This is a decomposed felspar one of the constituent minerals of granite which has accumulated in vast quantities in certain localities, having been no doubt washed down by rains from the weathered and exposed surface of granitic rocks. At one time the use of this substance was unknown in England, but now about 38,000 tons, worth about £50,000, are annually exported from the south of England for the Staffordshire potteries, and for the manufacture of mosaic tesseræ, buttons, artificial gems, and the like. The best pipe-clay is obtained from Poole in Dorsetshire, and the isle of Purbeck; it is employed in the manufacture of tobacco-pipes and fine pottery, and also sometimes used for the fulling or scouring of woollens.

Fullers' earth is a soft, dull, unctuous kind of clay, usually of a greenish-brown colour. It is found in various parts of Eng land, particularly in Surrey, Hampshire, and Bedfordshire, the lighter-coloured beds being the most esteemed. It is used in the fulling of cloth, from its property a property common to all soft aluminous minerals-of absorbing oil and grease. At one time it was deemed of so much importance to the national trade in woollen, that its exportation was prohibited; but now soap is chiefly used instead, and fullers' clay has fallen in importance. What the present consumption may be, it is impossible to say but about forty years ago not less than 7000 tons were annually made use of.

Ochre.

This is a painter's term for a native earthy mixture of alumina, silica, and oxide of iron. It is found of various hues, but chiefly of a yellow or reddish-brown, and is employed as an ingredient in painters' colours, and in the polishing of metal articles. It is obtained from various places, particularly from Shotover Hill, near Oxford; from the coal-measures of the east of Fife; and from: Italy. The quantity raised in Britain is unknown, but about 5000 hundredweights are said to be annually imported.

Clay-Slate.

Clay-slate, of which roofing and writing-slate are the most familiar examples, is very extensively diffused, and as extensively made use of in the British islands. Clay-slate belongs to one of the lowest or oldest formations, is essentially composed of alumina and silex, has a peculiarly laminated or fissile structure, and is usually of a dark lustrous blue, bluish-green, or purplish colour. The principal quarries are in Wales, where they give employment to nearly 5000 hands; in the north of England and west of Scotland; the most extensive being in Caermarthen near Bangor, in Borrowdale in Cumberland, and at Easdale and Ballachulish in Argyleshire. The beds of clay-slate are often of

great thickness, but only certain portions are sufficiently compact to be of commercial importance. The principal consumer of this material is the slater, though considerable quantities are also used as pavement in cellars and warehouses, for shelves in dairies, and the like. The finer-grained varieties are polished for school-slates; and those of attractive colours are now manufactured into flower-pots, vases, fancy-tables, and other ornamental objects.

SILICEOUS SUBSTANCES.

Silex or silica is one of the most important and most generally diffused of the mineral ingredients that enter into the composition of the rocky crust of the globe. Rock-crystal, quartz, chalcedony, and flint, may be regarded as nearly pure silica; and all the varieties of sandstone, quartz-rock, and granite, are in a great measure composed of it-many sandstones, for example, being pure granular quartz or silica, with a slight argillaceous

cement.

Quartz and quartz-rock, though of importance as forming the bases of other rocks, are of themselves of no great commercial value. The purer varieties of rock-crystal are occasionally cut as ornamental stones; and of late, the transparent and colourless varieties have been pretty generally adopted by opticians as spectacle lenses. Their extreme hardness renders them more durable than glass, and less liable to be scratched, while they are altogether cooler and more agreeable. The so-called Brazilian pebble, used for this purpose, is of pure silica, and is sometimes found in crystals as large as a cocoa-nut.

Flint.

The common nodular flints found in the chalk-formation are nearly pure silica, exhibiting but a trace of alumina, oxide of iron, and lime. The formation of flint within a mass so different in composition as chalk, is still, in some respects, an unsolved problem in geology. It occurs in nodular masses of very irregular forms and of variable magnitude, some of these not exceeding an inch, others more than a yard in circumference. Although thickly distributed in horizontal layers, they are never in contact with each other, each nodule being completely enveloped by the chalk. Externally, they are composed of a white cherty crust; internally, they are of gray or black silex, and often contain cavities lined with chalcedony and crystallised quartz. When taken from the quarry, they are brittle, and full of moisture, but soon dry, and assume their well-known hard and refractory qualities. Flints, almost without exception, enclose remains of sponges, alcyonia, echinida, and other marine organisms, the structures of which are often preserved in the most

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delicate and beautiful manner. From these facts, it would seem that flints are simply an aggregation of silex around some organic nucleus, the same as ironstone nodules or septaria are aggregations of clay and carbonate of iron. The uses of flint are various: calcined and ground to a powder, it is used in the manufacture of the finer sorts of pottery; it also enters into the composition of flint-glass; and before the invention of the percussion-cap, gun-flints were in universal use. Flints also form excellent building materials, because they give a firm hold to the mortar by their irregularly rough surfaces, and resist, by their hardness, every vicissitude of weather. The counties of Kent, Essex, Suffolk, and Norfolk, according to Dr Ure, contain many substantial specimens of flint masonry.

Sandstones.

Sandstone, or freestone, as it is sometimes called, occurs in innumerable varieties, differing in colour, in composition, fineness of grain, and compactness. Thus we have some red, from the presence of iron oxide; some silvery and glistening, from the presence of minute scales of mica; others white, yellow, and mottled; and some almost jet-black, from the presence of bituminous or carbonaceous matter. As to mineral composition, there is no other class of rocks so varied; for though quartz grains give to them their family character, clay, lime, mica, carbon, iron, and the like, mingle with them so capriciously, that it is impossible to find any two strata of sandstone exactly of the same composition. Again, their texture is equally if not still more varied; in some the grains being as large as peas, in others quite impalpable; some being so soft and friable, as to be rubbed down by the hand, and others so hard and compact, that nothing but the chisel of the stone-cutter can touch them. The principal use of sandstone is in building, and for this purpose good durable strata are found in almost every formation, from the greywacke up to the recent tertiaries. In England, where bricks form the more available material for the construction of houses, there are comparatively few freestone quarries of much importance. Those of Portland Isle, which have furnished the stone for St Paul's and other public buildings in London, those of Bath, and of Gateshead Fell, near Newcastle, are the most extensive and valuable. In Scotland, freestone of excellent quality is to be found in most localities, and consequently it is the prevailing architectural material. The best strata are those underlying the coal-formation-such as are quarried in the neighbourhood of Glasgow, near Linlithgow, in the neighbourhood of Edinburgh, and in several parts of Fifeshire. The blocks from the quarries of Craigleith, Granton, Cullelo, &c. which all belong to the same suite of strata, almost rival marble in their whiteness, compactness, and durability. The principal buildings of the New Town of Edinburgh are

constructed of this material, and certainly no city in the world can boast of similar erections. Good building sandstone is also obtained from the old red formation, such as is quarried at Kingoodie and other places near Dundee, the rock being at once exceedingly durable, and producing blocks of any dimensions.

Many sandstones are likewise used as pavement, those being sought for that purpose which are at once compact and thinbedded or schistose. By far the most valuable of this kind are the Forfarshire gray micaceous flagstones, now so generally employed as foot-pavement in all our large towns. A very extensive trade in these is carried on at Arbroath and Montrose, the flagstones being now squared and dressed by machinery at the quarries. Another excellent material, still more durable, but exceedingly hard and refractory, is also obtained from Caithness, which, when well laid down, appears to the unpractised eye more like plates of cast-iron than slabs of stone. Pavement of average quality is likewise obtained from the coal-measures, but being of a softer and more absorbent texture, is not so well adapted for out-door purposes. All these beds are highly fissile or schistose, occurring in laminæ or layers of from one to fourteen inches in thickness; and thus accounts for the fact, that at one time the thinner sorts were used for roofing, under the name of tile-stones or gray-slate.

Besides building and paving, several sorts of sandstone are employed for grindstones, millstones, whetstones, and the like. Thus the quarries of Gateshead Fell, near Newcastle, situated on the millstone grit, or quartzose sandstones of the lower coalmeasures, furnish the grindstones known in all parts of the world as "Newcastle grindstones." Good millstone and whetstone beds are found in various other places, as are also varieties fit for the wheels of glass-cutters and cutlers. The stones chiefly used in Sheffield are procured at Wickersley in Yorkshire. The celebrated burr millstones of France are obtained from the upper fresh-water siliceous limestones of the Paris basin, and are not strictly sandstones in the usual acceptation of that term.

Sand.

On narrowly inspecting the immense masses of sand borne down by our rivers, piled up along our shores, or scattered in dunes and strata over the surface of the country, it will be found that the great bulk of it is composed of siliceous particles, evidently derived from decomposed quartz-rock, granite, sandstone, and the like. As might be expected, most sands are mingled with clay, lime, and other earthy impurities; and it is according to their siliceous character, and degree of purity from earthy ingredients, that they become of value in the arts. Thus sharp, well-sifted sand is an indispensable ingredient in well-prepared mortar, without which the builder, the plasterer, and fresco-painter could not proceed a single step: the commoner

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sorts are widely used in paving, in the construction of ovens, kilns, annealing furnaces, and the like, where heat is wished to be retained; and some peculiar varieties are much used in the preparation of moulds for the casting of iron, brass, and other metals. Good siliceous sand is an indispensable ingredient in all sorts of glass, now one of the most important manufactures in the civilised world. The most valuable sands for this purpose are those of Aumont, near Senlis, in France, and those of the Isle of Wight, and of Lynn in Norfolk, in England; though of course each glass-making country possesses sands fit for the same uses if properly washed and sifted.

Granitic Rocks.

This term may be considered as embracing not only the true igneous granite, but the gneissose and mica-slate rocks which, though stratified, partake of the same mineral character, and are usually associated with it. In all of them silica is a predominant ingredient, imparting those hard and durable qualities which render them of economical importance. Ordinary granite is a crystalline compound of quartz, felspar, and mica; but other minerals, such as hornblende, hypersthene, &c. occasionally mingle with it, thus producing a number of varieties. The small-grained grayish granite of Aberdeen is essentially a compound of quartz, felspar, and mica; that of Peterhead is the same compound, rendered red by the oxide of iron contained in the felspar crystals. Granitic compounds are very widely distributed, forming the fundamental rock of our principal mountain chains. The Grampians in Scotland, the Cumberland and Cornish hills in England, the Wicklow mountains in Ireland, the Alps in Switzerland, the Pyrenees in Spain, the Dovrefelds in Norway, the Ural in Russia, the Abyssinian and other African ranges, and the Andes in South America, are all less or more composed of rocks partaking of a granitic character.

The economical uses to which granitic rocks are applied are by no means unimportant. Compact granite, from its extreme hardness, is largely employed in the construction of docks, piers, lighthouse foundations, bridges, and other structures where durability is the main object in view. Waterloo Bridge in London, the Liverpool and other English docks, are built of granite. It is the ordinary building stone in Aberdeen, and is largely used in the metropolis for paving. The Pyramids, though internally constructed of limestone, are externally coated with granite. Pompey's Pillar, and other ancient Egyptian structures, are com posed of it; the column of Alexander, and the pedestal of the colossal statue of Peter the Great, in the Russian capital, as well as several monumental monolithes in other countries, are also of granite. Within these few years the granite of Aberdeenshire has been brought into use as an ornamental stone; and machinery has been erected, we believe, both at Aberdeen and Peterhead,