Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/27



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-whin, 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 immediately below the coal, is called Blue-thilL

Hard blue metal is one of the most common measures in the coal-field; it is a mixture of shale and sandstone, sometimes containing 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 flagstone plate: a greyish-white argillaceous sandstone, hard and breaking into sharp wedge-shaped fragments. It is quarried 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 therefore not very hard. It crops out on Byker Hill, Whickham Banks, and Gateshead Fell, where it is about 11 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