282 



is exceedingly easy of fusion ; and so much so, that it is used as a 

 flux for other kinds *. 



Pieces of this mineral charcoal are frequently to be seen, in the 

 deep stratum of blue ferruginous clay dug for tile-making, near 

 Hackney-road : an examination of this substance, and of that which 

 forms the stratum in which it is found, seems to point out the cir- 

 cumstance, from which the important change it has experienced 

 has proceeded. 



Under about a yard of mould, and as much yellow clay, there is 

 in general found about this spot, a layer of variously sized gravel, and 

 silicious sand, extending from twelve to fifteen feet in depth, and 

 coloured in every part by a yellow oxide of iron. Beneath this is the 

 stratum of blue ferruginous clay, which is now dug to the depth of 

 twenty -two feet without showing any appearance of termination. 

 This clay, which seems to have been the muddy deposit of a stand- 

 ing water through many ages, has in almost every part of it small 

 particles of pyrites, which are sometimes formed into pretty large 

 nodules ; these larger masses, in general, investing pieces of charred 

 wood. Similar pieces of wood, but without this investiture of 

 pyrites, are here also frequently met with : the whole mass of this 

 structure appearing to be made up of clay, intermixed with pyrites 

 and altered vegetable matter. 



The change which this wood has suffered appears to have been 

 produced by the heat resulting from the decomposition of the 

 pyrites in which it had been imbedded. 



Yours, &c. 



* Phil. Mag. No. xlix. June, 1802. 



