Page:A Treatise on Geology, volume 1.djvu/181

 CHAP. VI. 'ends' or ' backs,') and, in addition, a minute fissility, generally in one certain direction across the bed, which does not occur in the shales above or below. It is a sort of crystallisation. Ironstone sometimes shows concentric laminæ, and often sparry divisions, when it becomes a septarium.

A very singular structure is frequently noticed in the argillaceous iron ores of a coal district, without however  being peculiar to them, which is represented in fig. 49. The substance of the iron ore is formed into conical sheaths, involving one another, and marked by concentric undulations and radiating striae. Large spheroidal masses of iron ore, weighing at least a ton, are thus found, in connexion with the coal, at Ingleton, in Yorkshire; and in the coal fields of Staffordshire and South Wales it is a well known form of aggregation. This structure also occurs in many other formations, as in the slate of Skiddaw, the lias, oolites, &c., though with considerable variations. It is usually called 'cone in cone,' 'cone coralloid,' conical limestone, conical ironstone, &c.

A different but yet closely allied phenomenon, noticed by Mr. Dillwyn in the substance of the coal of Swansea and other parts of South Wales, which we have also seen at Ingleton, is represented in fig. 48. Such a mass of coal, however solid, is found to separate not along a plane, parallel to the bed, but with deep hollows, and acute sinuous ridges, sulcated on their slopes, and