Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/509

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499
Clovelly, North Devon.

destitute of metallic veins. It alternates with transition limestone, and where it does so, occasionally contains organic remains. It alternates also, in one instance at least, with large beds of a species of culm. Its veins contain few or no extraneous substances besides common quartz and, though less frequently, calcareous spar.[1]

Transactions of the Geological Society, 1st series, vol. 2 plate page 0659.png

The latter (the Killas of Cornwall) has generally the silky character ascribed to clay slate.[2] It is traversed almost throughout its whole extent by frequent veins, or rather dykes, of a porphyritic rock, which forms one variety of the Elvan of the Cornish miners, and which does not occur either in the dunstone or shillat. It has been found to contain at St. Agnes, the topaz, and in various places the tourmaline and the garnet: the veins are frequently occupied by chlorite, by mica, and by crystallized felspar. A considerable part of the tin of Cornwall is obtained from the killas, and the grey ore of cobalt has repeatedly been found in it. All these minerals have usually been considered as characteristic of the earlier rocks, and none of them are, I believe, stated by the writers of our most esteemed mineralogical systems to occur in that to which they assign the name of grauwacke[3].

It is not without the greatest diffidence in my own knowledge of the subject and powers of observation that I venture to dissent, even on a mere point of nomenclature, from the opinion of my able and experienced friend Dr. Berger. I cannot however but conceive that

  1. In the neighbourhood of Bideford, a little to the west of Ilfracombe, the rock is said to be much traversed by this mineral.
  2. May it not be added that its siliceous varieties are of a texture much finer and more compact than that of Dunstone?
  3. The following substances also which appear (although less positively) to be restricted to primitive formations by the school of Werner, occur in the killas; wolfram, native silver, horn silver, schiefer spar, actinolite. I believe that the chiastolite has also been found in it.
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