Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/23

 Rh red and green felspar, which are found at Castle Cornet. The same highly coloured felspars are also occasionally intermixed so as to form a constituent part of the granite, which thus becomes exceedingly beautiful.

Among the various pebbles which I picked up on the beach, I observed a black siliceous schistus and hornblende slate, but from whence they had been detached I could not discover: the latter however is known to be common in gneiss as well as in granite rocks.

At Grande Rocque are large masses of sienite, which are quarried to form building stones. It is the only sock of this nature on the island, and its produce is fully equal in beauty to that of the celebrated quarries of Mont Mado in Jersey, although it cannot be raised in such large masses. The felspar is the predominant ingredient, and it is either white or flesh-coloured. It is traversed by veins of a similarly constituted stone, but more minutely compacted and of a brick red colour. In some places indeed the veins seem to consist of a felspar, with grains of quartz; and hornblende imbedded, approaching in its nature to a petunse porphyry. It is here an universal rule that where the granite are traversed by veins of a similar nature, the vein is the most compact of the two. As the hornblende is sometimes wanting in these stones, and as mica is sometimes present, we meet with many other granitic varieties. I observed in one place lumps of argillaceous porphyry stuck in granite, as has been noticed by Baron Born.

The predominant rock towards the bay of St. Sampson's, is a grey or black granitel, consisting of quartz and hornblende mixed in various proportions. Detached masses of this rock are also found in the higher grounds, as well as among the gneiss of the southern coast.