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IV.—On the Geological History of the Cornish Serpentinous Rocks
Published online by Cambridge University Press: 01 May 2009
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Botallaek Mine.—The dark hornblendic slates of the sea-border of the parish of St. Just are known to many geological tourists. They have been well described by the late Mr. J. A. Phillips,1 who regards them as consisting of altered killas ; and the justice of this conclusion will not I think be questioned by any who have studied the rocks in situ; traces of an original lamellar structure are visible even in hand-specimens. Phillips (op. cit. p. 322) gives the following analyses of the rock, a being from near the surface, and b from a depth of 130 fathoms, or far below the sea-level; while c is the analysis of a typical Cornish killas (from Polgooth Mine, 100 fathoms from surface):—
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References
page 220 note 1 Quart Journ. Geol. Society, 1875, vol. xxxi. pp. 319–343.Google Scholar
page 221 note 1 Loc. cit. See also “ On the Composition and Origin of the Waters of a Saltspring in Huel Seton Mine,” Phil. Mag. July, 1873.
page 222 note 1 Quart. Journ. Geol. Soc. 1878, vol. xxxiv. p. 487.Google Scholar
page 223 note 1 See Geol. Mag. Dec. III. Vol. III. 1886, p. 365.Google Scholar
page 223 note 2 Eruptive Rocks from the Neighbourhood of St. Minver,” by Rutley, F., F.G.S., Quart. Journ. Geol. Soc. 1886, vol. xlii.Google Scholar
page 224 note 1 Jn a note Mr. Rutley says : “ Prof. Bonney, who favoured me with an opinion upon these sections, regards much of the substance which I have here called serpentine as a palagonitic material.” But palagonite is a constituent of recent volcanic tufa, and hardly likely to be found in so old a rock as this. Moreover, it is fusible, while this mineral, like serpentine, is very infusible. Further, Mr. Kutley says the serpentinous matter has a hardness of 3 or slightly less, while the hardness of palagonite is from 4 to 5.—J. H. C.
page 224 note 2 Op. cit. p. 397.
page 224 note 3 See Geol. Mag. August, 1886, p. 362.Google Scholar
page 224 note 4 Allport, , “ Metatnorphic Rocks surrounding the Land's End Mass of Granite,” Quart. Journ. Geol. Soc. 1876, vol. xxxii. p. 423.Google Scholar