STATHAM ON THE GEOLOGY OV 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 diflfereut 
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 elvan-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 Came, 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, silica 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 fi-om 4.56 to 27.06 of that metal, 
according to the analysis of Kobell and Turner. 
