Greenstone, also called Trap or Whinstone, is a mixture of felspar and hornblende.
It has sometimes a granular crystalline structure, and at other times it is very compact without apparent grains.
It is extremely hard and tough, and makes capital road metal - is very often split up by joints, so that it is well suited for paving setts, but not for large blocks. Its colour is too sombre for the walls of houses.
Some of the stratified varieties are dangerous as building stones, being liable to decomposition on exposure to the weather, even where there is no frost.
Penmaenmawr Stone from N. Wales is largely used throughout the country for paving setts. It is very easily split by cutting a fine line with an axe in the direction required, and then giving the stone a few smart taps with a hammer.2
Bardon Hill Stone from Leicestershire is also much used for road metal in the central counties.
Stone of this description is also found in Cornwall, near Edinburgh, in Argyleshire, at Carlin Knowse and other places in Fifeshire, and also in County Wexford.
Whinstone is found in Wigtownshire, near Selkirk, in Kincardineshire, near Haddington, near Edinburgh, at Falkirk, in Perthshire, Fifeshire, Inverness-shire, Ross-shire, and other places in Scotland.
Basalt resembles greenstone, but is composed of lime felspar, augite,3 olivine, and titano-ferrite.4
1 Dana and Wray. 2 Seddon.
3 Black and greenish-black crystals of anhydrous silicate of magnesia
4 Titanic iron.
"It varies in colour from greyish to black. In the lighter coloured felspar predominates; in the darker iron or a ferruginous augite."1 It is often of a dark green.
This stone affords a great resistance to crushing, and is eminently adapted for paving curbs, etc.
This material is found also in the counties of Armagh, Antrim, and Londonderry.