Page:A tour through the northern counties of England, and the borders of Scotland - Volume I.djvu/187

 hereabouts. I before mentioned the sudden disappearance of the limestone to the northward, occasioned by its rapid dip, which introduces in its room gritstone, and shale, or shiver. Of these substances the latter is nearly of a black colour, varying in quality and texture; of extreme hardness in its stratum, but soon shivering when exposed to the atmosphere; sometimes impregnated with vitriol and iron; sometimes saturated with carbonic acid; and sometimes containing petroleum. The limestone mountain, called Tre-Mountain, to the south of the shiver, is full of marine exuviæ; enchrini,entrochi, screws, high-waved cockles, &c. as well as quartz crystal, and elastic bitumen attached to the limestone. It contains also that singular calcareous substance, peculiar to this spot, called Blue-John, found in detached masses of irregular forms and different sizes, from that of an apple to nearly a ton in weight; and worked by the manufacturers at Castleton, Buxton, Derby, and other places, into beautiful pillars, vases, and other ornamental forms. The miners say, Nature intended it for lead, but that accident has made it what it is. The scarcity of it, at present (for it appears to be nearly exhausted) has raised its price on the spot to 20l. per ton. You are not to imagine, however, that all the elegant articles sold in the shop for Blue John are worked