Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/150

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Mr. William Phillips on the Veins of Cornwall.


Elvan is one of the three grand distinctions made in regard to rocks by the Cornish miner. Whatever is not grouan (granite) or kill as (schist), is of course with him elvan. So that, in fact, it is extremely difficult to say what it is or even what it is not, with the exception of granite and schist. The substance to which it is most commonly applied occurs frequently in Cornwall, not as forming tracts of country, but interposed between the schist, in what is termed by the miner a channel. I know of no instance of its thus occurring in granite. The situation of these channels is not horizontal: they generally dip at various angles with the horizon, and in various directions. The colour of elvan is bluish-grey, or yellowish. It does not always disturb the contents of the metalliferous vein, which generally continues through it, though the load is mostly narrower than when in the schist; but it sometimes has the effect of dividing it into small branches. By the accompanying section, of Pleasure, Fancy, and North Herland veins, Pl. 8. fig. 10. it will be seen that a large channel of elvan took a course opposed, though not opposite, to that of the metalliferous vein, and that two other channels took similar directions by the section of the Manor Old Vein, fig. 11. The examination of some specimens induced me to consider the elvan of these mines to consist of crystals of quartz and felspar imbedded in compact felspar; in one specimen, the latter was intermixed with compact quartz.