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 aggregated, the larger of which contain smaller ones included within them. The singularity and uniformity of structure, observable in each concretion, seem to render the animal origin of this substance very probable.
The ironstone is first met with, at the depth of about five hundred 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 extent 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. yellow-stone, blue-flat, white-flat, pennystone, 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
