These rocks are, however, seldom used for building or engineering works, except in the immediate neighbourhood of the place where they are found.
The Porphyries "generally occur as dykes and eruptive masses intersecting the older schists and slabs, and are usually much fissured and jointed, and for this reason incapable of being raised in massive monoliths like the granite."1
There are two principal varieties found in Great Britain. Each consists of a general mass or base, through which are scattered crystals varying in size from small grains to 3/4 inch in length. The stone breaks with a smooth surface and conchoidal fracture.
Felstone Porphyry consists of a base, which is an intimate mixture of quartz and orthoclase, known as Felsite, with independent crystals of felspar.
Quartziferous Porphyry has a base consisting of a granular crystalline compound of quartz and felspar, with individual crystals of felspar and quartz.
"Both varieties appear in many tints - red, flesh-coloured, fawn-coloured, black, bluish-black, and bluish-green; and both varieties may contain, in subordinate quantities, other crystals than those enumerated above.1
"Incapable of being raised in large blocks, they are polished only for minor ornaments; their principal use in Britain being for causeway-stones and road metal, for which their hardness and toughness render them specially suitable.
"Though chiefly used for road material, in some districts they are employed in the building of country mansions, farm sheds, and walls; and when properly dressed and coursed make a very fair structure (especially the fawn-coloured sorts), and are perfectly indestructible."1
Some of the darker varieties are too sombre for building purposes, except when used for ornamental purposes to relieve surfaces of lighter stone.
Elvan (a term originally peculiar to Cornwall and Devon) is found in dykes or veins traversing the granite or slate; the dykes varying in width from a few feet to 300 or 400.
It usually differs from granite in the absence of mica and in the fineness of its grain. It sometimes contains schist.
"It is much used as a building stone in Cornwall, and is found to be very durable,"2 also as road material in competition with Guernsey granite.
Stone locally known as Elvan is also met with in County Wexford.
Gneiss is composed of the same constituents as granite, but the mica is more in layers, and the rock has therefore a stratified appearance.
The rock splits along the layers with facility, and breaks out in slabs from a few inches to a foot in thickness.
Mica Schist, sometimes called Mica Slate, is composed chiefly of mica and quartz in thin layers: the mica sometimes appears to constitute the whole mass.
Its colour is grey or silvery grey, and it has a shining surface, owing to the quantity of mica present.
Hornblende Schist, or Hornblende Slate, is usually black, composed principally of hornblende, with a variable quantity of felspar, and sometimes grains of quartz.
It resembles mica schist, but has not so glistening a lustre, and seldom breaks into thin slabs. It is tougher than mica schist, and is an excellent material for flagging.1