Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/412

 running nearly due east and west, and varying in a manner apparently 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 general 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 irreconcilable with any other theory than that which supposes them to be of contemporaneous 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 minute 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 tourmaline, 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