STATHAM — ON THE GEOLOGY OF THE SCILLY ISLES. 



23 



which, at low water, joins Taylor's Island to the main and of St. 

 Mary's, I detected the existence of a trachytic vein about twelve feet 

 wide, crossed by joints in the granite running south-west by south. 

 On each side of this dyke the granite is altered in character, and, from 

 being of the white coarse character so common in this island, assumes 

 a brownish, speckled, and porphyritic nature. I imagine that the 

 heat occasioned by the irruption of this trachytic vein has fused the 

 surrounding granite, or in some way altered the character of its 

 constituents,* causing them to re-arrange themselves in a different 

 form ; for the granite upon Taylor's Island is found to contain 

 crystals of tourmaline replacing the mica. The dyke probably ex- 

 tends a good way inland, as I noticed, at a deep well, cut in the rocks, 

 near Newford Down, that such porphyritic granite seemed to abound 

 about all those parts ; indeed, the blocks lying in all directions, do 

 not at first sight present the appearance of granite at all; and it 

 requires a close inspection to be assured that they are really granite. 



On the north side of Porthloo Bay, likewise at low water, I found 

 portions of a porphyritic ridge running parallel with the line of coast, 

 stretching out to Newford Island, at north-west by west. But by far 

 the most interesting relic of igneous action is to be found in the por- 

 phyritic dyke, or el van-course, in Watermill Bay, near New Quay, to 

 the north-east of the island. Attention was called, in one of the 

 earliest volumes of the Transactions of the Geological Society of Corn- 

 wall, to this remarkable geological feature, in a paper entitled " The 

 Geology of some parts of Cornwall and the Scilly Isles." The writer 

 expresses therein his belief that the mass of surrounding granite was 

 " decidedly stratified ; " and, so far as mere appearances are concerned, 

 any unscientific observer would readily concur in such an opinion. Mr. 

 Joseph Carne, in the able paper to which I have already made allusion, 

 thus adverts to this interesting phenomenon. " At Watermill Bay 

 in St. Mary's, the pebbles on the beach indicate the contiguity 

 of porphyry and porphyritic granite ; and on the south side of it, 

 between the rivulet and the curious little quay called New Quay, 



* The constituents of Felspar are, according to Rose, sUica 65.91, alumina 

 21.00, lime or magnesia 0.11, potash 10.18, or soda 3.50. Those of Tourmaline, 

 according to Rammelsberg, silica 37.80, lime or magnesia 1.42, alumina 30.56, 

 soda 2.09, iron 0.50, or manganese 2.50, other substances 9.90. The iron might 

 be procured from the mica, which contains from 4.56 to 27.06 of that metal, 

 according to the analysis of Kobell and Tinner. 



