Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/297

 of steatitic slaty marle, very friable, of a snow white colour and great specific gravity.

Associated with this limestone and in a similar position, I also discovered beds of sahlite, a mineral of considerable rarity in this country. These vary in thickness from two inches to a foot and more, and are sometimes intimately mixed with thin laminæ of the fibrous variety of tremolite. This mineral, from its superior hardness, resists the action of the water of the river better than the limestone does with which it is interstratified, and may therefore be distinctly traced projecting from those beds of which the calcareous portions have been more or less worn away.

The beds of sahlite may be seen extending for a space of from fifty to a hundred yards along the bank of the river at the same place. Considerable variety of aspect occurs throughout their course, from a greater or less intermixture of calcareous matter with the pure sahlite. Thus, like the more common specimens of this mineral it sometimes effervesces with acids, while in other cases it appears to contain no uncombined calcareous matter. When it is unmixed it resembles at first sight the whiter varieties of felspar, exhibiting a large platy fracture, and a similar lustre. In the angle of its fragment as well as in its specific gravity, hardness, and habitudes with the blow-pipe, it perfectly resembles the better known varieties of this mineral. As it becomes more and more mixed with the calcareous matter its platy fracture first becomes less regular and gradually disappears, the mixed mineral however still retaining a specific gravity greater than that of the neighbouring limestone. Its sp. gr. varies from 3,153 to 3,246, the most purely lamellar being the lightest. It is invariably white with a slight tinge of grey.