Jet, of which necklaces, ear-rings, and other ornaments are made, is but a variety of coal, as common in its origin and nature as that which we pile on our fires. It is occasionally found in the lignite beds of England, but principally in Germany and Prussia, where it occurs associated with amber, which is regarded as a fossil gum, while jet seems to be the trunk and branches of trees more completely bituminised and freer from earthy impurities than cannel or other coals.-Amber, a wellknown yellow resin-like substance, is believed, as stated, to be a fossil gum or resin; and its connexion with deposits of lignite seems to confirm that opinion. It is solid, brittle, commonly transparent, and when rubbed, becomes electrical. It is found in various countries, more particularly on the Adriatic and Sicilian shores; on the Baltic, between Memel and Dantzic, where there are regular mines of it; and in Japan, Madagascar, and the Philippine Islands. It is used chiefly in the manufacture of beads and necklaces, and in the preparation of varnishes. The largest known specimen of amber was found near the surface of the ground in Lithuania, about twelve miles from the Baltic: it weighs eighteen pounds, and is in the royal cabinet at Berlin. Other curious specimens have been detected enclosing insects, and even drops of water-these apparently having been enclosed when the gum was exuding in a fluid state from the living tree.

Naphtha-Petroleum-Asphalte.

Naphtha, petroleum, mineral pitch, and asphalte, may in a great measure be regarded as one and the same substance in different degrees of concentration and purity. Thus naphtha, on exposure to the air, soon loses its limpid appearance, and passes into petroleum; and petroleum, under similar treatment, shrinks into a viscous slaggy state, undistinguishable from mineral pitch. Natural naphtha is a limpid, or but slightly-coloured bitumen, highly inflammable, and of a strong bituminous, but not disagreeable odour. It is found at Baku on the Caspian, at Hit on the Euphrates, and at other places in Mesopotamia; it occurs abundantly in the lower districts of the Birman empire; is found at various places in the north of Italy, as Piacenza, Modena, &c.; and in some districts of North America. It generally exudes from fissures in the rocky strata, or is collected in shallow wells, dug in the clays and shales where it occurs. A similar liquid can be obtained by distilling petroleum, coal-tar, and other bitumens; but the artificial product has a more penetrating and unpleasant odour. Naphtha has the property of dissolving most of the essential oils and resins, and is at present largely used as a solvent of caoutchouc. It is also used for lamps; and the cities of Parma and Genoa are said to be lighted with the produce of the wells in the duchies of Modena and Parma,

Petroleum, or rock-oil, is another liquid bitumen, of a brownish colour and variable consistency, and yielding a strong disagreeable odour. It is found exuding from various secondary strata, but chiefly in coal districts, where it is evidently a product of that formation. It occurs in small quantities in various localities of Britain, but abundantly in other countries of Europe, in Persia, the Birman empire, in Texas, and in the islands of Trinidad and Barbadoes. On exposure to the air, petroleum thickens, and assumes a darker hue, in which state it is generally known by the name of mineral pitch, or Barbadoes tar. On further exposure, and especially when mingled with earthy impurities, it passes into a solid state, then becoming the common asphalte or bitumen of commerce. In its ordinary liquid state it is burned for light; worked into balls with earth and gravel, it is used in eastern countries as fuel; and mingled with grease, it is occasionally employed as a substitute for tar in coating vessels.

Asphalte, so called from its adhesive nature, differs from mineral pitch in being solid and brittle at the ordinary state of the atmosphere. It melts easily, and is highly inflammable, leaving, when pure, little or no ash after combustion. It is found in most of the localities where petroleum springs occur, being nothing more than their accumulated produce. The chief supplies are obtained from the shores of the Dead Sea, from Barbadoes, from Trinidad, where it occupies a basin or lake about three miles in circumference, and from Clermont, Seyssel, and Bourg in France, where it occurs in limestone and calcareous shales. Asphalte was employed by the ancients in some of their cements, and also in the process of embalming. It is now extensively used in the formation of pavement, roofing, and other economical purposes. Melted and mingled with properly sifted gravel, or iron slag, it forms a very durable and unexpensive pavement, being liable to be softened, however, during intense heats.

CALCAREOUS SUBSTANCES.

Under this head we include such economic minerals as contain a notable proportion of calx or lime in their composition. Common limestone, magnesian and lithographic limestones, marble, chalk, marl, gypsum, and alabaster, are familiar examples. Some of these have evidently been deposited from calcareous waters; others are as evidently the production of animalcules, like the coral insect; and some are almost wholly composed of the shells of molluscs, and of other calcareous exuviæ. Whatever may

have been their several origins, they have all undergone certain chemical and structural changes since their formation-thus rendering them less or more compact and crystalline, producing a dull massive rock or a brilliant marble, an opaque gypsum or a translucent alabaster.

Common Limestone.

Limestones fit for building and agricultural purposes are found in every formation, from the oldest to those of the most recent origin. The rock is generally dug in open quarries, but occasionally, when it dips rapidly, and is worth the expense, it is followed downward by mining-the greater part of the stratum being excavated, and only portions left at intervals to support the superincumbent material. It is then broken into fragments of moderate size, and conveyed to a kiln, where, being placed in alternate layers with coal or turf, it is roasted, thereby expelling its water and carbonic acid. In this state it is known as shell or unslaked lime, and requires to be drenched with water to convert it into a powdery quicklime. As quicklime, it is used by the farmer; but it requires to be further slaked and mingled with a certain proportion of good sharp sand to render it suitable for mortar. Besides building and agricul tural purposes, a large quantity of lime is used as a flux in metallurgic processes, such strata being sought for this pur pose as contain but a small per centage of impurities. Considerable quantities are also used in the purification of gas, in soap-making, leather-dressing, dyeing, medicine, and in many other economical processes. The supply of limestone in our own country is inexhaustible; it is worked in beds from one foot to one hundred feet in thickness; the mountain or carboniferous limestone which underlies the coal-formation often exceeding that thickness, and ranging unbroken for many miles in

extent.

Marble.

Marble is but a technical term for any species of limestone sufficiently pure and compact to be susceptible of a fine polished surface. No matter what the colour, whether white or black, whether studded with the strange forms of fossils, or streaked with the most fantastic veinings, marble is but a carbonate of lime, containing only a few subordinate impurities, which do no more than affect its colours and markings. The best varieties are obtained from the primary and transition formations, in which they occur compact, crystalline, and not unfrequently replete with party-coloured veinings. Pretty enough marbles for slabs and other architectural purposes are sometimes obtained from the secondary formations, these being, in general, curiously marked with the shells, encrinites, and other corals which are imbedded in the mass. None of these, however, are susceptible of the same degree of polish as the primary marbles, some of which, like that of Carrara, seems almost translucent. Most countries of any extent have varieties of native marbles, which, though inferior to those of Italy and the Archipelago, might still be more extensively used than they are, were it not for the expense in cutting and

polishing, and, above all, the rapidity with which many of them become weathered and tarnished.

Sculptors and architects generally arrange the marbles of a country into some such divisions as the following:-One-coloured, as the black and white; variegated, when marked with irregular spots and veins; madreporic, when studded with encrinal or coral markings; shell, when only a few shells are interspersed through the mass; lumachelli, entirely composed of shells; cipolin, containing veins of greenish talc; breccia, marbles formed of angular fragments of different composition and colour; and puddingstone, when the fragments are round instead of angular. The celebrated marbles of Greece and Rome, such as the Parian, the Pentelic, the Carrara, &c. were of one uniform colour, and only occasionally marked with grayish or greenish veins. Besides these, which were chiefly employed in sculpture, and in the decoration of their public edifices, the ancients indulged in a variety of fancy marbles for minor ornamental purposes-such as black, red, green, yellow, spotted, and veined. The localities of some of these ancient marbles are lost, but inexhaustible supplies of first-rate statuary and architectural marbles can still be obtained from the Archipelago, from Carrara, Genoa, Corsica, Sicily, and other parts of Italy. At Carrara alone, about 1200 men are employed at the different quarries, and at the mills for sawing the marble. The annual rental is calculated at about £28,000, and the value of the yearly exportations of the raw material at not less than half a million. So accessible are these quarries, and so free from flaws is the rock in some portions, that blocks of more than 200 cubic feet can be detached by means rude and primitive compared with quarrying in Britain. The value of the material differs according to the quality and size of the block, large blocks being from £2 to £3 per cubic foot; a price scarcely half of what was sometimes paid during the usurpation of Italy by Napoleon.

Many marbles of excellent quality are found in France; in England they are abundant in the counties of Derby, Devon, and Anglesea, the last being of a green colour; in Scotland, at Assynt, Ballachulish, and in the islands of Tyree, Skye, and Jura; and in Ireland, at Kilkenny and other places. The Kilkenny marble is black, and encloses shells of a whitish colour, which, when cut across and polished, present various circular markings, which add to the beauty of the slab. The United States also furnishes some excellent architectural marbles, principally of primary formation. One range, which passes unbroken through several of the States, is perhaps one of the most extensive and valuable primary limestones in the world. It is of a pure white colour, and of a highly crystalline texture, affording blocks of more than fifty feet long and eight feet thick. It is employed in several of the States' public buildings-as, for example, the City Hall of New York, and Girard College, Philadelphia.

Magnesian Limestone-Magnesia.

Magnesian limestone, which appears extensively in England, Germany, and other continental countries, occurs often in beds of great thickness, immediately above the coal-measures, just as the mountain or carboniferous limestone lies immediately beneath. It is usually of a cream-yellow colour, and of very variable consistency, some layers being soft and powdery, others irregularly crystalline and concretionary, and some compact and homogeneous. The compact granular variety is generally known by the name of Dolomite, after Dolomieu, a French geologist. Magnesian limestone is, for the most part, used as the ordinary carbonates of lime; that is, for agricultural and building purposes-some of the English quarries furnishing an exceedingly durable material. The new houses of parliament, for example, are built of a magnesian limestone; that of Bolsover Moor, in Derbyshire, having been selected after the most rigid scientific tests of a commission of inquiry. Besides these uses, some of the more compact and homogeneous schists are employed for litho graphic blocks, the chief supply for that purpose being derived from Germany, though lithographic schists are also obtained from the white lias limestone in England.

Magnesian limestone is so called from its containing a notable per centage of magnesia-a well-known medicinal earth, commonly obtained by burning the carbonate of magnesia. The calcined magnesia of the druggist is procured either from this source, or from the bittern of sea-salt, or from the waters of certain springs impregnated with the sulphate of magnesia. Natural carbonate of magnesia is found in Piedmont, in Moravia, in the United States, and in the East Indies. It exists as a component part of many mineral substances, making them feel soft and soapy to the touch.-Meerschaum (German, foam of the sea), a substance in great repute among tobacco-pipe fanciers, is an earthy carbonate of magnesia, extremely light, and of a yellowish-brown colour. It is found in various parts of southern Europe, particularly in Greece and Turkey, where, besides being fashioned into pipe-bowls, it serves also the purposes of a fulling-earth. Germany, however, is the great seat of the meerschaum pipe manufacture, whence France and England obtain their supplies.

Chalk.

Chalk, another well-known mineral, is a carbonate of lime of a white or whitish-gray colour, having a soft meagre feel and earthy fracture. It is the last or youngest of the secondary rocks, and constitutes an important geological feature of England-the chalk-hills which form the white cliffs of our southern shores having conferred the ancient name of Albion (alba, white) upon our island. Calcined like common lime, it is used for manure