394 Elements of Mineralogy. 



being afterwards covered with frefh lava, aricf 



this, after mouldering by that of ftill latter 



eruptions, affords fome ground for calculating 



the age of the volcano at leaft within certain 



limits. 



The beds of lava are deepeft and narrowefl 

 in the proximity of the crater, and broader 

 and fhallower as theyare morediftant, unlefs 

 fome valley intervenes ; pumiceftone and 

 ames lie ftill more diftant. From thefe obfer- 

 vations extinguifhed volcanos are traced. 

 Many excellent inveftigations of this fort 

 may be feen in Mr. Sou/aviis hiftory o the 

 fouth of France. 



Bafaltic mountains (common in Sweden) 

 feem to owe their origin to lub-inarine vol- 

 canos. 



Purifications. 



The moft remarkable obfervations relative 

 to petrifactions are, 



i ft - That thofe of fhells are found on or 

 near the furface of the earth ; thofe of fifh 

 deeper, and thofe of wood deepeft, Shells 

 mjpecie are found in immenfe quantities at 

 confiderable depths. 



2%. That thofe organic fubftances that re- 



fift 



