PETRALOGY. 



A TREATISE ON ROCKS. 



INTRODUCTION 

 TO THE SECOND VOLUME. 



HAVING in the former volume comprised all the Domains Accidential 

 which may be called Substantial, as depending upon the pre- 

 dominant substance, under various modes of combination, it 

 is now necessary to enter on another field, that of the Acci- 

 dential Rocks, which must of course be arranged according 

 to their various accidences*. These accidences being, so to 

 speak, infinitely diversified, and independent of any Mode in 

 the sense used in the former volume, and often even of Struc- 

 tures and Aspects, it was necessary to adopt new denomi- New terms, 

 nations. Even the Domains now become what might be 

 called Dominions in the natural kingdom, as they no longer 

 imply the preponderant or predominant substance, but grand 

 divisions arising from natural accidences, as the Volcanic and 

 Decomposed Rocks. 



But while the term Domain still seemed unobjectionable, 

 it became necessary to abandon the other subdivisions, which 

 being derived from the substances, and their qualities, could 

 have no place here. Instead of denominations strictly arising 

 from the very essence of the subject discussed, the subdivi- 

 sions themselves became, so to speak, accidential and arbi- 



* Pliny has natures acddcntia ; Cicero accidentia for res attribute. Acci- 

 dence is here used in contradistinction to accident, which, in common English, 

 implies a moral event or incident, not an accidental circumstance in nature. 

 Accidence is here a natural casualty, an adventitious attribute. 



VOL. ii. a 



