Chapter XIII

. of "South America" (1846) and in the "Manual of Scientific Enquiry" (1849), but are perhaps nowhere so clearly expressed as in this correspondence. His most important contribution to the question was in establishing the fact that foliation is often a part of the same process as cleavage, and is in nowise necessarily connected with planes of stratification. Herein he was opposed to Lyell and the other geologists of the day, but time has made good his position. The postscript to Letter 542 is especially interesting. We are indebted to Mr. Harker, of St. John's College, for this note.)

Down, August 23rd [1846?].

I must just send one line to thank you for your note, and to say how heartily glad I am that you stick to the cleavage and foliation question. Nothing will ever convince me that it is not a noble subject of investigation, which will lead some day to great views. I think it quite extraordinary how little the subject seems to interest British geologists. You will, I think live to see the importance of your paper recognised. (536/2. Probably the paper "On Slaty Cleavage." "Quart. Journ. Geol. Soc." Volume III., page 74, 1847.) I had always thought that Studer was one of the few geologists who had taken a correct and enlarged view on the subject.

LETTER 537. TO D. SHARPE. Down [November 1846].

I have been much interested with your letter, and am delighted that you have thought my few remarks worth attention. My observations on foliation are more deserving confidence than those on cleavage; for during my first year in clay-slate countries, I was quite unaware of there being any marked difference between cleavage and stratification; I well remember my astonishment at coming to the conclusion that they were totally different actions, and my delight at subsequently reading Sedgwick's views (537/1. "Remarks on the Structure of Large Mineral Masses, and especially on the Chemical Changes produced in the Aggregation of Stratified Rocks during different periods after their Deposition." "Trans. Geol. Soc." Volume III., page 461, 1835. In the section of this paper dealing with cleavage (page 469) Prof. Sedgwick lays stress on the fact that "the cleavage is in no instance parallel to the true beds."); hence at that time I was only just getting out of a mist with respect to cleavage-laminae dipping inwards on mountain flanks. I have certainly often observed it--so often that I thought myself justified in propounding it as usual. I might perhaps have been in some degree prejudiced by Von Buch's remarks, for which in those days I had a somewhat greater deference than I now have. The Mount at M. Video (page 146 of my book (537/2. "Geol. Obs. S. America." page 146. The mount is described as consisting of hornblendic slate; "the laminae of the slate on the north and south side near the summit dip inwards.")) is certainly an instance of the cleavage-laminae of a hornblendic schist dipping inwards on both sides, for I examined this hill carefully with compass in hand and notebook. I entirely admit, however, that a conclusion drawn from striking a rough balance in one's mind is worth nothing compared with the evidence drawn from one continuous line of section. I read Studer's paper carefully, and drew the conclusion stated from it; but I may very likely be in an error. I only state that I have frequently seen cleavage-laminae dipping inwards on mountain sides; that I cannot give up, but I daresay a general extension of the rule (as might justly be inferred from the manner of my statement) would be quite erroneous. Von Buch's statement is in his "Travels in Norway" (537/3. "Travels through Norway and Lapland during the years 1806-8": London, 1813.); I have unfortunately lost the reference, and it is a high crime, I confess, even to refer to an opinion without a precise reference. If you never read these travels they might be worth skimming, chiefly as an amusement; and if you like and will send me a line by the general post of Monday or Tuesday, I will either send it up with Hopkins on Wednesday, or bring it myself to the Geological Society. I am very glad you are going to read Hopkins (537/4. "Researches in Physical Geology," by W. Hopkins. "Phil. Trans. R. Soc." 1839, page 381; ibid, 1842, page 43, etc.); his views appear to me eminently worth well comprehending; false views and language appear to me to be almost universally held by geologists on the formation of fissures, dikes and mountain chains. If you would have the patience, I should be glad if you would read in my "Volcanic Islands" from page 65, or even pages 54 to 72--viz., on the lamination of volcanic rocks; I may add that I sent the series of specimens there described to Professor Forbes of Edinburgh, and he thought they bore out my views.

There is a short extract from Prof. Rogers (537/5. "On Cleavage of Slate-strata." "Edinburgh New Phil. Journ." Volume XLI., page 422, 1846.) in the last "Edinburgh New Phil. Journal," well worth your attention, on the cleavage of the Appalachian chain, and which seems far more uniform in the direction of dip than in any case which I have met with; the Rogers doctrine of the ridge being thrown up by great waves I believe is monstrous; but the manner in which the ridges have been thrown over (as if by a lateral force acting on one side on a higher level than on the other) is very curious, and he now states that the cleavage is parallel to the axis-planes of these thrown-over ridges. Your case of the limestone beds to my mind is the greatest difficulty on any mechanical doctrine; though I did not expect ever to find actual displacement, as seems to be proved by your shell evidence. I am extremely glad you have taken up this most interesting subject in such a philosophical spirit; I have no doubt you will do much in it; Sedgwick let a fine opportunity slip away. I hope you will get out another section like that in your letter; these are the real things wanted.

LETTER 538. TO D. SHARPE. Down, [January 1847].

I am very much obliged for the MS., which I return. I do not quite understand from your note whether you have struck out all on this point in your paper: I much hope not; if you have, allow me to urge on you to append a note, briefly stating the facts, and that you omitted them in your paper from the observations not being finished.

I am strongly tempted to suspect that the cleavage planes will be proved by you to have slided a little over each other, and to have been planes of incipient tearing, to use Forbes' expression in ice; it will in that case be beautifully analogical with my laminated lavas, and these in composition are intimately connected with the metamorphic schists.

The beds without cleavage between those with cleavage do not weigh quite so heavily on me as on you. You remember, of course, Sedgwick's facts of limestone, and mine of sandstone, breaking in the line of cleavage, transversely to the planes of deposition. If you look at cleavage as I do, as the result of chemical action or crystalline forces, super-induced in certain places by their mechanical state of tension, then it is not surprising that some rocks should yield more or less readily to the crystalline forces.

I think I shall write to Prof. Forbes (538/1. Prof. D. Forbes.) of Edinburgh, with whom I corresponded on my laminated volcanic rocks, to call his early attention to your paper.

LETTER 539. TO D. SHARPE. Down, October 16th [1851].

I am very much obliged to you for telling me the results of your foliaceous tour, and I am glad you are drawing up an account for the Royal Society. (539/1. "On the Arrangement of the Foliation and Cleavage of the Rocks of the North of Scotland." "Phil. Trans. R. Soc." 1852, page 445, with Plates XXIII. and XXIV.) I hope you will have a good illustration or map of the waving line of junction of the slate and schist with uniformly directed cleavage and foliation. It strikes me as crucial. I remember longing for an opportunity to observe this point. All that I say is that when slate and the metamorphic schists occur in the same neighbourhood, the cleavage and foliation are uniform: of this I have seen many cases, but I have never observed slate overlying mica-slate. I have, however, observed many cases of glossy clay-slate included within mica-schist and gneiss. All your other observations on the order, etc., seem very interesting. From conversations with Lyell, etc., I recommend you to describe in a little detail the nature of the metamorphic schists; especially whether there are quasi-substrata of different varieties of mica-slate or gneiss, etc.; and whether you traced such quasi beds into the cleavage slate. I have not the least doubt of such facts occurring, from what I have seen (and described at M. Video) of portions of fine chloritic schists being entangled in the midst of a gneiss district. Have you had any opportunity of tracing a bed of marble? This, I think, from reasons given at page 166 of my "S. America," would be very interesting. (539/2. "I have never had an opportunity of tracing, for any distance, along the line both of strike and dip, the so-called beds in the metamorphic schists, but I strongly suspect that they would not be found to extend, with the same character, very far in the line either of their dip or strike. Hence I am led to believe that most of the so-called beds are of the nature of complex folia, and have not been separately deposited. Of course, this view cannot be extended to THICK masses included in the metamorphic series, which are of totally different composition from the adjoining schists, and which are far-extended, as is sometimes the case with quartz and marble; these must generally be of the nature of true strata" ("Geological Observations," page 166).) A suspicion has sometimes occurred to me (I remember more especially when tracing the clay-slate at the Cape of Good Hope turning into true gneiss) that possibly all the metamorphic schists necessarily once existed as clay-slate, and that the foliation did not arise or take its direction in the metamorphic schists, but resulted simply from the pre-existing cleavage. The so-called beds in the metamorphic schists, so unlike common cleavage laminae, seems the best, or at least one argument against such a suspicion. Yet I think it is a point deserving your notice. Have you thought at all over Rogers' Law, as he reiterates it, of cleavage being parallel to his axes-planes of elevation?

If you know beforehand, will you tell me when your paper is read, for the chance of my being able to attend? I very seldom leave home, as I find perfect quietude suits my health best.

(PLATE: CHARLES DARWIN, Cir. 1854. Maull & Fox, photo. Walker & Cockerell, ph. sc.)

LETTER 540. TO C. LYELL. Down, January 10th, 1855.

I received your letter yesterday, but was unable to answer it, as I had to go out at once on business of importance. I am very glad that you are reconsidering the subject of foliation; I have just read over what I have written on the subject, and admire it very much, and abide by it all. (540/1. "Geological Observations on South America,"