GENUINE PETRIFACTIONS. 197 



substance is saturated throughout with mineral matter ; 

 every part of it has undergone a change ; sometimes 

 flint has filled up every interstice, and on slicing and 

 polishing it, the most delicate texture of the original 

 appears. Thus wood, commonly petrified by flint or 

 chalcedony, may be cut so thin, that, with a powerful 

 glass, the ramifications of the vessels and the structure 

 of their tissues may be seen, and the particular kind 

 of tree to which the specimen belonged determined, 

 although for ages it may have been cased up in stone. 



Genuine petrifactions, in which the parts of an organ- 

 ized body are changed into mineral matters, are supposed 

 to have been frequently effected suddenly by the com- 

 bination of gaseous fluids with the principles of the 

 body thus transmuted. It seems, indeed, certain that 

 the change of animal and vegetable substances into 

 silex, must, in the majority of instances, have been 

 almost instantaneous, for the most delicate parts are 

 often preserved, such as the soft bodies of mollusca, 

 and even the pollen of plants. 



Having made these general remarks, w^e may now 

 proceed to fossil shells, commencing with the nummu- 

 lites, which form a singular genus ; they are so called 

 from their resemblance to a piece of money. They vary 

 in size from that of a crown -piece to one exceedingly 



