a genus of stones belonging to the class of siliceous earths. It is of a grey colour; the texture obscurely laminar, or rather fibrous; but the laminae or fibres consist of a congeries of grains of a quartzy appearance, coarse and rough. The specific gravity is 2729; it effervesces with acids, and strikes fire with steel. Mr Kirwan found it to contain a portion of mild calcareous earth, and a small proportion of iron. It is used as a whetstone for coarse cutting tools. It is found about Newcastle, and many other parts of England, where there are large rocks of it in the hills.