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VII.—On the Geological History of the Cornish Serpentinous Rocks
Published online by Cambridge University Press: 01 May 2009
Extract
My argument that at Porthalla there is a “passage” from hornblende-schist to serpentine; or rather that some beds of a common series have been changed into serpentine, others into hornblende-schist, and others again into a substance of intermediate character, is, I think, much strengthened by the fact that many such “apparent passages” are admitted to exist by all those who have examined the Lizard Coast with any degree of detail. De la Beche's description of that seen near the Lizard Town is as follows, and it would apply equally well to the others. “The hornblende slate,” he says, “supports the great mass of the Lizard serpentine with an apparent passage of the one into the other in many places—an apparent passage somewhat embarrassing,” that is, from his point of view; from mine it is perfectly natural. He goes on to say: “Whatever the cause of this apparent passage may have been, it is very readily seen at Mullion Cove, at Pradanack Point, at the coast west of Lizard Town, and at several places on the east coast between Landewednack and Kennick Cove, more especially under the Balk … and at the remarkable cavern and open cavity named the Frying-Pan, near Cadgwith.” At Kynance some of the laminse of serpentine are not more than one-tenth of an inch in thickness for considerable distances.
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References
page 360 note 1 Report on Cornwall, etc. See also Memoirs of the Geological Survey.
page 361 note 1 I do not find picrite mentioned in my edition of Von Cotta. Dana says (p. 258), Picryte from Moravia “consists half of chrysolite (olivine) along with felspar, diallage, hornblende and magnetite.” Rutley defines it (Study of Rocks, p. 265) as “blackish-green crystalline rock with a compact black matrix containing porphyritic crystals and grains of olivine. The matrix may consist of hornblende, diallage, or biotite, associated with magnetite and calc-spar. The olivine constitutes nearly half the bulk of the rock.” Prof. Bonney regards it as a compound of olivine, a small variable amount of felspar, such as anorthite or labradorite, enstatite or a pyroxenic mineral with biotite.
page 361 note 2 This district was surveyed by MrWhitley, N., of Truro, in 1841Google Scholar, and described in the Report of the Koyal Institution of Cornwall. The serpentine was also described by the Rev. Rogers, Canon in the Trans. Royal Geol. Soc. of Cornwall, vol. vi. p. 41, 1846Google Scholar.
page 362 note 1 Phil. Mag. Feb. 1871.
page 362 note 2 Min. Mag. Nov. 1877.
page 363 note 1 Phil. Mag. Feb. 1871.
page 363 note 2 For this analysis, which was given in the Mineralogical Magazine, loc. cit., I am indebted to Mr. Phillips. He remarks, “I find more magnesia than you do, but my specimens have undergone more extensive change than yours, as you will see from the fragment which I inclose.” He also observes, “My recent analysis of some remarkably fresh rock from the Sanctuaries gave 1·75 per cent, of magnesia.”
page 364 note 1 The Hensbarrow Granite District, 1876.
page 365 note 1 Quart. Journ. Geol. Soc. 128, 11 1876, p. 422Google Scholar.
page 365 note 2 Phil. Mag. 1871.
page 365 note 3 Quart. Journ. Geol. Soc. vol. xxx. p. 557.
page 366 note 1 A shallow basin or crucible of this rock has been recently found in close juxtaposition with a “celt mould” of hard sandy slate at Altarnum, a few miles from the Polyfant quarries. It was probably used for melting the bronze used by the ancient Britons. It is now in the Museum of the Eoyal Institution of Cornwall at Truro.