ing its name from the Greek malachè, the marsh-mallow, in allusion to its colour. It occurs in copper-mines in reniform, concretionary, and stalactitiform masses, more or less compact, and in concentric bands of various shades. When cut and polished, it is highly prized for ornamental purposes brooches, snuff-boxes, vases, inlaid work, &c.; but its softness renders it of less value than it would otherwise be to the lapidary and jeweller. The finest specimens are obtained from Siberia, the Urals, and Burra Burra in Australia.

For other ornamental stones, not strictly regarded as “precious," the student is referred to Chapter V.

VI.-PASTES, OR ARTIFICIAL GEMS.

From their rarity and value, the gems and precious stones very early became the objects of imitation, and this often with considerable success. As chemistry advanced the imitations became more perfect, and now factitious gems are frequently produced which require all the skill of an expert to detect. These artificial products are made of very pure, fusible, transparent, and dense glass, termed strass or paste, with the addition of metallic oxides to impart the necessary tints. This strass consists of silica, alumina, oxide of lead, and potash, with traces of borax and arsenious acid to increase its clearness and brilliancy. The success of an artificial stone depends chiefly upon the exact imitation of the tint of the real stone, but also in no small degree upon the cutting, polishing, setting, and foiling. Being essentially a glass, the artificial products differ from the natural in hardness, specific gravity, and power of conducting heat, and may be detected by their inferiority in these important properties. The hardest glass rarely exceeds 5, while the gems range from 7 to 10; glass seldom exceeds 2.5 in specific gravity, the gems range from 2.6 to 4.5; glass has not the same cold feel when touched by the tongue, its conductivity being inferior to that of the precious stones.

In the preparation of pastes, the ingredients are separately reduced to a fine powder, then mixed and sifted, next carefully fused, and ultimately allowed to cool very slowly. The more tranquil and continuous the fusion, and the more gradual the cooling, the greater is the density and beauty of the product. The proportions of the admixtures and their treatment are strictly matters of chemistry and technology; but we may notice a few to show that they are wholly mineral and metallic,

and in this respect come within the cognisance of Economic Geology. The imitation of the diamond is obtained by pure silex 100 parts, red oxide of lead 150, calcined potash 35, calcined borax 10, and oxide of arsenic i part; the topaz by 1000 strass, 40 antimony, and I purple of cassius; the ruby by 1000 strass, 5 peroxide of manganese, and a trace of purple of cassius; the emerald by 1000 strass, 8 oxide of copper, and 0.2 oxide of chromium; the sapphire by 1000 strass and 15 oxide of cobalt; the amethyst by 1000 strass, 8 peroxide of manganese, 5 oxide of cobalt, and 0.2 purple of cassius; the beryl by 1000 strass, 7 glass of antimony, and 0.4 oxide of cobalt; the carbuncle by 1000 strass, 500 glass of antimony, 4 purple of cassius, and 5 peroxide of manganese; and so on with many others-different fabricators using different proportions, according to their success in the imitation.

Occurring in drusy cavities as geodes and as accessory minerals, the gems and precious stones do not bulk largely in the rocky crust, but appear (as the Arabic poet has it) merely as "the blossoms of the mineral kingdom;" hence the high esteem in which they have ever been held, and the uniform values they have maintained. In early times they were procured principally from the East, and the term "Oriental" was and is still regarded as a mark of distinction; but in recent times they have been obtained from the Urals, from Mexico, Brazil, and Southern Africa, in equal purity and perhaps in greater abundance. Notwithstanding these new sources of supply, their money value has been little affected; the diamonds of Brazil, and more recently those of the Cape, though increasing the numbers, scarcely, if at all, diminishing the price of these, the most brilliant of mineral productions. This arises partly from the greater demand, and partly from the greater wealth of modern society; and as these are ever increasing factors, there is little likelihood of the gems and precious stones falling much in value, any more than they are likely to fall in favour for their brilliancy and beauty.

The practical geologist has thus every incentive to search; and as new regions are every year being more minutely explored, new sources of supply may reward his diligence, just as we have seen within the current century the gold-fields of California and Australia, and the diamond-fields of the Cape, made known through the keener and more intelligent observation of their first discoverers. The Earth is an exuberant and undenying mother, but she does not thrust her bounties upon her children; and if they would enjoy these, they must make

intelligent endeavour to discover and reasonable effort to secure them. The structure of the globe is better known now than it was fifty years ago, the relations of the rock-formations are more fully understood, and their respective constituents more minutely determined; and thus the researches of the geologist become more definite, and his commercial success more certain. The discovery of a coal-field may, in many respects, be more important than that of a diamond-field; but while it takes the skill and labour of generations to develop the resources of the one, the treasures of the other may be revealed during the toil of a single summer. It is this suddenness, this condensation of wealth within the least possible sphere of time and labour, which becomes the great incentive to gem-hunting; and though often hazardous and uncertain, there is no reason why correct observation and sound deduction should not reduce such uncertainty to a minimum.

Works which may be consulted.

Dana's 'System of Mineralogy;' Bristow's 'Glossary of Mineralogy ;' Jackson's Minerals and their Uses;' Greg and Lettsom's 'Mineralogy of Great Britain and Ireland.'

XVIII.

THE METALS AND METALLIC ORES.

THERE is no chapter in Geology more interesting than that which deals with the Metals and Metallic Ores. These substances lie at the foundation of all the higher arts and industries, and little progress can be made, even in civilisation, without some acquaintance with their nature and uses. Man restricted to tools and implements of wood, bone, or stone, can never successfully combat with the forces of nature, or modify them to his service and comfort. He is essentially a savage. But the moment he can arm himself with a weapon of metal, or handle a metallic tool, he gains an ascendancy over external nature, and his course, mentally as well as physically, is thereafter onwards and upwards. He always passes through the successive stages of stone, bronze, and iron; and not till he has arrived at the last can he be said to possess tools, implements, and machinery sufficient for the arts and industries of civilised existence.

To the metals man owes his finest and most efficient tools, implements, and machinery; his most beautiful and durable ornaments; his most brilliant dyes and pigments; his most convenient medium of exchange; and, indeed, very much of that power which, as an intelligent being, he exercises over the domain of nature. They run through all his arts and industries his endless machinery, instruments, and apparatushis steam-engines, railways, ships, and telegraphs. In fine, there are few or none of his economic processes in which they do not directly or indirectly bear a part. As they are found in nature, it is usual to speak of them as Native Metals and Metallic Ores-that is, as metals occurring in a pure and simple state, or as metals chemically combined with other substances -thus forming oxides, sulphides, carbonates, silicates, and the like. In the present chapter we shall direct attention principally to their geological recurrence, their abundance, and the

facilities with which they can be procured, leaving the processes by which they are reduced, smelted, alloyed, and manipulated, to the metallurgist and technologist.

I.-NATIVE METALS.

Comparatively few of the metals occur in a free or uncombined state that is, as pure and simple elementary substances. Those most frequently found are-gold, platinum; palladium, silver, mercury, copper, arsenic, antimony, bismuth; and those less frequently and doubtfully-iron, lead, zinc, and tin. There also occur, though less abundantly, a series of double-metals, or amalgams of gold and mercury, of silver and mercury, of gold, silver, and mercury, of platinum and iridium, of iridium and osmium, and the like; but these we need not especially refer to.

Gold.

Gold, so well and widely known, occurs in various shades of gold yellow, has a hardness from 2.5 to 3.0, and a specific gravity from 16.0 to 19.5, according to its purity. It has extreme permanence in air and fire, being little tarnished by any amount of exposure, and melts at 2016° Fahr. It is also extremely malleable and ductile, its malleability being such that it may be beaten into leaves not more than 280,000 of an inch in thickness, and its ductility so great that one grain is capable of being drawn out into 500 feet of wire. It readily forms alloys with other metals; and in coinage, as well as in the arts, is generally so alloyed (with copper, silver, &c.) to improve its hardness, and so render it better able to resist the tear and wear of circulation, handling, and cleaning. If 24 carats be taken as the standard of purity, any stated number below 24 will indicate the amount of admixture. Gold is not acted upon by the common acids, but yields to chlorine and nitro-muriatic acid, forming a chloride of gold which is soluble in water.

Geologically, gold is a widely distributed metal, and occurs, with few exceptions, in quartz-veins which traverse the metamorphic or older schistose and slaty rocks. It appears in minute disseminated particles, in scales, strings, arborescent plates, and in nuggets from a few grains to many pounds in weight. When not found in situ in the veinstone, it is usually distributed in stream-drifts of sand and gravel which have been wasted and worn, and transported in course of ages from the mountain-veins to the valleys below. It was from such drifts