402 Rev. J. J. Conybeare on the Veins of St. Agnes. 



running nearly due east and west, and varying in a manner appa- 

 rently capricious, both as to its thickness and inclination. The 

 appearance of these dykes will be best understood by reference to 

 the annexed drawings,* which are faithfully copied from sketches 

 made on the spot by Mr. Buckland ; nor can I describe their gene- 

 ral character better than in the language of that gentleman. " The 

 elvans all along this coast occur in beds and veins of every possible 

 thickness, from forty feet to half an inch, sometimes overlying, 

 but more frequently traversing the killas in various directions, 

 under such circumstances as are apparently irreconcileable with any 

 other theory than that which supposes them to be of contempora- 

 neous formation with the rock containing them, the result of 

 some play of affinities which allowed a part of the mass to assume 

 a crystalline texture, while its coarser and more abundant portions 

 were left to arrange themselves in the slaty or tortuous form which 

 characterizes the killas." These elvans are for the greater part of 

 porphyritic structure, the base being in most instances a very mi- 

 nute aggregation of quartz, pale chlorite, and possibly some felspar : 

 the first of these is usually the predominant ingredient, the imbedded 

 substances are felspar, quartz occasionally crystallized in small 

 double hexagonal pyramids, and chlorite in small patches. In one 

 quarry, a little to the west of St. Agnes, we found the same variety 

 of elvan passing (by the addition of tourmalin, and the decrease or 

 loss of all its ingredients excepting the quartz) into a rock much 

 resembling that of Roche, in the same county. At Cligga Point 

 itself besides the elvan, we observed a small formation of what 

 would probably be considered by the Wernerian school, as the 

 newer granite, incumbent upon the schist at an angle of at least 

 80 degrees. Its singular stratification will best be understood from 



* PI. TS. 



