SLATE. 



81 



more earthy in its texture. It forms beds of various 

 degrees of thickness, and sometimes lofty and .ser- 

 rated mountains, which are covered with verdure 

 on their declivities. Slate rocks vary much in qual- 

 ity even in the same mountain ; some being hard 

 and flinty, from containing a quantity of silicious 

 earth ; others being soft and shelly, composed chief- 

 ly of alumine. Indeed, flinty slate differs from com- 

 mon slate only in containing a larger quantity of 

 silex ; and when it loses the slaty character, it is 

 called hornstone ; and if it contain crystals of feld- 

 spar, it is called hornstone porphyry. Whetstones 

 and hones are made of talcy slate containing silex ; 

 and their excellence is proportioned to the fineness 

 of the silicious particles and their uniform diffusion. 



Slate is universally regarded by geologists as a 

 sedimentary rock, formed by the deposition of mi- 

 nute particles of the primary rocks, in the state of 

 mud, which has subsequently been consolidated by 

 heat and pressure. In no other way can we account 

 for the impression of animal and vegetable remains 

 which are often found low down in the rock. 



Although slate splits into thin laminae, yet there 



are also contained in 

 Fig. 28. it certain joints or 



fissures, traversing it 

 in straight, well-de- 

 termined lines, caus- 

 ing it to cleave in a 

 direction transverse 

 to that of the beds, 

 making with the la- 

 minae an angle usual- 

 ly of from forty to 

 sixty degrees, as seen in figure 28. This aflfords to 

 the quarryman great aid in extracting blocks of a 

 symmetrical shape. The same vertical joints or 

 cleavings are met with also in limestone, and even 

 ia granite. If- we make a paste of clay and 5tar«?^ 

 G 



