Great Coal-field of Shropshire. 197 



ny stone, and also of a singular substance, here called curlstone. This 

 latter is a bluish-grey limestone intimately mixed with clay ironstone, 

 and occurs in distinct concretions of the size of a cubic foot or more, 

 bearing a rude resemblance to the capital of a Corinthian column, 

 each of which is again subdivided into irregular cones, laterally ag- 

 gregated, the larger' of which contain smaller ones included within 

 them. The singularity and uniformity of structure, observable in 

 €ach concretion, seem to render the animal origin of this substance 

 very probable. 



The ironstone is first met with, at the depth of about five hun- 

 dred feet, in the twenty-eighth bed from the surface, the thickness 

 of which is nearly twenty-seven feet. Being often wanting in the 

 upper part of this bed, it is named when found in this situation 

 chance iron-stone : that which is met with in the lower part of the 

 bed is of much more general occurrence throughout the whole ex- 

 tent of the coal-field, and is called ball-stone. The other beds of iron 

 ore are five in number, and are distinguished by the following names, 

 viz. yelloiv-stoney hlue-jiat^ white-flat^ pen?iystone, and crawstone. 

 These all form subordinate beds in indurated clay, each bed being 

 composed of balls or of broad flat masses. 



The slaty-clay, called by the colliers basses^ is of a bluish-black 

 colour and a slaty texture ; it usually contains pyrites and is always 

 either intimately mixed with coal or combined with petroleum ; in 

 the former case it passes insensibly into slaty coal, and in the latter 

 into Cannel coal ; so that the real beds of coal in some parts are 

 found to degenerate into basses, and on the other hand the basses 

 often contain very tolerable coal. 



The beds of coal usually present a mixture of slate coal and pitch 

 coal, rarely of Cannel coal : none of it possesses the quality of caking. 

 Several beds are so penetrated by pyrites that the coal which they 



