242 Proceedings of the Itoyal Society 
cavities. This charcoal contains only about 3^ per cent, of incom- 
bustible ash. 
The surface of the fossils is covered with a shining coat of 
very bituminous caking coal, which on the principal part of the 
stem varied from only a 20th to a 10th in thickness, but at the 
lower end of that now at the British Museum, increased to half 
an inch, and at last to two inches and a half. This coaly covering 
contains only 4, 3, 2, and sometimes only 1*1 per cent, of mineral 
matter; which is not the same as the fossilising matter of the 
included wood, but is chiefly siliceous in nature, being at least 
insoluble in acids. The crust is not altered bark, for bark could not 
fail to undergo, in part at least, fossilisation by the material which 
has fossilised the wood. Moreover, the coaly crust is found round 
fragments and on broken points where bark could never have existed. 
The rock of the quarry is a very pure quartzy sandstone, hard, 
tough, and quite free from earthy carbonates or iron. But for 
some feet around the fossils, and also here and there throughout 
the quarry, where there is no fossil near, the rock has quite a 
different appearance, has a higher density, is more sharp-edged, 
much tougher, and harder to pulverise, and becomes yellow under 
exposure to the air. These changes are owing to the siliceous 
particles of the sandstone being bound together by carbonate of 
lime, carbonate of magnesia, and carbonate of protoxide of iron, 
forming together from 10 to 38 per cent, of the rock, and bearing 
much the same relation in proportion to each other as in the 
mineral material of the fossils, — consequently derived from the 
same fluid which fossilised them. 
Thus the interesting fact is presented of these great trees and 
the rock in which they are imbedded having been both similarly 
mineralised, so to speak, by the same fossilising fluid, while there 
is between them a thin uniform coating of bituminous coal, which 
has refused admission to any of the fossilising agents. After 
rejecting various theories to account for this exemption, the only 
one which stands the test of facts is, that a part of the process of 
fossilisation consists in a slow process, analogous in its results to 
the destructive distillation of wood, the result of which is charcoal 
left behind, and bitumen gradually forced outwards, and collected 
on the exterior surface. 
