CHAMBERS'S INFORMATION FOR THE PEOPLE. 



indurated argillaceous rock, and consists, like 

 ordinary clay, principally of silica and alumina, 

 with varying proportions of oxide of iron, and 

 potash, soda, or magnesia. It has originally been 

 deposited as fine muddy sediment, which has 

 not only been afterwards hardened by metamor- 

 phic action, but the direction in which it splits 

 has been changed by great lateral pressure. That 

 is to say, slaty rocks do not, like most sedimen- 

 tary strata, split in the planes of stratification, but 

 at every possible angle to these, by what is 

 called the 'natural split' or ' cleavage.' It is this 

 fissile structure, along with fine texture, which 

 gives slate its peculiar value. The most extensive 

 slate quarries in the British Islands are those 

 of Penrhyn, Llanberris, and Festiniog, in North 

 Wales. In this district, about 350,000 tons of 

 slates are annually produced, the value of which 

 is estimated at .700,000. At Borrowdale, in 

 Cumberland, and at Easdale and Ballahulish, in 

 Argyllshire, there are also large slate quarries, 

 as well as lesser ones about Dunkeld and many 

 other localities. 



Since the repeal of the duty on slates carried 

 coastways, in 1831, this material has received 

 many new applications. For roofing purposes, j 

 the Scotch slates are stronger but coarser looking 

 than the Welsh. Sometimes, however, the latter 

 are used for public buildings in very large thick 

 slabs, in which case they are very strong. In 

 the neighbourhood of some slate quarries, the 

 walls of houses are built of the more compact 

 portions of the rock Slate is also used in thick 

 slabs for pavement, for shelves, and extensively 

 now in the construction of tanks and cisterns. 

 These larger pieces, after they are quarried, are 

 sawn and polished by ordinary stone-cutting 

 machinery ; but common-sized roofing slates are 

 split by hand with a mallet and broad steel chisel, 

 and dressed with a knife. This material has of 

 late years been applied to the production of what 

 is called ' enamelled slate,' a purpose for which 

 it is singularly well adapted. School slates and 

 pencils are made from fine-grained slate. 



SILICIOUS SUBSTANCES. 



Silica or silicic oxide is one of the most abun- 

 dant ingredients in the crust of the globe. Quartz, 

 or rock-crystal, is an example of pure silica ; and 

 agate, chalcedony, flint, and sandstone are com- 

 posed of little else than this substance. Silica is 

 the oxide of silicon or silicium, and is a feeble 

 acid. 



Rock-crystal Quartz. Transparent quartz is 

 called rock-crystal, a material of much use for 

 some purposes, yet not of much intrinsic value, 

 unless it happen to be finely crystallised or pos- 

 sess attractive colours, as in the varieties called 

 amethyst and Cairngorm stones. Clear and 

 colourless rock-crystal, when in large pieces and 

 free of flaws, is cut by the Chinese and others 

 into hollow cups and vases, and also by the 

 Japanese into very perfect spheres. These, from 

 the amount of labour required to work so hard a 

 substance, are very costly, but they are objects of 

 great beauty. As rock-crystal is harder as well as 

 cooler than glass, it is better adapted for the 

 lenses of spectacles and opera-glasses, a purpose 

 for which it is now a good deal used. Opaque 

 quartz is an abundant substance, of little value. 



394 



Aventurine is a quartz filled with golden-coloured^ 

 scales, probably of mica. 



Flint. Common flint is found in nodules of 

 various sizes, occupying distinct layers, chiefly in> 

 the Upper Chalk. The nodules are usually quite 

 separate, and surrounded with chalk ; sometimes,, 

 however, flint is found in flat tabular layers. 

 Silicious bodies, resembling the chalk-flints, are- 

 found in other limestone rocks, and are probably 

 due to the same origin. Different authors have 

 accounted in various ways for the formation of 

 flints, but all that has been certainly ascertained is 

 that, in almost all cases, they are silicified sponges.. 

 This has been proved by Dr Bowerbank. Flint 

 is nearly pure silica coloured dull amber, gray, or 

 black, by some organic substance which disappears 

 on calcination, leaving a white mass of silica. 

 This property gives flint a peculiar value for white 

 pottery, in the manufacture of which it receives 

 its chief application. It was formerly used in 

 making crystal, hence the name 'flint' glass. In 

 the chalk districts it is employed as a building- 

 stone and road-metal. Before the introduction of 

 lucifer-matches, flint was in universal demand for 

 striking fire with steel, and its use for this purpose 

 on guns is not even yet obsolete those sent to- 

 the interior of Africa being usually supplied with 

 them. In prehistoric times, it was the chief 

 material of which cutting implements were made. 

 Flint is slightly harder than quartz, and has a 

 characteristic conchoidal fracture. 



Sandstones. Sandstone, as well as limestone,, 

 when it cuts freely in any direction, is called free- 

 stone. Any rock which is formed of sand com- 

 pacted firmly together is a sandstone, or some- 

 times, if it is very coarse, a gritstone. Usually, 

 the particles of which it is composed are water- 

 rolled grains of quartz, and according to the 

 colour of these, we have stone of endless shades 

 of colour, from a nearly pure white to a decided 

 black, including yellow, brown, red, gray, and 

 green. It is the glistening nature of the quartz, 

 particles which gives sandstone its peculiar beauty,, 

 as contrasted with ordinary limestone. In large 

 towns, one can sometimes scarcely tell a lime- 

 stone front from one done in cement ; but this is. 

 never the case with a sandstone. Particles of 

 various minerals, such as mica, felspar, oxide of 

 iron, lime, or clay, are frequently disseminated 

 through sandstones, and according as one or 

 other prevails, we have micaceous, felspathic,. 

 calcareous, argillaceous, or ferruginous sandstone. 

 The durability of a sandstone depends much upon 

 the nature of the substance which cements the 

 grains together if silicious, the stone is more 

 likely to be lasting than if it is calcareous, but 

 hardly any test except long experience can be 

 trusted to determine whether a building-stone is 

 durable or not Sandstones differ much in hard- 

 ness, but many of the softer kinds harden when, 

 exposed to the atmosphere. 



In England, good sandstone for building pur- 

 poses is comparatively rare, that found at Darley 

 Dale in Derbyshire, and at the quarries about 

 Mansfield in Nottinghamshire, being exception- 

 ally good. In Scotland, on the other hand, it is 

 most abundant, all the principal towns being built 

 of sandstone, either from the Carboniferous or Old 

 Red Sandstone formations, with the exception of 

 Aberdeen, which is chiefly of granite. Craigleith 

 Quarry, near Edinburgh, produces one of the best 



