593 



APPENDIX. 



VOLCANIC ROCKS, 



THE general history of volcanoes is too uni- 

 versally known to require any notice in this place. 

 But, although it might be supposed easy to pro- 

 nounce on the volcanic nature of any rock, from 

 its connection with such mountains, geologists 

 are by no means agreed respecting the substances 

 to which they attribute this origin. That uncer- 

 tainty has arisen from various causes. Many 

 observers have differed respecting those rocks 

 which they suppose to have formed parts of the 

 mountain previous to the existence of fire, and 

 those which they chuse to attribute to its action. 



Q Q 



