282 
ingly 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. 
