—This should have more felspar than quartz, and have as little mica as possible; the closer the grain the better. Coarse-grained granites soon decompose.
Trappean Rocks.
—Some of these are excellent for road metal. Basalts of dark colour and close grain should be selected. Greenstones with similar characteristics are good; as is also Whinstone.
Gneiss.
—Is inferior to granite; it has mica in layers and is not a good road metal.
Clay Slates.
—These are useless, as they crumble on exposure or degenerate into mud.
Limestone.
—The Metamorphic, Silurian, and Carboniferous limestones may be used if crystalline in appearance, but the Lias and Oolitic are of little use.[28]