MODE XII. SLATE. 105 



Brown jasper, it is believed, may also form 

 mountains or rocks ; but' the other kinds, as the 

 Egyptian, the jaspagate, &c. are only found in 

 small pieces, commonly globular. 



STRUCTURE II. COLUMNAR. 



This structure is very rare, and scarcely occurs 

 except at D unbar, in the south of Scotland, where 

 the interstices of the pillars are filled with sili- 

 ceous cement. 



MODE XII. SLATE. 



Texture, eminently schistose or slaty, com- Characters, 

 monly straight, sometimes curved or undulating, 

 of a very fine or impalpable grain. 



Hardness, from marmoric to basaltic. Frag- 

 ments, sharp, splintery, sometimes rhomboidal. 



Weight, granitose. 



Lustre, sometimes dull, sometimes silky. 

 Opake. 



The colour is most generally bluish, but some- 

 times greenish, or a purple red; also yellowish, 

 and sometimes with stripes or spots of a darker 

 colour. It is the thonschiefer, or clay-slate, of Names. 

 Werner, the argillaceous schistus of many English 



