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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.

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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 (umianthus, undefiled) and asbestos (asbestos, unconsumable). Cloth of this kind was used by the ancients to wrap the bodies

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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.

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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. Obsidiannamed, 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.

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Tripoli, &c. We include under this head several siliceous earths and slates extensively employed in the polishing of metallic surfaces. The most familiar of these are tripoli (so called from Tripoli in Barbary, whence it was originally procured), polishing-slate, semiopal, and some of the porcelain earths. The uses of these substances are well known: it is their peculiar origin that confers on them an especial scientific value and interest. It has been established by Ehrenberg that these, and several other rocky masses, are not the results of ordinary deposition, but an aggregation of the siliceous shells of the minutest animalcules. This is a curious fact: the remains of creatures individually invisible to the naked eye forming rocks which, in the course of time, were to figure in the economical applications of the human race!

SALINE SUBSTANCES.

Under this section we comprehend such products as rock-salt, alum, saltpetre, and the like, which are found either as native salts, or are procured by artificial processes from certain earthy, substances with which they are combined in nature. Some of these salts are of vast economical importance, and appear to be as indispensable to the progress of civilised life as either coal or iron.

Rock-Salt.

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The common culinary salt of every-day use is chemically a muriate of soda, or, more strictly, a chloride of sodium, every hundred parts of which are composed of sixty chlorine and forty soda. It exists abundantly in sea-water, constituting more than a thirtieth part of its weight; it is discharged by salt or brinesprings--which arise from different geological formations, and are situated in different countries—to the extent of from 20 to 30 per cent. ; and it is found in various degrees of purity in beds and irregular masses, from 20 or 30 to more than 120 feet in thick

Native chloride of sodium is never found in a state of absolute purity, but is always less or more combined with certain salts of lime, magnesia, soda, iron, and alumina; to free it from these impurities, and render it fit for culinary purposes, is the duty of the salt-boiler and refiner. At one time salt was largely, and is still some extent, derived by evaporation from sea-water, which, being exposed in large flats to the sun, or in shallow pans to the action of heat, and subjected to certain clarifying processes, produced the coarse-grained varieties commonly known as bay-salt. This process is now all but abandoned in Britain, and is only had recourse to in some southern and tropical countries, where the arts of life are still in a rude and primitive condition. Subsequently the article was obtained from brine-springs, such as those of Droitwich in Worcestershire; and

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still more recently from the mineral rock-salt, which abounds in the new red sandstone and upper secondary formations. This important mineral product occurs in Cheshire and Worcester in England, at Altemonte in Calabria, Halle in the Tyrol, Cardona in the Pyrenees, Wieliczka in Poland, and in several districts of North America. As brine-springs always issue from saliferous deposits, and are doubtlessly derived from the solution of the solid masses by subterranean waters, we shall restrict our description to the solid rock-salt, taking the mines of Cheshire as a sufficiently illustrative example. These mines, together with the brine-pits of Worcester, not only supply sufficient salt for the consumption of almost the whole of Britain, but furnish, besides, an article of export to the extent perhaps of two millions of tons.

It has been stated that the chief deposits of English rock-salt are confined to the new red sandstone formation, where it alternates with its argillaceous and gypseous marls.

“ In Cheshire,” says Professor Ansted, “the rock occurs in large quantities in the condition of an impure muriate of soda, and associated with a peculiar marl; it is sometimes massive, and sometimes existing in large cubical crystals; and the beds containing it usually alternate with considerable quantities of gypsum, although this latter mineral is not worked to profit. The appearance of the rock-salt is by no means of that brilliant character, nor has it the delicate transparency and bright reflecting surface, which the reader may perhaps suppose characteristic of it. It is usually of a dull red tint, and associated with red and palish-green marls ; but it is still not without many features of great interest; and when lighted up with numerous candles, the vast subterranean halls that have been excavated present an appearance richly repaying any trouble that may have been incurred in siting them. At Nantwich, and the other places in Cheshire where the salt is worked, the beds containing it are reached at a depth of from 50 to 150 yards below the surface. The number of saliferous beds in the district is five; the thinnest of them being only six inches, but the thickest nearly forty feet; and a considerable quantity of salt is also mixed with the marls associated with the purer beds. The method of working the thick beds is not much unlike that of mining the thicker seams of coal. The roof, however, being more tough, and not so liable to fall, and the noxious gaseswith the exception of carbonic acid gas—totally absent, the works are more simple, and are far more pleasant to visit. Large pillars of various dimensions are left to support the roof at irregular intervals, but these bear only a small ratio to the portion of the bed excavated, and rather add to the picturesque effect in relieving the deep shadows, and giving the eye an object on which to rest. The intervening portions are loosened from the rock by blasting; and it may be readily understood that the effect of the explosions heard from time to time, and re-echoing through the wide spaces, and from the distant walls of rock, give a grandeur and impressiveness to the scene not often surpassed. The great charm, indeed, on the occasion of a visit to these mines, even when they are illuminated by thousands of lights, is chiefly owing to the gloomy and cavernous appearance, the dim endless perspective, broken by the numerous pillars, and the lights, half-disclosing and half-concealing the deep recesses which are formed and terminated by these monstrous and solid projections. The descent to the mines is by a shaft used for the general purposes of drainage, ventilation, and lifting the miners and produce of the mine. The shafts are of large size in the more important works, and the excavations very considerable, the part of the bed excavated being in some cases as much as several acres. Over this great space the roof, which is twenty feet above the floor, is supported by pillars, which are not less than fifteen feet, thick. The Wilton mine, one of the largest of them, is worked 330 feet below the surface; and from it, and one or two {of the adjacent mines, upwards of 60,000 tons of rock-salt are annually obtained, two-thirds of which are immediately exported, and the rest is dissolved in water, and afterwards reduced to a crystalline state by evaporating the solution.” The modes of working rock-salt are much the same in all countries; while the fineness and purity of the manufactured material depends - upon the rapidity with which the brine is evaporated, and the nature of the clarifying agents employed.

The formation of rock-salt is a subject which has much engaged the attention of speculative geologists. The sandstones and marls with which it is associated are evidently derived from deposition in water; but the irregularity of the salt beds, the fact of their occurring in masses of vast thickness, and the soluble nature of the compound, all point to a somewhat different -origin. At present, salt lakes and superficial accumulations of salt occur in various parts of the world, and these have furnished data for reasoning as to the saliferous deposits of earlier eras. Salt lakes are chiefly, derived from salt springs, and, being subjected to the vapourising influence of the sun, which carries off only fresh vapour, their waters become in time saturated with 1 saline matter. But water can hold only a fixed amount of salt in solution, and so soon as this amount is attained, the salt begins to fall to the bottom by its own gravity. In the course of ages these layers will form a thick bed, interstratified, it may be, with mud, or other earthy sediment; and should the lake be ultimately dried up, the salt will constitute a deposit something analogous to the rock-salt of the new red sandstone. Such is the process which some geologists have advanced to account for the formation of rock-salt--supposing that portions of the irseas of deposit were occasionally cut off by igneous disturbances from connexion with the main ocean, and subjected to a

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