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To John Phillips   18 January [1856]1

Down Bromley Kent

Jan. 18th—

Dear Phillips

I have been looking over my Chapter on Cleavage & Foliation in my Geolog. Observ. on S. America 1846, & with that candour so characteristic of authors I really think it worth your looking over.— Some remarks in first part of Chapt. p. 140, are, perhaps, worth skimming over; but the concluding remarks p. 162 give my results.— I see I give one case p. 147 of cleavage not strictly coincident with mountain range. I may remark that I think you may trust my observations, as I made a vast number always with compass (corrected for var.) & note-book, as I was at time deeply interested in subject, & astounded at cleavage being quite distinct from stratification. The case p. 144 of the confused cleavage, where two great series crossed each other; the separate hillocks running in different directions, each with its own folia parallel to its longer axis seems interesting.

The foliation of the Chonos Group p. 157 seems to me a grand case, & here it is not parallel to line of coast: the apparent crossing (p 159) of a subsequently formed chain is worth notice I think.— In my general conclusions, I allude to well-known fact of cleaved clay-slate when metamorphosed by neighbouring granite, becoming foliated in the planes of cleavage; & I now suppose that this is the explanation of most of the cases of widely extended foliated rocks having the same strike, described by me, for instance that of the Chonos group; at the same time, I think, it should never be forgotten that rocks which have been liquefied by heat, sometimes have their crystallized materials so arranged, as almost to deserve to be called foliated, of which I saw grand instance in Eastern Cordillera of Chile.—2

In a brief description of the Falkland Isld. in Journal of Geolog. Soc. (read in March 1846) Vol. 2 (?) p. 270,3 I give a case of a range of stratified quartz, changing its course, & with it the cleavage of the clay-slate at its base: I remember making numerous careful observations on this head.— At p. 271, I give from Capt. Sulivan4 (a careful observer) a case, which I have never myself seen; of cleavage in a set of folded beds, in some vertical, in others at right angles to each bed.—

I ought to apologise for troubling you with so long a note more especially as I do not know how much you are concerned with the vaguely denominated foliated rocks.—

Yours very sincerely | Ch. Darwin

The remark p. 163 of Geolog. Observat. in S. America of difference in Mineralogical composition in the planes of cleavage; I do not remember to have seen noticed: I am sure of its accuracy, & presume it is the first step towards metamorphosism or the segregation of separate minerals in the planes of cleavage.—

One other remark,—for years & years the existence of grauwacke with Clay-Slates in various parts of world has perplexed me; as clay-slates seem to have been formed in deep & tranquil seas— Do you think that the same pressure which causes the cleavage, & some movement along the planes of cleavage, can in part have actually broken up the rock & mingled different varieties together,, like fragments of ice in a glacier, & like these subsequently recemented together by pressure.— Do just think of this.—

My Geolog. Observ. in S. America must be in Bodleian Library5

If you care to have copy of my 3 vols. together, I wd. with pleasure give you order for one. on Smith & Elder.6

1
Dated on the basis that Phillips was preparing his report on cleavage for the meeting of the British Association for the Advancement of Science to be held in August 1856 (Phillips 1856).
2
Phillips’s intention in Phillips 1856 was to distinguish the phenomenon of cleavage in rocks from that of stratification, a distinction that CD had also taken pains to make in South America (see also Correspondence vol. 4, letter to Charles Lyell, [on or before 20 January 1847], n. 2). In his discussion of the relationship between the planes of cleavage and the inclination of strata, Phillips cited CD’s observations (Phillips 1856, pp. 375–6), and in commenting on the symmetry between cleavage planes and axes of elevation he quoted from South America, p. 162: ‘The cleavage laminæ range over wide areas with remarkable uniformity, being parallel in strike to the main axes of elevation, and generally to the outlines of the coast’ (Phillips 1856, p. 375).
3
‘On the geology of the Falkland Islands,’Quarterly Journal of the Geological Society of London. Proceedings of the Geological Society, pt 1, 2 (1846): 267–74 (Collected papers 1: 203–12). The page reference is to a passage on quartz.
4
Bartholomew James Sulivan, lieutenant in the Beagle, made two surveying voyages to the Falkland Islands, 1837–9 and 1842–6.
5
The Bodleian is the University Library of Oxford. Phillips was keeper of the Ashmolean Museum at Oxford and reader in geology. He became professor of geology in 1857.
6
In 1851, Smith, Elder & Co. had published a combined edition of Coral reefs, Volcanic islands, and South America entitled Geological observations on coral reefs, volcanic islands, and on South America etc. (Geology of the ‘Beagle’).

Manuscript Alterations and Comments

1.5 p. 162] interl
1.5 give] altered from ‘gives’
2.1 & here] after del illeg
2.1 it] interl
2.2 apparent] interl
2.4 becoming] altered from ‘become’
2.9 sometimes] interl
2.9 crystallized] interl
3.2 270,] after del ‘267,’
3.6 vertical] after del ‘at’
7.2 in various parts of world] interl
7.5 together,] interl
7.6 these] over ‘them’
7.6 subsequently] interl

Please cite as “DCP-LETT-1822,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-1822