Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/21

 Rh Castle Cornet is built is a gneiss, often approaching so near to granite as to render its place in a nomenclature doubtful. It is every where crossed and intersected by veins of quartz, of trap, and of felspar, curved and mixed in various ways, but tending, upon the whole, to the north or north-east. More rarely there are found in it veins of brick red and bright green felspar, and pebbles of the same substance, or with hornblende imbedded, are found on the beach, as well as coarse agates passing into quartz and hornstone.

Proceeding from the Castle southwards, gneiss is found to constitute the cliffs on the eastern side, often in a state of decomposition, and covered with a great depth of debris. These strata, which extend all along the south coast to Rocquaine Bay, seem to tend from NE to SW; and have various inclinations, but most generally 10° or 15°, dipping to the south. On the southern side of the island they are intersected by veins of white, flesh-coloured, and red felspar, of various breadths. In some places the felspar veins pass into granite. Veins of quartz, and veins of a granitel consisting of quartz and felspar, also traverse it. A few veins of trap are also found intersecting it at Rocquaine, which are occasionally superseded by trap porphyry, or by the same substance containing minute grains of quartz. In this tract there are wrought two or three quarries of a black granitel, consisting of hornblende and quartz, and very hard.

The constitution of this stone varies much through the extent which it traverses. In some places it is a true granite, in others the mica disappears, in others again this latter ingredient becomes so abundant, that the stone passes into micaceous schistus. Occasionally also hornblende enters into its composition, as has been noticed by others: when this is the case it sometimes loses the foliated structure and passes into sienite. Sometimes all the other ingredients are excluded, and felspar alone remains. There may