The next group to be dealt with is that of the igneous rocks. These are known as granites, syenites, porphyrys, etc.

Many of these rocks may be more clearly defined under the head of Plutonic rocks, having cooled under pressure at some depth from the surface of the earth, subsequent disturbance and upheavals forcing them to the surface as intrusive veins and masses.

Granites are made up of varying proportions of quartz, felspar, and mica in irregular grains or fragments. The quartz may be black, white, or grey. The felspar is of a crystalline nature, sometimes as potash felspar (orthoclase), and at others as soda felspar (oligoclase). The colours may be either yellow, pink, or white. The mica is usually of a grey colour, sometimes nearly black.

The proportions are more or less - felspar, 40 per cent.; quartz, 30 per cent.; and mica, 15 to 20 per cent. These quantities vary, increased proportions of quartz or mica being encountered in different types of the mineral.

A mineral somewhat akin to granite and termed syenite contains horneblende in place of quartz, together with potash felspar and mica; while most British granites contain all four constituents - quartz, felspar, horneblende and mica, and should more properly be called syenitic granites.

Gneiss

This is a species of granite, but with the component materials in layers or foliated.

The best known and most generally useful granites are from the following quarries -

Cheeswring, Cornwall

This is a porphyritic stone of a light grey colour and fine grain, useful for heavy structures where great strength and durability are necessary. Weight, 165 lbs. per cubic foot.

Hay Tor

A fine-grained stone of great durability. Blocks of any size can be obtained. Colour, blue or greyish blue. Weight, 165 lbs. per cubic foot.

Mount Sorel

A rough durable stone of great strength. Colour, pink. It is syenitic in composition. Weight, about 164 lbs. per cubic foot.

Shop, Cumberland

A fine crystalline stone of the porphyritic variety. Very tough and durable, and capable of a high polish. In colour red. Weight, 165 lbs. per cubic foot.

Rubislaw, Aberdeen

This is a fine-grained granite of the syenitic variety. It takes a high polish, is almost indestructible, and is applicable to the finest work. In colour it is a bluish grey. Can be had in large blocks or lengths for columns up to 25 feet. Weight, about 167 lbs. per cubic foot.

Peterhead

One of the finest granites in the British Isles, is most useful for ornamental and polished work, such as columns, pilasters, plinths, etc. Colour, a warm pink. Weight, 165 lbs. per cubic foot.

Isle Of Mull

This district furnishes a fine pink granite, to be had up to any size required. In composition it much resembles the Aberdeen granites. Weight, 164 lbs. per cubic foot.

Inverary, Argyleshire, supplies one of the hardest granites to be found. It is more of the nature of a porphyry than the Aberdeen varieties, and is much more difficult to work. Weight, 165 lbs. per cubic foot.

Ireland supplies several classes of good useful granites, some of them of a fine handsome type and pleasing colour. Amongst the best known quarries are those of Wicklow, Down, Carlow, Mayo, and Galway. The quarries at Dalkey, near Dublin, furnish a good stone. Weight, 169 lbs. per cubic foot.