To Charles Lyell   [on or before 20 January 1847]

South America, p. 167. This letter probably continues a discussion begun during Lyell’s visit to Down on 16 January 1847. See letter to Charles Lyell, [23 January 1847], and Correspondence vol. 3, letter to J. D. Hooker, [December 1846 – January 1847].

‘Cleavage’ and ‘foliation’ were the terms used to denote the regular fissures of slaty rocks and the divisions of metamorphic schists, respectively. In South America, p. 165, CD stated that, with regard to foliation, ‘Mr. Lyell and most authors believe, that the constituent parts of each layer were separately deposited as sediment, and then metamorphosed.’ This was the point of contention between CD and Lyell. In CD’s view, ‘in most cases foliation and cleavage are parts of the same process: in cleavage there being only an incipient separation of the constituent minerals; in foliation a much more complete separation and crystallization’ (South America, p. 166). He suspected that, ‘the planes of cleavage and foliation are intimately connected with the planes of different tension, to which the area was long subjected, after the main fissures or axes of upheavement had been formed, but before the final consolidation of the mass and the total cessation of all molecular movement’ (ibid., p. 168): ‘this difference in the tension affecting the crystalline and concretionary processes’ (ibid., p. 167). At the meeting of the British Association for the Advancement of Science in 1856, John Phillips, discussing the debate on cleavage and foliation, stated that in some cases foliation was still regarded by ‘Lyell and most writers … as the stratification, or traces of the stratification, of the metamorphic rocks of gneiss and mica-schist’ (Phillips 1856, p. 383).