Northumberland and Durham. 1 7 



Shale or slate-clay is found throughout the Coal field, possessing 

 various shades of colour and degrees of induration. Hard black 

 and dark grey shale is called Black metal by the miners ; it is used 

 by the manufacturers of potters' saggers and fire-bricks, but for the 

 latter purpose Thil-zuhin, or hard bituminous shale forming the 

 floor of the coal seams, is preferred.* Shale of a blueish grey 

 colour is called Blue metal. A blue bituminous shale, lying imme- 

 diately below the coal, is called Blue-thil. 



Hard blue metal is one of the most common measures in the 

 coal-field ; it is a mixture of shale and sandstone, sometimes con- 

 taining scales of mica ; is much harder than Blue metal, and from 

 its waved structure breaks into sharp wedge-shaped fragments. Its 

 colour varies from ash-grey to iron-grey. 



Clay-stone (of Jameson) is not very common ; it varies in colour 

 from black to ash-grey, and is the Black-stone or Blue-stone of the 

 miners, (vide St. Anthony's section,) it is fine-grained in texture, 

 and breaks into angular fragments. 



The following are the principal varieties of sandstone that occur. 



White Jlagstone plate : a greyish-white argillaceous sandstone, 

 hard and breaking into sharp wedge-shaped fragments. It is quar- 

 ried for flag-stone at Heworth and on Gateshead Fell, where it is 

 about two fathoms thick. 



Grindstone sill or post : a light yellowish or buff-coloured fine- 

 grained sandstone, loosely aggregated, and theiefore not very hard. 

 It crops out on BykerHill, "Whickham Banks, and Gateshead Fell, 

 where it is about 1 1 fathoms thick. It is quarried for the well 

 known Newcastle grindstones, and from its softer parts filtering 

 stones are made. In many places the upper part of this bed is 



* Stourbridge clay is imported for the glass-house pots. 



Vol. iy. c 



