SLATE. — ROOF SLATE. — FLINTY SLATE. 



85 



Siate rocks vary much in quality in the same mountain ; those 

 which contain a great quantity of siliceous earth pass into flinty slate. 

 When magnesia enters largely into the composition of slate rocks, 

 they are distinguished by their green colour, and pass into chlorite 

 or talcy slate, — a rock before mentioned as occurring also in prima- 

 ry mountains. Whetstone-slate, or hone, is a variety of talcy slate, 

 containing particles of quartz : when these particles are extremely 

 minute, and the slate has a uniform consistence and requisite degrees 

 of hardness, it forms hones of the best quality. Carbonaceous mat- 

 ter is first discovered in slate rocks, and increases in quantity as they 

 approach the secondary strata. Drawing-slate is said to contain 1 1 

 per cent, of carbon ; where the carbon is very abundant, the slate 

 has a dark colour, and is generally soft. Impressions of vegetables 

 are found in some slate rocks that were formerly regarded as prima- 

 ry ; the slate rocks in the vicinity of Mont Blanc, and Mont Cenis, 

 contain impressions of ferns. Slate contains occasionally impressions 

 of fuci, or sea weed. 



That fine variety of slate which is used for roof-slate, seldom 

 forms entire mountains, but is generally imbedded in slate rocks of 

 a coarser kind : the beds of roof-slate are sometimes of considerable 

 thickness, and generally rise at an elevated angle. If geologists had 

 not been induced, by an attachment to theory, pertinaciously to ad- 

 here to opinions once received, they could not have failed to recog- 

 nise the effect of crystallization in the cleavage of slate, as evidently 

 as in the laminar divisions of felspar. 



Those varieties of roof-slate are preferred for the covering of 

 buildings, that are the least absorbent of water, and have the smooth- 

 est surface, and split into the thinnest plates ; they are, however, 

 frequently made too thin to be durable, and too light to resist the 

 force of the wind, during storms. 



Quarries of slate are worked extensively in Westmoreland, York- 

 shire, Leicestershire, North Wales, Cornwall, and Devonshire. The 

 foreign localities of slate are so numerous, that it would be superflu- 

 ous to name them. 



Mountains of slate are seldom so precipitous as those of granite, 

 but have often a sharp serrated outline. They are covered with 

 verdure on their declivities, as they contain less silex, and a more 

 equal admixture of the earths favourable to vegetation. 



Flinty slate, as before observed, differs from common slate by 

 containing a greater quantity of siliceous earth; and, as its name 

 implies, it partakes of the nature of flint. Slate and flinty slate not 

 only pass into each other, but frequently alternate. When the latter 

 ceases to have the slaty structure, it becomes hornstone, or what the 

 French denominate petrosilex. If it contains crystals of felspar, it 

 becomes hornstone porphyry : all these varieties may be observed 

 alternating with each other in the same rocks in Charnwood Forest, 

 and in North Wales and in Cumberland. 



