Silica 61.91 Alumina 21.73 Protoxide of Iron 4.73 Lime 0.09 Magnesia 0.59 Potash 3.16 Soda 0.25 Chloride of sodium 0.08 Organic matter 0.70 Water 6.73 99.97 These shales or clunches, now indurated, were originally the soft soils from which the roots of plants of the coal-measure period drew their nourishment; and they still retain the impression of such roots in great abundance, as any one may satisfy himself who takes a piece in his hand to examine it. The vegetable matter derived from roots, and from the plants themselves, give them a dark colour; but this burns away in the firing, and the bricks come out of the kiln nearly a pure white. It is almost invariably found that where the vegetation of a seam of coal grew on the spot, and was not transported, as was the case in some instances, that one of these underclays is to be found still retaining very beautiful casts of the roots of plants, and not unfrequently the seeds as well as the plants themselves that grew above them. As fire-clays, they are little if at all inferior to the famous Stourbridge clays, and they supply an invaluable material for crucibles, for bricks for the interior of our blast and puddling furnaces, for the kilns of our potteries, and for various other purposes where intense heat is required.