The equally well-known freestone called Bath Stone has a more decided oolitic structure, is softer, and of a richer colour than Portland Stone. Bath Stone, whether from the immediate neighbourhood of Bath, or from other parts of Somersetshire, is from the Great Oolite, one of the English Lower Oolites. It is much used for doorways and windows in conjunction with brickwork and rough stone work, but in the west of England it is the chief stone used for dwelling-houses, public buildings and churches, and the beautiful city of Bath may be said to be entirely built of the stone bearing its name.

Another of the Lower Oolites, the Inferior Oolite, also furnishes an excellent stone. This has been largely quarried at Leckhampton, Doulting and Dundry, and the town of Cheltenham and the Cathedral of Wells excellently display its architectural capabilities. In some localities, as at Ham Hill and Sherborne, the stone of the Inferior Oolite has a very warm and rich colour, as seen in the buildings of Yeovil and the surrounding country.

The Lower Oolites, in addition, yield a hard, shell limestone called Forest Marble, and two thin-bedded fissile limestones which have been named, from their localities, Stonesfield Slate and Collyweston Slate.

Although the limestones of the Lias are not worked for building stones, they furnish valuable building materials; the Marlstone of the middle Lias, and the "Blue Lias" of the Lower Lias, making excellent lime, that of the latter forming hydraulic cement.

The Middle Oolites contain the Coral Rag, which has been, for five hundred years, largely used as a building stone, and is still extensively quarried.

The Purbecks at the top of the Upper Oolites furnished the builders of the middle ages with a highly valued

stone, for it is largely found in the ecclesiastical edifices. of the "Early English" period, the finest illustrations being in Salisbury Cathedral, and in London, in the Temple Church, where not only the slender shafts, but the recumbent figures of the Knight Templars are of

Purbeck marble.

The newer Cretaceous rocks also furnish largely used limestones. The Kentish Rag of the Lower Greensand, the Totternhoe stone, the Merstham and Godstone firestone, and the harder beds of the Lower Chalk, are all well known as building stones. Some of these were used in our most ancient buildings, as St. Albans Abbey, Westminster Abbey, the Tower of London, and Windsor Castle. While the soft Upper Chalk itself has for centuries been washed for whiting, and burnt for lime; the flints it contains have, apart from other uses, been extensively employed, both as building and as road material.

The still newer Tertiaries furnish, in the Isle of Wight, a fine white freshwater limestone, that was worked so anciently as the period of the building of Winchester Cathedral, for which it was largely used.

The Palæozoic rocks contain some remarkable limestones. From the Permians the magnesian limestone, or dolomite, is obtained. Of this stone the Houses of Parliament were built, and although its success under the atmospheric conditions of London is not great, yet in Yorkshire some of the oldest buildings show it as a highly durable material.

The Carboniferous limestones are chiefly worked for fluxes or lime burning, but they yield some very beautiful so-called marbles, as the encrinital and shelly marbles of Derbyshire, the black and variegated marbles of Ireland,

the Mona Marble of Anglesea, and the marble of the Isle of Man used for the black steps of St. Paul's.

The Devonian rocks are famed for their limestones, giving the beautiful Devonian marbles, now so much used for interior architectural decoration, and the Plymouth limestone of which the great breakwater in Plymouth Sound is built.

Though the Silurian limestones are not used as building stones, they are of high value for the other economic purposes to which limestone is applied.

Both the Silurian and the still older Cambrian rock systems yield the very important building material, slate. This is largely worked in North Wales, and the quarries there are of great size. Slate is a changed or metamorphosed clay, the alteration having been produced by, amongst other influences, great lateral pressure which has induced a new structure giving a cleavage across, and not parallel with, the plane of original deposition. The finest slates split into very thin plates as may be seen in the Museum of Practical Geology, Jermyn Street.

Some of the non-stratified rocks, igneous and metamorphic, also consist of most valuable stone. Granites are every year becoming more extensively used both for plain and ornamental work. When polished, from their beauty, variety and durability, their suitability for exterior decoration is pre-eminent, and when unpolished they are admirably adapted for bold and massive works. Granite in England is abundant in Cornwall, Devonshire, Cumberland, and Leicestershire, in many localities in Ireland and Scotland, and in Lundy Island and the Channel Islands. The colour of granite chiefly depends on that of the felspar it contains, which, with quartz and mica, constitute ordinary granite. Granite containing hornblende is exceedingly tough and durable, and hence

the hornblende granites of Mount Sorrel, in Leicestershire, are very largely used for road metal. The ancient granite monuments of Egypt, of which Cleopatra's Needle on the Victoria Embankment is one, are of this stone. The pink granite of Aberdeenshire has recently been greatly used for polished work, and the richly coloured porphyritic Shap granite is becoming more widely known, but the very handsome Luxullianite, also porphyritic, is seldom seen. From this last named granite the sarcophagus for the body of the Duke of Wellington, in the crypt of St. Paul's, was made. Other valuable ornamental stones from this class of rocks, some true porphyries, are obtained from British localities.

Besides those from the granitic rocks, Cornwall furnishes a very beautiful stone in the Serpentine of the Lizard. This rock, although much resembling marble in appearance, is of a quite different composition, being a silicate of magnesia and entirely devoid of lime. It is much used for interior architectural adornment as well as for ornamental objects.

Many other stones from British rocks well adapted for building, for ornament, and for road material, might be enumerated, since, perhaps, no part of the world of equal area contains such varied rocks as the British Islands.

J. LOGAN LOBLEY.

CHAPTER LXXXIV

CLAYS.

BY PROFESSOR J. LOGAN LOBLEY, F.G.S.,

Professor of Physiography and Astronomy, City of London College; Lecturer on Geology, Crystal Palace School of Engineering; Vice-President of the City of London College of Science Society; Member of the General Committee of the British Association; Member of the Geologists' Association; Hon. Member Hamp. F. C.; etc. Author of "The Study of Geology," Geology for All," "Mount Vesuvius," Hampstead Hill," "The Inter-relations of the Field Naturalist's Knowledge," "The Cretaceous Rocks of England," "The Causes of Volcanic Action," "The Origin of Gold," etc.

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CLAYS underlie the soil of a large proportion of agricultural land, and though possessing the prominent main characteristic of more or less plasticity with water in common, they vary very considerably, so that amongst the numerous varieties that are distinguishable, while some are little used for any economic purpose, others have a high commercial value for fictile manufactures, and one for a textile industry.

The recent development of the terra-cotta manufacture, and its extensive architectural employment as a building material, give additional value and interest to many clays that were hitherto only regarded as being suitable for bricks or tiles. Since the general disuse of stucco, too, ornamental and high-class brickwork has largely increased, and fine bricks of good colour are consequently in much greater request. The present is, therefore, a most favourable time for landowners to work the clays on their estates with profit.