Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/411

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XXII. Memoranda relative to the Porphyritic Veins, &c. of St. Agnes in Cornwall.


By The Rev. J.J. CONYBEARE.


member of the geological society


[Read December 3rd, 1813.]


I have drawn up the following remarks on the rocks in the vicinity of St. Agnes, in Cornwall, almost wholly from the notes of my friend and fellow-traveller, Mr. Buckland, an accident having prevented my accompanying him to the most interesting spot mentioned in them.


Transactions of the Geological Society, 1st series, vol. 4 plate page 0557.jpg

It is well known that a considerable mining district takes its name from this small town, which is situated on the coast about eleven miles north-east of Truro. The lodes are principally worked for tin, though copper is occasionally raised. The prevailing rock is killas, and the nearest point at which granite has been noticed is in the Gwennap cluster of mines about six miles distant. Having observed in the walls of the neighbourhood several specimens of those porphyritic and granitic rocks, which are distinguished in the country by the name of elvans, we were induced to enquire more particularly into their Geological relations, and the frequency of small promontories on the coast afforded us a favourable opportunity of ascertaining their position in several spots.

Between St. Agnes and Cligga point, (a considerable headland about four miles to the east of it) no less than five of these promontories occur, each of which is traversed by a dyke of elvan,