USEFUL MINERALS. 



In England, Derbyshire, Staffordshire, and Devon- 

 shire are richest in useful marbles. Their colours 

 are white, gray, dove, blue, black, and red more 

 or less mixed ; and according to the peculiar way 

 they are marked with fossils, they are known as 

 bird's-eye, entrochal or encrinital, dog-tooth, 

 shelly, and breccia marbles. Many of the Devon- 

 shire specimens are wholly formed of fossil corals, 

 and are called madrepore marbles. Ireland is 

 peculiarly rich both in pure and in what is known 

 as serpentinous marble. The latter, which is often 

 of great beauty the green Connemara marble, for 

 example is an intimate mixture of serpentine and 

 limestone. The famous Italian verd-antique is 

 also a rock of this kind. Black and coloured 

 marbles occur in Kilkenny, Carlow, Galway, 

 Mayo, and several other localities. In Scotland, 

 there is a beautiful pink marble, dotted over with 

 dark green spots, found in the island of Tiree. 

 Marbles also occur in Skye and Jura, on the 

 mainland of Argyllshire, and in Sutherlandshire. 



In most foreign countries there are marbles 

 more or less valuable ; some, indeed, as Italy and 

 Spain, have them in such profusion, that the 

 supply is almost inexhaustible, and the wealth of 

 colour and pattern endless. The best of the 

 Italian marbles are those of the Apuan Alps, which 

 rise around Carrara, Massa, and Seravezza. The 

 beautiful white saccharoid marble from the Carrara 

 quarries is now used by sculptors all over the 

 world. Of this material, about seventeen thou- 

 sand cubic feet, amounting in value to nearly 

 thirty thousand pounds, are annually exported; 

 but this is a mere bagatelle compared to the 

 quantity of inferior marbles exported yearly from 

 this district for architectural and furniture pur- 

 poses, which amounts to forty millions of cubic 

 feet. One third of this goes to America, one 

 third to England and France, and the remainder 

 to other European countries. The beautifully 

 coloured and variegated marbles found here and 

 elsewhere in Italy are chiefly consumed in the 

 country. Some of the finest of these are nearly 

 twice as valuable as the best statuary marble. 

 The most famous of the marbles used by the 

 ancient Greeks were those of Paros and Pen- 

 telicus, both white marbles. We have not space 

 to say anything about the marble trade of other 

 countries, few of which export to any extent. Of 

 late years, however, considerable quantities of 

 Belgian slabs have found a market in England 

 on account of their cheapness. Extensive and 

 valuable beds of white and other marbles are 

 found in the United States and Canada. 



Some marbles are so peculiar in their nature as 

 to receive special names. Among such are the 

 beautiful Algerian onyx marble, which is a stalag- 

 mitic carbonate of lime ; the fire marble of 

 Carinthia, which has a rich play of deep red and 

 other colours ; and the ruin or landscape marble 

 found at Bristol, the figuring of which, when 

 polished, looks like a representation of ruins. 



Being slightly soluble in water, and especially 

 in carbonic acid water, marble is unfitted for 

 external work in most European countries. But 

 some varieties, such as the brecciated Italian 

 marbles, which are formed of calcareous fragments 

 united by a silicio-calcareous cement, and have 

 also an admixture of hornblendic substance, resist 

 the decomposing action of the atmosphere, and 

 retain their polish for years. 



Marble is cut into blocks or slabs by thin plates 

 of soft iron set in a frame, and driven by a steam- 

 engine. Sand and water are at the same time 

 freely supplied to the cutting irons, whose motion 

 is exactly that of an ordinary saw. Mouldings 

 are formed with cutting tools shaped to the 

 required form. Columns, vases, and balusters 

 are first rudely chiselled to a round shape by 

 hand, and then turned on a lathe, a process which 

 leaves fine ridges on the object. These, as well 

 as the saw-marks of flat surfaces, are rubbed 

 down with sand and water, and finally a polish is 

 brought up on the marble with fine emery and 

 putty powder (oxide of tin). Granite, serpentine, 

 alabaster, and other ornamental stones are cut 

 and polished in a similar way. 



Chalk As already stated, chalk is of the same 

 chemical composition as limestone, of which it 

 is merely a variety. It forms the last of the 

 secondary or mesozoic rocks, and is largely repre- 

 sented in the south and south-east of England. 

 Chalk, although usually too soft, is yet employed 

 in some districts to a considerable extent as a 

 building-stone. It is largely used to prepare lime 

 for mortar, cements, and artificial stone. When 

 purified, it yields whiting or Spanish white; and 

 the finer kinds of it are of great service in schools 

 for writing on black-boards. Drawing-chalks are 

 not composed of this material, but of various 

 others, according to their colour. Thus, red chalk 

 is a peroxide of iron; black chalk is a kind of 

 clay, coloured with carbon ; and French chalk, 

 used for marking metal, is a silicate of magnesia. 



Magnesian Limestone Magnesia. Dolomite or 

 magnesian limestone forms a large portion of the 

 upper strata of Permian rocks, as developed in 

 Yorkshire and other parts of England, and in 

 Germany. It occurs also in other countries and 

 in other formations. Dolomite is usually crys- 

 tallised either in large or small granules ; but it 

 is very variable in its lithological character, being 

 either compact, earthy, friable, concretionary, or 

 in thin laminations. Its most common colour is 

 buff yellow, but it is also found of various other 

 tints. Magnesian limestone is used for agricultural 

 and other purposes much in the same way as 

 ordinary limestone. It is, however, a much more 

 valuable building-stone, especially when it is of 

 homogeneous texture, and contains the carbonates 

 of lime and magnesia (calciujn and magnesium 

 carbonates) in nearly equivalent proportions. But 

 it stands best, and will even endure well for cen- 

 turies, in a country atmosphere. In large towns, 

 it appears to be unable to resist the corroding 

 action of the atmosphere, which usually contains 

 a minute quantity of mineral acid. The Houses 

 of Parliament are built of dolomite, chiefly from 

 Bolsover Moor, in Derbyshire ; and although the 

 main portion of this costly structure is only a 

 quarter of a century old, it already shews such 

 unmistakable signs of decay, that silicious solu- 

 tions of various kinds are being tried to preserve 

 the rich carving. 



In a typical magnesian limestone, nearly a half 

 of its bulk consists of magnesium carbonate 

 that is, the metal magnesium in union with 

 oxygen and carbon. In the neighbourhood ot 

 Newcastle, Epsom salts (magnesium sulphate) 

 are made from dolomite, by treating it with sul- 

 phuric acid ; and in the same district, carbonate 

 of magnesium, also used in pharmacy, is obtained 



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