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 for mation, 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 conse quently 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 66 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

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 composed 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,

for the purpose of polishing it like marble, to which many prefer it, for chimney slabs, vases, pedestals, pillars, &c. When uniform and compact in grain, it is susceptible of a very high polish, and has this advantage over marble, that it is not easily stained or scratched, nor at all acted upon by acids.

Serpentine, or the granitic rock generally so called, is one of very varied composition and quality. The noble serpentine of the mineralogist is a green translucent rock, rather soft, but susceptible of a good polish; and if found in sufficiently large blocks, would make not a bad substitute for marble. We have before us a specimen of a beautiful leek-green variety from New Zealand, where it is said to occur eight or ten feet thick, and capable of being raised in blocks of any size. Should this be the case, the houses of our brethren who have made these islands their adopted home, need be in no lack of interior decorations. Potstone, the lapis ollaris of the ancients, is another granitic product, easily worked into form, and formerly used for culinary vessels; whence its common designation.

Mica-Talc-Asbestos.

Mica, talc, asbestos, and other kindred minerals which are the products of the granitic and primary rocks, may be appropriately considered in this place. The silvery-looking, scaly substance which occurs in ordinary granite is mica, so called from its glistening aspect. It is sometimes found in crystals more than a foot square, and when of this size, is split into thin plates, and, from its transparency, used in certain cases as a substitute for glass. It stands a higher degree of heat, without splintering, than glass, and is well adapted for ship-lights, not being liable to fracture during the firing of cannon. The large sheets exposed for sale by the mineral-dealers are generally brought from Siberia; hence the term Siberian glass.-Talc, when crystallised, has much the same appearance, but on trial will be found to be less transparent, softer, and non-elastic. The larger crystals are sometimes applied to the same purposes as mica, but the principal use of the mineral is in porcelain paste, and in polishing alabaster figures. It is also said to be an ingredient in rouge for the toilet, having the property of communicating softness to the skin. Talc-slate, the other form in which this mineral occurs, is a massive mineral, breaking up in tabular fragments; it has a white streak, and greasy or soapy feel. It is employed in the porcelain and crayon manufactures, and is used as a marking material by carpenters, tailors, and others.-Asbestos or amianthus is a soft mineral, occurring in separate filaments of a silky lustre, and consisting essentially of silica, magnesia, and lime. When steeped in oil, it may be woven into cloth, which is incombustible, and may therefore be purified by fire; hence the terms amianthus (amianthus, undefiled) and asbestos (asbestos, unconsumable). Cloth of this kind was used by the ancients to wrap the bodies

of the dead about to be burned, to prevent their ashes being mixed with those of the funeral pile. In the United States of America asbestos is sometimes used as a lamp wick.

Basaltic Rock.

Under this head we include all the basalts, greenstones, whinstones, and traps which make up the sum of the igneous rocks of the secondary formations. They are essentially siliceous-quartz, hornblende, hypersthene, augite, and so forth, entering largely into their composition. Some of the basalts and greenstones dress well under the hammer, and though of a dingy colour, make an excellent building stone, their durability being equal to that of granite itself. Ordinary greenstone or whinstone is a very valuable rock in many districts of Scotland, where it furnishes material at once for houses, fences, drains, and roads. Indeed no rock is better adapted, or more extensively used, for causewaying, and for macadamised roads it is unrivalled. Large quantities are, or at least used to be, shipped from the Firth of Forth for the kerbstones and causeways of the streets of London. We have seen some ornamental pedestals in basalt which took on a pretty fair polish; and an elaborately-carved Bhuddist idol, of considerable size, now in the museum at St Andrews, is of the same material. Some of the trap-rocks stand fire to perfection, and this has suggested their use as oven-soles, where such varieties can be procured.

Volcanic Products.

The mineral products ejected from volcanoes are chiefly lava, obsidian, pumice, scoriæ, and a light impalpable dust, in all of which silica and alumina are the main ingredients. Some of the compacter sorts of lava are hardly to be distinguished from the trap-rocks of the secondary formations, and may consequently be employed for the same economical purposes. Obsidian-so named, according to Pliny, from one Obsidius, who first brought it from Ethiopia-is a true volcanic glass, of various colours, but usually black, and nearly opaque. In Mexico and Peru it is occasionally fashioned into adzes, hatchets, and other cutting instruments, or into ring-stones. So closely does it resemble the slag of our glass furnaces, that in hand specimens it is almost impossible to distinguish the natural from the artificial product. It consists chemically of silica and alumina, with a little potash and oxide of iron. Pumice, a well-known volcanic product, is extremely light and porous, and of a fibrous texture; it is harsh to the touch, is usually of a grayish colour, and has a shining pearly lustre. Like obsidian, it is principally composed of silica and alumina, with traces of potash, soda, and oxide of iron. Pumice is quarried and exported in large quantities from the Lipari and Ponza islands, off the coast of Sicily. It is used for polishing metals and other purposes in the arts.