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To Daniel Sharpe   16 October [1851]1

Down Farnborough Kent

Oct 16th.

My dear Sharpe

I am very much obliged to you for telling me the results of your foliaceous tour2 & I am glad to hear you are drawing up an account for the Royal Soc.3 I hope you will have a good illustration or map, of the waving line of junction of the slate & schist with uniformly directed cleavage & foliation. It strikes me as crucial.4 I remember longing for an opportunity to observe this point. All that I say is that when slate & the metamorphic schists occur in the same neighbourhood, the cleavage & foliation are uniform: of this I have seen many cases, but I never observed slate actually overlying mica-slate.— I have, however, observed many cases of glassy clay-slate included within mica-schist & gneiss.

All your other observations on the arches &c seem very interesting. From conversations with Lyell &c, I recommend you to describe in a little detail the nature of the metamorphic schists;5 especially whether there are quasi-sub-strata of different varieties of mica-slate or gneiss &c.; & 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 (& described at M. Video) of portions of fine Chloritic schists being entangled in the midst of a gneiss district.6 Have you had any opportunity of tracing a bed of marble? this I think from reasons given at p. 166 of my S. America would be very interesting.7

A suspicion has sometimes occurred to me (I remember more especially when tracing the clay-slate at the C. of Good Hope,8 turning into true gneiss) that possibly all the metamorphic schists necessarily once existed as clay-slate, & that the foliation did not arise or take its direction in the metamorphic schists, but resulted simply from the preexisting cleavage.— The so-called beds in the metamorphic schists, so unlike common cleavage laminæ, 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 Roger’s Law, as he reiterates it, of cleavage being parallel to his axes-planes of Elevation?9

If you know before hand, will you tell me when your Paper is read; for the chance of my being able to attend.10 I very seldom leave home, as I find perfect quietude suits my health best.

Pray believe me, dear Sharpe | Your’s sincerely | C. Darwin

Footnotes

1
The year is ascertained from Sharpe’s note at the top of the letter: ‘Ansd 23 Oct 1851’. Sharpe’s reply has not been found.
2
Sharpe had recently completed a tour of Scotland to examine evidence of foliation and cleavage in metamorphic rocks.
3
Sharpe 1852. The paper, received on 20 November 1851, was read on 12 and 19 February 1852 (Abstracts of the papers communicated to the Royal Society of London 6 (1850–4): 152). CD reviewed it for publication in the Philosophical Transactions of the Royal Society (see letter to the Royal Society, 16 March [1852]).
4
A coloured map of Scotland (Plate XXIV), showing the direction of the foliation and cleavage in the slates, schists, and gneiss, was included in the published version. It indicates the conformity of the direction of foliation and cleavage ‘which is perhaps the strongest evidence that can be adduced to show the identity of the causes which produced the two phenomena’ (Sharpe 1852, p. 451) and provides ‘striking evidence of the connection of the physical geography with the foliation of the rocks’ (Sharpe 1852, p. 456). This supported CD’s and Sharpe’s view that cleavage and foliation were parts of the same process produced by unequal tensions in rocks resulting from upheavement of strata, and unrelated to sedimentary processes.
5
For CD’s reason for wishing Sharpe to describe metamorphic schists in more detail, see letter to Charles Lyell, [8 April 1851]. Sharpe 1852, p. 457, specifically argues against Lyell’s views on foliation as expressed in C. Lyell 1851a, stating: The remarks already made on the analogy between foliation and cleavage confirm Mr. Darwin’s view, that “foliation and cleavage are parts of the same process;” for on no other supposition can we explain the conformity between the two where seen in contact, and their being combined in the same arch of elevation. Now as cleavage is almost always transverse to the bedding and obviously a change produced in the beds after their deposition, it follows that foliation also is distinct from bedding or sedimentary stratification.
6
South America, p. 166.
7
‘The mere presence of true strata in the midst of a set of metamorphic schists, is no argument that the foliation is of sedimentary origin, without it be further shown in each case that the folia not only strike, but dip throughout in parallel planes with those of the true stratification’ (South America, p. 167).
9
See Correspondence, vol. 3, letter to Daniel Sharpe, [1 November 1846], where CD recommended that Sharpe read Rogers 1846. CD’s copy is in the Darwin Library–CUL. The law that Henry Darwin Rogers suggests concerned the ‘general fact of the approximate parallelism of the cleavage dip, and the anticlinal and synclinal planes of the closely folded strata; and also to point out the general law of their parallelism to the planes of maximum heat.’ (Rogers 1846, p. 423). Sharpe makes no reference to Roger’s law in his paper.
10
There is no evidence in CD’s Account book (Down House MS) of a visit to London on either 12 or 19 February 1852, when the paper was read.

Bibliography

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Rogers, Henry Darwin. 1846. On cleavage of slate-strata. Edinburgh New Philosophical Journal 41: 422–3.

Sharpe, Daniel. 1852. On the arrangement of the foliation and cleavage of the rocks of the north of Scotland. Philosophical Transactions of the Royal Society of London, pp. 445– 61.

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.

Volcanic islands: Geological observations on the volcanic islands, visited during the voyage of HMS Beagle, together with some brief notices on the geology of Australia and the Cape of Good Hope. Being the second 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. 1844.

Summary

Thanks DS for writing about his research on foliation and cleavage. Discusses nature of slate and metamorphic schists.

Makes suggestions for the paper DS is preparing for the Royal Society and raises questions for his consideration; CD hopes he can attend the Society meeting when the paper is read ["On foliation and cleavage of Scotland", Philos. Trans. R. Soc. Lond. 142 (1852): 445–62].

Letter details

Letter no.
DCP-LETT-1458
From
Charles Robert Darwin
To
Daniel Sharpe
Sent from
Down
Source of text
American Philosophical Society (Mss.B.D25.)
Physical description
ALS 3pp

Please cite as

Darwin Correspondence Project, “Letter no. 1458,” accessed on 16 April 2024, https://www.darwinproject.ac.uk/letter/?docId=letters/DCP-LETT-1458.xml

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

letter

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