16? 



tances which unite with the calcareous matter of which it is chiefly 

 formed. 



FREE-STONE, so called from its breaking on working in all direc- 

 tions with the same degree of freedom, is a compact lime-stone 

 of an earthy fracture, and is often met with in the search for coal. 

 When it is of a very considerable degree of thickness, it obtains, in 

 many coal countries, the name of post-stone, of which four varieties 

 occur, named, according to their colour, white, grey, brown or yellow, 

 and red post. These are sometimes called by the colliers bands ; 

 thus they will say grey free-stone bands, but the Scotch colliers term 

 them grey f ekes. 



WHIN-STONE, BASALTES, and other stones whose hardness is 

 such that their angular parts will scratch glass, and whose fracture 

 exhibits an appearance so much resembling that which might be 

 expected from sand half vitrified, as to have induced many to 

 suspect them to be of volcanic origin, also form strata of con- 

 siderable importance, from their hardness and quantity, in most 

 coal mines. These are commonly called cockle in Cornwall, and 

 skurdy in the north of Scotland. 



IRON-STONX, called also metal-stone, and by the Scotch colliers 

 dogger band, is a stratum, which is generally next above the roof 

 of the coal. It is very hard, compact, and solid, and of considerable 

 weight. Its colours are various shades of brown and grey, and 

 sometimes it is nearly black. It evidently consists of argillaceous 

 earth impregnated with iron. Nodules of a richer ore of the same 

 metal are frequently contained in it. These are called ball iron- 

 stone, cat's-head, and doggers. Masses of pyrites are also frequently 

 found in this stratum. 



SHALE, or slate-clay, called also shiver, black metal, and bleas, 

 and by the northern colliers blae or till, often also forms the roof of 

 coal mines. It is generally bluish, blackish, or of a reddish grey. 

 It is frequently marked with impressions of vegetables, It breaks 

 into long thin shivery lamina, and is very friable. It readily re- 



