740 FOSSIL PLANTS OF THE OAEBONIFEEOUS SYSTEM. 



and polished, to show its structure on the great scale, and will be 

 exhibited in the British Museum, the Edinburgh Museum, and the 

 Edinburgh Botanic Garden. 



" The composition of all these great fossils is substantially the 

 same. The great mass of each consists of carbonate of lime, carbonate 

 of magnesia, carbonate of protoxide of iron, and free carbon, the pro- 

 portions varying in different parts of the same fossil. The iron-car- 

 bonate and charcoal vary most in their amount. The charcoal, which 

 is left after the action of diluted acids, sometimes without any other 

 insoluble residuum, seems to form three per cent of the mass, unless 

 when collected, as it often is, in cavities. This charcoal contains 

 only about 3^ per cent of incombustible 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, 

 varies from only a 20th to a 10th of an inch in thickness, but 

 at the lower end of that now at the British Museum, it increases 

 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 rook 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 car- 

 bonate 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 embedded 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. 



