or subsidence. I can see that this does not apply so strongly to broken Europe, any more than to the Malay Archipelago. Yet, had I been asked, I should have said that probably nearly the whole of Europe was subjected during the Glacial period to periods of elevation and of subsidence. It does not seem to me so certain that the kinds of partial movement which we now see going on show us the kind of movement which Europe has been subjected to since the commencement of the Glacial period. These notions are at least possible, and would they not vitiate your argument? Do you not rest on the belief that, as Scandinavia and some few other parts are now rising, and a few others sinking, and the remainder at rest, so it has been since the commencement of the Glacial period? With my notions I should require this to be made pretty probable before I could put much confidence in your calculations. You have probably thought this all over, but I give you the reflections which come across me, supposing for the moment that you took the proportions of s.p.a.ce at rest and in movement as plainly applicable to time. I have no doubt that you have sufficient evidence that, at the commencement of the Glacial period, the land in Scotland, Wales, etc., stood as high or higher than at present, but I forget the proofs.
Having burnt my own fingers so consumedly with the Wealden, I am fearful for you, but I well know how infinitely more cautious, prudent, and far-seeing you are than I am; but for heaven's sake take care of your fingers; to burn them severely, as I have done, is very unpleasant.
Your 2 1/2 feet for a century of elevation seems a very handsome allowance. can D. Forbes really show the great elevation of Chili? I am astounded at it, and I took some pains on the point.
I do not pretend to say that you may not be right to judge of the past movements of Europe by those now and recently going on, yet it somehow grates against my judgment,--perhaps only against my prejudices.
As a change from elevation to subsidence implies some great subterranean or cosmical change, one may surely calculate on long intervals of rest between. Though, if the cause of the change be ever proved to be astronomical, even this might be doubtful.
P.S.--I do not know whether I have made clear what I think probable, or at least possible: viz., that the greater part of Europe has at times been elevated in some degree equably; at other times it has all subsided equably; and at other times might all have been stationary; and at other times it has been subjected to various unequal movements, up and down, as at present.
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