USEFUL MINERALS. 



as an amulet and in medicine ; the Gauls, largely 

 for decorating their arms ; and in the middle ages 

 jewels were frequently set with it. In Europe the 

 demand for coral is now only moderate, but the 

 annual value of what is sent to the East is 

 estimated at ,200,000. The cutting and working 

 of it are extensively carried on in Naples, Leghorn, 

 Genoa, and Marseille. 



ARGILLACEOUS SUBSTANCES. 



It will be convenient to group under this head 

 All the substances in which clay (argilla) is the 

 predominating ingredient. From the brick and 

 boulder clays of the most recent deposits, through 

 .all the formations, down to the oldest, argillaceous 

 compounds of one kind or another occur. These 

 include substances varying much in texture and 

 appearance, as plastic clay, fire-clay, shale, and the 

 clay-slate used for roofing. Plastic clays are 

 chiefly confined to the recent formations ; while 

 clay-slate, clay-stone, and argillaceous shale no 

 doubt represent the clays of the older geological 

 periods. Typical clay is an aluminium silicate 

 (silicate of alumina) ; but ordinary clay is usually 

 that substance in combination with some alkaline 

 silicate. Alumina, which is the base of clay, is 

 the oxide of the metal aluminium. See the sheet 

 on METALLURGY. 



Clay. Clay suitable for being made into com- 

 mon bricks, tiles, or coarse earthenware, is an 

 abundant material, but such clay is always mixed 

 \vith impurities, the principal of which are sand, 

 lime, magnesia, and oxide of iron. Common clay, 

 -when fired, is either red or buff in colour, owing 

 4o the presence of iron. The fine pottery clays 

 which burn white will be noticed presently. Fire- 

 clay, from which highly infusible furnace-bricks, 

 .gas-retorts, crucibles, and the like, are made, is 

 tfound in the coal-measures, in alluvial deposits, 

 and in other formations. A good fire-clay con- 

 sists of aluminium silicate, with little or no lime, 

 alkali, or iron, as these substances give it fusi- 

 bility. The Stourbridge fire-clay bricks and cru- 

 cibles are so famous for their refractory nature, 

 that this clay is exported to the most distant parts 

 of the world. But for high and long-continued 

 heats there are still more refractory bricks made 

 from the Dinas rock in the Vale of Neath, South 

 Wales. This substance, however, can scarcely 

 'be called a clay, as it is nearly all silica. 



Fine plastic clays which are pure enough to 

 .burn white in the kiln are valuable substances. 

 Such clays form the chief part of the raw material 

 required for the manufacture of pottery and porce- 

 lain, so largely carried on in Staffordshire and 

 many other parts of Great Britain. The leading 

 ikinds are pipe-clay, China clay, and Bovey clay. 

 Pipe-clay is tough, very plastic, of various colours 

 in the raw state, and consists of little else than 

 silica, alumina, and water. It is chiefly obtained 

 .in Dorsetshire from the Bagshot beds of the 

 Middle Eocene, and, from its being shipped at 

 iPoole, is sometimes known as Poole clay. From 

 ithat port, 67,579 tons were shipped in 1870 to 

 various parts of the country, but principally for 

 use in Staffordshire and London, in making 

 earthenware and stoneware. Much of it, how- 

 ever, is also employed in the manufacture of 

 tobacco-pipes, for domestic purposes, and by 

 modellers. China clay or kaolin, so largely used 



as an ingredient for the body of fine porcelain, is 

 the most valuable of all clays. It occurs in suffi- 

 cient quantity and purity for manufacturing pur- 

 poses in but few places Aue, near Schneeberg 

 in Saxony, Gomritz below Halle, St Yrieix in 

 France, and Cornwall in England, being the best 

 known European localities. It appears to be 

 found largely in China and Japan. Kaolin is no 

 doubt everywhere a product of decomposing rocks 

 rich in felspar. In Cornwall, it is obtained by 

 washing the decomposing felspar of granite, 

 but some of it occurs in deposits which have 

 accumulated by the action of rain. The com- 

 position of kaolin is the same as pipe-clay, with 

 the addition of small quantities of calcium and 

 magnesium silicates, and of free silica. It forms 

 a plastic mass with water, and when dried and 

 fired, remains unmelted at an intense white heat 

 China clay is the chief ingredient required for the 

 manufacture of porcelain, has of late been largely 

 used in paper-mills, and has also been recently 

 applied in the figuring of wall-papers. It is like- 

 wise employed in some places in the manufacture 

 of alum. In 1870, the produce of China clay in 

 Cornwall and Devonshire was one hundred and 

 twenty-three thousand tons, valued at more than 

 a hundred thousand pounds, the greater part of 

 which was prepared at St Austell and its neigh- 

 bourhood. Bovey clay, also a product of decom- 

 posing granite, is obtained from Bovey-Heathfield, 

 Devonshire, and, like other white clays, is exten- 

 sively used by potters. Above forty-eight thou- 

 sand tons were shipped from Teignmouth m 1870. 

 We may here add that the decomposing granite 

 itself, under the name of China stone or Cornish 

 stone, is also much used in the potteries. The 

 total produce of china clay, china stone, and 

 potter's clay in Cornwall and Devon in 1884, 

 amounted to 357,283 tons, valued at .263,576. 



Fullers' Earth. This substance is a soft, 

 unctuous, fusible clay or earth, which has the 

 power of removing grease or oil from wool. For- 

 merly, it was used in the woollen districts for 

 ' fulling/ but soap has now superseded it. 



Ochre. This term should perhaps be confined 

 to a clay coloured by hydrated oxide of iron, but it 

 is often applied to any earth containing iron, or 

 more rarely other substances, such as chromium 

 or uranium, which can be used as a pigment 

 Ochre is obtained near Oxford, in Cornwall, in 

 the east of Fife, and several other places ; also 

 in other countries. The ochres of especial interest 

 to the painter, however, are those of Siena in 

 Italy, which are found of yellow, brown, and red 

 colours. The colours prepared from this substance 

 are of remarkable stability, as may be well seen in 

 many pictures by the old masters. There is a very 

 pretty way of decorating the interior of houses 

 practised in India, by means of coloured earths 

 and clays, which is worth mentioning here. In- 

 stead of merely smearing the floor and wall with 

 pipe-clay and ochre, as we do, the natives, by 

 means of hollow wooden rollers, perforated with 

 various patterns, and filled with earths of different 

 colours, produce various designs on such surfaces. 

 This is quickly done, and can therefore be easily 

 renewed whenever the pattern becomes soiled. 



Clay-slate. The rocks which yield the best 

 roofing-slate are the Cambrian and Silurian, 

 although some is also obtained from the Devonian 

 and Old Red Sandstone. Clay-slate is a highly 



393 



