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II.—Note on the Serpentine of Duporth, in St. Austell Bay, Cornwall

Published online by Cambridge University Press:  14 March 2018

Extract

The Geological Survey Map of CornwalI shews, to the south of the town of St. Austell, several patches of “greenstone,” extending from near the ancient earthwork called Trethullan Castle, on the west, to the sea at St. Austell Bay in the east ; a distance of over four miles.

The western end of the most extensive of these patches swells ont on the top of the hill to the north-east of St. Mewan Church, so as to cover several hundred acres (including some of the best land in the parish). This was described by Mr. J. Arthur Phillips some years since in the Philosophical Magazine. Mr. Phillips speaks of the rock as being distinctly crystalline, consisting of felspar (sometimes triclinic) ; semi-transparent yellowish-brown crystals, probably hornblende ; a green fibrous mineral also believed to be a variety of hornblende ; many black grains of oxide of iron and a few hexagonal crystals, probably apatite ; and he says the rock exhibits unmistakeable evidence of extensive alteration. Speaking of its extension eastward, he says “it extends in diminished proportions in a southeasterly direction to the sea at Duporth.”

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1877

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References

page 222 note * Phil . Mag. Feb., 1871.

page 223 note * Phil. Mag., Feb., 1871.

page 224 note † For this analysis I am indebted to Mr. Phillips, who has courteously sent me his results since the reading of the above paper. 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 enclose.” He also observes “My recent analysis of some remarkably fresh rock from the Sanctuaries gave 1.75 per cent. of magnesia.

page 224 note ‡ .33 lost in water bath.

page 224 note § .5 lost at 120° C.

page 224 note ǁ 2.73 lost at 120° C.