840 SLATES 



1. Mica-schist, sometimes called Mica-slate. This is a rock occupying a vast 

 extent, in some mountain chains : it is of a schistose texture composed of the minerals 

 mica and quartz, the mica being generally predominant. 



2. Roofing-slate. This substance is closely connected with mica-slate; so that unin- 

 terrupted transitions may be found between these rocks in many mountain chains. It is 

 a simple schistose mass, of a bluish-grey or greyish-black colour, of various shades, 

 and a shining, somewhat pearly internal lustre on the faces, but of a dead colour in 

 the cross fracture. 



This slate is extensively distributed in Great Britain. It skirts the Highlands of 

 Scotland, from Loch Lomond by Callender, Comrie, and Dunkeld ; resting on, and 

 gradually passing into mica-slate throughout the whole of that territory. Eoofing- 

 slate occurs on the western side of England, in the counties of Cornwall and Devon ; 

 in various parts of North Wales and Anglesea ; in the north-east parts of Yorkshire, 

 near Ingleton, and in Swaledale ; as also in the counties of Cumberland and Westmore- 

 land. It is likewise met with in the counties of Wicklow and other mountainous 

 districts of Ireland. 



All the best beds of roofing-slate improve in quality as they lie deeper under the 

 surface ; near to which, indeed, they have little value. This variety of slate is found 

 in the Cambrian, Silurian, and Devonian formations. 



A good roofing-slate should split readily into thin even laminae : it should not be 

 absorbent of water either on its face or endwise, a property evinced by its not increasing 

 perceptibly in weight after immersing in water ; and it should be sound, compact, and 

 not apt to disintegrate in the air. The slate raised at Eisdale, on the west coast of 

 Argyleshire, is very durable. The slates of Penrhyn and other quarries in North 

 Wales are very celebrated ; those of Delabole in Cornwall are also well known and 

 much esteemed. 



Cleaving and Dressing of the Slates. The splitter begins by dividing the blocks, cut 

 lengthwise, to a proper size, which he rests on end, and steadies between his knees. 

 He uses a mallet and a chisel, which he introduces into the stone in a direction 

 parallel to the cleavage planes. By this means he reduces it into manageable pieces, 

 he gives to each the requisite length, by cutting cross grooves on the flat face, and 

 then striking the slab with the chisel. It is afterwards split into thinner sections, by 

 finer chisels dexterously applied to the edges. The slab is then dressed to the proper 

 shape, by being laid on a block of wood, and having its projecting parts at the ends 

 and sides cut off with a species of hatchet or chopping-knife. It deserves to be 

 noticed that blocks of slate may lose their property of divisibility into thin laminae. 

 This happens from long exposure to the air, after they have been quarried. The 

 workmen say, then, that they have lost their waters. For this reason, the number of 

 splitters ought to be always proportionate to the number of block-hewers. Frost 

 renders the blocks more fissile ; but a supervening thaw renders them quite refractory. 

 A new frost restores the faculty of splitting, though not to the same degree ; and the 

 workmen therefore avail themselves of it without delay. A succession of frosts and 

 thaws renders the quarried blocks quite intractable. 



3. Whet slate, or Turkey hone, is a slaty rock, containing a great proportion of quartz, 

 in which the component particles, the same as in clay-slate and mica-slate, but in 

 different proportions, are so very small as to be indiscernible. 



4. Polishing slate. Colour, cream-yellow, in alternate stripes ; massive ; composition 

 impalpable; principal fracture, slaty, thin, and straight ; cross fracture, fine earthy ; 

 feels fine, but meagre ; adheres little, if at all, to the tongue ; is very soft, passing into 

 friable; specific gravity, in the dry state, 1'6 ; when imbued with moisture, 1'9. It 

 is supposed to have been formed from the ashes of burnt coal. It is found at Planitz 

 near Zwickau, and at Kutschlin near Bilin in Bohemia. 



5. Drawing slate, or Slack chalk, has a greyish-black colour ; is very soft, sectile, 

 easily broken, and adheres slightly to the tongue; spec. grav. 2*11. The streak is 

 glistening. It occurs in beds in primitive and transition clay-slate ; also in secondary 

 formations, as in the coal-measures of most countries. It is used in crayon -drawing. 

 Its trace upon paper is regular and black. The best kinds are found in Spain, Italy, 

 and France. Some good black chalk occurs also in Caernarvonshire and in the island 

 of Islay. 



6. Adhesive slate has a light greenish-grey colour, is easily broken or exfoliated, 

 has a shining streak, adheres strongly to the tongue, and absorbs water rapidly, with 

 the. emission of air-bubbles and a crackling sound. 



7. Bituminous shale is a species of soft, sectile slate-clay, much impregnated with 

 bitumen, which occurs in the coal-measures. See KIMMEEIDQE SHALK, and SHAJ.ES. 



8. Slate-clay has a grey or greyish-yellow colour ; is massive, with a dull glim- 

 mering lustre from spangles of mica interspersed. Its slaty fracture approaches at 

 times to earthy ; fragments, tabular ; soft, sectile, and very frangible ; specific gravity, 



