Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/458

 sinking pits during the dry season in the channels of torrents. The nodules which are thus found lie intermixed with other rolled pebbles, and weigh from a few ounces to two or three pounds. Their colour when recent is blackish olive passing into grey. The preparation which they undergo is, first, exposure to the sun for several weeks, and then calcination. This latter process is performed by packing the stones in earthen pots, and covering them with a layer five or six inches thick of dried goat's dung; fire is then applied to the mass, and in twelve hours time the pots are sufficiently cool to be removed. The stones which they contain are now examined, and are found to be some of them red, others pink, and others nearly colourless, the difference in their respective tints depending in part on the original quantity of colouring matter, and in part, perhaps, in the difference in the heat to which they have been exposed.

A communication from J. Taylor, Esq. Member of the Geological Society, on some remarkable appearances in coak, was read.

The coak in question is produced from two varieties of Newcastle coal, known in the market by the name of Tanfield moor and Pontop. The coal is charred in an oven of brickwork, of very simple construction, each charge being sufficient to cover the floor to the thickness of 18 or 20 inches. The combustion begins at the surface and proceeds gradually downwards. When all the bituminous matter has been driven off; the mouth of the oven is opened for the purpose of drawing the charge, at which period the coak presents the appearance of a glowing pavement rifted into perpendicular columnar masses, the bases of which rest on the floor of the oven. Adherent to the sides of these rifts are occasionally found