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.] 



J. 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. 



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. Hav- 

 ing 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 oppor- 

 tunity 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 pro- 

 montories occur, each of which is traversed by a dyke of elvan, 



