60 LYELL'S ELEMENTS OF GEOLOGY. 



Process of Petrifaction. 



The disintegration of mica also, another mineral which enters 

 largely into the composition of granite and various sandstones, 

 may yield silex which may be dissolved in water, for nearly 

 half of this mineral consists of silica, combined with alumine, 

 potash, and about a tenth part of iron. The oxidation of this 

 iron in the air is the principal cause of the waste of mica. 



We have still, however, much to learn before the conversion 

 of fossil bodies into stone is fully understood. Some phenomena 

 seem to imply that the mineralization must proceed with con- 

 siderable rapidity, for stems of a soft and succulent character, 

 and of a most perishable nature, are preserved in flint ; and 

 there are instances of the complete silicification of the young 

 leaves of a palm-tree when just about to shoot forth, and in that 

 state which in the West Indies is called the cabbage of the 

 palm.* It may, however, be questioned whether in such cases 

 there may not have been some anti-septic quality in the water 

 which retarded putrefaction, so that the soft parts of the fturied 

 substance may have remained for a long time without disintegra- 

 tion, like the flesh of bodies imbedded in peat. 



Mr. Stokes has pointed out examples of petrifactions in which 

 the more perishable, and others where the more durable portions 

 of wood are preserved. These variations, he suggests, must 

 doubtless depend on the time when the lapidifying mineral was 

 introduced. Thus in certain silicified stems of palm-trees the 

 cellular tissue, that most destructible part, is in good condition, 

 all signs of the hard woody fibre having disappeared, and the 

 spaces once occupied by it being hollow or filled with agate. 

 Here petrifaction mustfnave commenced soon after the wood was 

 exposed to the action of moisture, and the supply of mineral 

 matter must have failed, or the water have become too much 

 diluted before the woody fibre decayed. But when the latter is 

 alone discoverable, we must then suppose that an interval of 

 time elapsed before the commencement of lapidification, during 

 which the cellular tissue was obliterated. When both structures, 

 namely the cellular and the woody fibre, are preserved, the pro- 

 cess must have commenced at an early period, and continued 

 without interruption till it was completed throughout.! 



* Stokes, Geol. Trans, vol. v. p. 212. second series. t Ibid. 



