448 Extracts from the Minute Book of the Geologieal Society. 



sinking pits during the dry season in the channels of torrents. The 

 nodules which are thus found lie intermixed with other rolled peb- 

 bles, and weigh from a few ounces to two or three pounds. Their 

 colour when recent is blackish olive passing into grey. The prepa- 

 ration 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. 



1816, January 5. 



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 bitumi- 

 nous 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 perpen- 

 dicular columnar masses, the bases of which rest on the floor of the 

 oven. Adherent to the sides of these rifts are occasionally found 



