708 



THE WONDERS OF GEOLOGY. Lect. VII. 



takes place. A combination of gaseous fluids with the 

 constituent principles of the animal or vegetable substances, 

 changing the latter into stone, without modifying the 

 arrangement of the molecules so as to alter the external 

 form, seems the only mode by which such transmutations 

 can have been effected. The production of congelation, by 

 a simple abstraction of caloric, is akin to this change ; but 

 petrifaction is induced by the introduction of another prin- 

 ciple. As to density, the most subtile gas may acquire the 

 greatest solidity ; as, for example, in the union of oxygen 

 with metallic substances. 



The following observations of Mr. Stokes,* on fossil 

 wood partly petrified by carbonate of lime, throw light on 

 this subject. The specimen which gave rise to these 

 remarks was a piece of beech-wood, from a Roman aque- 

 duct in Germany, in which were several insulated portions, 

 converted into carbonate of lime, while the remainder was 

 unchanged. " Sometimes," observes Mr. Stokes, " the 

 most minute structure is preserved, as in the vessels of 

 palms and coniferse, which are as distinct in the fossil as 

 in the recent trees. From this state of perfection, we have 

 every degree of change, to the last stage of decay : the 

 condition of the wood, therefore, had no influence on the 

 process. The hardest wood, and the most tender and suc- 

 culent, as for instance, the young leaves of the palm, are 

 alike silicified. In some instances, the cellular tissue has 

 been petrified, and the vessels have disappeared ; here sili- 

 cification must have taken place soon after the wood was 

 exposed to the action of moisture, because the cellular 

 structure would soon decay ; the process was then sus- 

 pended, and the vessels decomposed. In other examples, 

 the vessels alone remain ; a proof that petrifaction did not 

 commence till the cellular tissue was destroyed. The 

 specimens where both cells and vessels are silicified, show 

 * Transactions of the Geological Society, vol. v. p. 207. 



