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Darwin Correspondence Project

To Charles Lyell   [21 January – 11 February 1855]

27 York Place, Baker St

Sunday

My dear Lyell

I have received your letter from Down, & I have been studying my S. American Book.

I ought to have stated more clearly, but undoubtedly in W. Tierra del Fuego, where Clay-slate passes by alternation into a grand district of Mica-schist, & in the Chonos islds. & La Plata, where glossy slates occur within the metamorphic schists, the foliation is parallel to the cleavage, ie parallel in strike & dip; but here comes, I am sorry & ashamed to confess a great hiatus in my reasoning, I have assumed that the cleavage in these neighbouring or intercalated beds was (as in more distant parts) distinct from stratification;1 if you choose to say that here the cleavage was or might be parallel to true bedding, I cannot gainsay it, but can only appeal to apparent similarity,—to the great areas of uniformity of strike & high angle,—all certainly unlike as far as my experience goes, to true stratification.— I have long known how easily I overlook flaws in my own reasoning, & this a flagrant case.—

I have been amused to find (for I had quite forgotten) how distinctly I give suspicion (p. 155) (top of page) the idea, before Sharpe, of cleavage, (not foliation) being due to the laminæ forming parts of great curves;2 I well remember the fine section at the end of a region where the cleavage (certainly cleavage) had been most uniform in strike & most variable in dip.—

I made with really great care (& in M.S in detail)3 observations on a case which I believe is new, & bears on your view of metamorphosis (p. 149 at bottom)4 In a clay-slate porphyry regions, where certain thin sedimentary layers of tuff, had by self-attraction, shortened themselves into little curling pieces, & then again into crystals of feldspar of large size, & which consequently were all strictly parallel. The series was perfect & beautiful. Apparently also the rounded grains of quartz had in other parts aggregated themselves into crystalline nodules of quartz. [DIAGRAM HERE]

I have not been able to get Sorby5 yet, but shall not probably have anything to write on it.—

I am delighted you have taken up subject, even if I am utterly floored.

Your’s most truly | C. Darwin

P.S (Do read this P.S.) | I have a presentiment it will turn out that when clay-slate has been metamorphosed the foliation in the resultant schists has been due generally (if not as I think always) to the cleavage, & this to a certain degree will “save my bacon,” (please look at my saving clause p. 167.)6 but when other rocks, then that stratification has been the ruling agent,—the strike, but not the dip, being in such cases parallel to any adjoining clay-slate.— If this be so, preexisting planes of division, we must suppose, on my view of cause determine the lines of crystallization & segregation, & not planes of division produced for the first time, during the act of crystallisation, as in volcanic rocks. If this shd ever be proved, I shall not look back with utter shame at my work.—7

Footnotes

The formations are discussed in South America, pp. 162–8.
Sharpe 1847. In South America, p. 155, CD stated: ‘I suspect that the varying and opposite dips [of the cleavage planes] may possibly be accounted for by the cleavage-laminæ … being parts of large abrupt curves, with their summits cut off and worn down.’ The passage is cited in C. Lyell 1855, p. 615.
CD’s notes and drawings relating to cleavage made during the Beagle voyage are in DAR 39.2.
Described in South America, p. 149.
Henry Clifton Sorby had published an article, ‘On the origin of slaty cleavage’ (Sorby 1853), which Lyell cited in C. Lyell 1855, pp. 610–12. Sorby’s observations on the differential distortion of alternating beds of finer and coarser materials confirmed the mechanical origin of slaty cleavage (Sorby 1853, p. 144). See also CD’s letter to Daniel Sharpe, 12 November [1854], nn. 4 and 5.
The ‘saving clause’ (South America, p. 167) reads: As in some cases it appears that where a fissile rock has been exposed to partial metamorphic action, for instance from the irruption of granite, the foliation has supervened on the already existing cleavage-planes; so, perhaps, in some instances, the foliation of a rock may have been determined by the original planes of deposition or of oblique current-laminæ.
CD’s and Sharpe’s views on the mechanical origin of cleavage and foliation had been accepted by most geologists, including Lyell, by 1855. In the fifth edition of the Manual, Lyell cited new proofs in favour of attributing cleavage to mechanical force (C. Lyell 1855, p. 603). On the question of how far the planes of foliation agree with those of the sedimentary strata, he concluded that ‘in rocks where no cleavage has intervened, foliation and the planes of stratification will usually coincide’ (p. 614).

Bibliography

Sorby, Henry Clifton. 1853. On the origin of slaty cleavage. Edinburgh New Philosophical Journal 55: 137–50.

South America: Geological observations on South America. Being the third part of the geology of the voyage of the Beagle, under the command of Capt. FitzRoy RN, during the years 1832 to 1836. By Charles Darwin. London: Smith, Elder & Co. 1846.

Summary

Relationship of schists to alternating beds of slate in western Tierra del Fuego and the Chonos Islands.

Comments on Sharpe’s theory of curved cleavage planes.

Example of metamorphosis in a "clay-slate porphyry region". Importance of previous lines of cleavage and stratification in foliation of metamorphosed rock.

Letter details

Letter no.
DCP-LETT-1633
From
Charles Robert Darwin
To
Charles Lyell, 1st baronet
Sent from
London, York Place, 27
Source of text
American Philosophical Society (Mss.B.D25.112)
Physical description
8pp

Please cite as

Darwin Correspondence Project, “Letter no. 1633,” accessed on 7 December 2019, https://www.darwinproject.ac.uk/letter/DCP-LETT-1633.xml

Also published in The Correspondence of Charles Darwin, vol. 5

letter