To Daniel Sharpe   12 November [1854]

Sharpe’s paper ‘On the structure of Mont Blanc’ (Sharpe 1855) was read at the Geological Society meeting of 15 November 1854. Sharpe discussed the relationship of the foliation of the crystalline rocks to the cleavage of the surrounding stratified slates.

The Athenæum, no. 1405, 30 September 1854, pp. 1172–82, carried a report of the British Association for the Advancement of Science meeting in Liverpool. Edward Forbes was president of section C (geology).

In his paper, Forbes stated that CD considered foliation in mica slate ‘identical with cleavage, and due to the same cause,—the one passing into the other’ (Athenæum, no. 1405, 30 September 1854, p. 1178, later published as Forbes 1854b). In Forbes’s opinion foliation was ‘a superinduced structure quite distinct from cleavage or lamination’ (Forbes 1854b, p. 82).

Sorby 1853. Henry Clifton Sorby presented observations that supported a mechanical origin for the cleavage in the slate rocks of North Wales. He suggested ‘there is abundance of evidence to prove that rocks having slaty cleavage have been greatly compressed in a line perpendicular to cleavage, and elongated to a certain extent in the line of its dip’ (Sorby 1853, p. 144).

The view of Charles Lyell and his predecessor, James Hutton, was that the lines of what CD and Sharpe called cleavage and foliation represented different layers of the original deposits which had been contorted and metamorphosed by later geological action. According to the Athenæum account, Lyell’s comment on Forbes’s distinction (see n. 3, above) was that ‘Plutonic action … often left the planes of stratification still visible. The unaltered sedimentary beds were frequently affected by irregularities as great as those of the altered rocks… . If these were rendered metamorphic, there would be danger of attributing to chemical action peculiarities which existed whilst the beds were yet unaltered.’ (Athenæum, no. 1405, 30 September 1854, pp. 1178–9). Lyell, apparently influenced by the papers of Sharpe and Sorby, revised his view in favour of the mechanical explanation of cleavage in the fifth edition of the Manual of elementary geology (C. Lyell 1855, pp. 607–17). In his preface, outlining the principal alterations and additions in that edition, he stated: ‘New proofs are cited in favour of attributing cleavage to mechanical force, p. 610., and for inferring in many cases a connection between foliation and cleavage, p. 615. At the same time, the question—how far the planes of foliation usually agree with those of sedimentary deposition, is entered into, p. 614.’ (ibid., p. ix).