196 



GENUINE PETRIFACTIONS. 



acid is dissipated by the atmosphere, and the marble, 

 slowly precipitated, assumes a crystalline form, and pro- 

 duces coherent stones. The acid originates in the 

 action of volcanic fires on the calcareous rocks of 

 which the Apennines are composed ; and carbonic 

 acid being thus evolved, rises to the source of the 

 springs derived from the action of the atmosphere, 

 gives them their impregnation, and enables them to 

 dissolve calcareous matter." 



When objects are exposed to the action of a petri- 

 fying spring, there is often only an incrustation. On 

 breaking the specimen, the substance enclosed has un- 

 dergone no change but that of decay, to a greater or 

 less extent. In a true petrifaction, on the contrary, the 



Genuine Petrifaction. 



