INTRODUCTION. 



be called CIRCUMSTANTIAL, or ACCIDENTIAL j but this last 

 division is of little moment. 



The first six domains of Petralogy comprise, 1. The Si- 

 derous Rocks, or those in which iron predominates, not in 

 the comparative quantity when analysed, but in the quality 

 and essential difference which it imparts. 2. The Siliceous, 

 denominated as usual from the quantity of silex. 3. The 

 Argillaceous. 4. The Magnesian : these two are ;again de- 

 nominated from predominance. 5. The Calcareous. 6. The 

 Carbonaceous. 



The remaining six domains, derived from circumstances or Accidential. 

 accidences, are, 7- The Composite, or Aggregated Rocks, as 

 calcareous spar with schorl, quartz and garnets, felspar and 

 siderite or hornblende, &c. This domain has often been con- 

 founded with the granites, however alien from that descrip- 

 tion. 8. The Diamictonic, or rocks in which the substances 

 are so completely mingled, that it is difficult, even upon an 

 analysis, to pronounce which preponderates. 9. The Ano- 

 malous, or those which contradict the common order of 

 nature, and present unexpected and unusual combinations. 

 Some of these domains, though they afford few objects at 

 present, may, in the progress of the science, be greatly 

 enriched and enlarged ; and the utility of such divisions will 

 be more perceptible as the study advances towards perfection, 

 the greatest obscurity at present arising from the want of 

 necessary subdivisions. 



The remaining three domains are generally admitted in 

 geological works, namely, 10. The Transilient Rocks, an 

 interesting series, in which one substance gradually passes 

 into another, as granite into porphyry, trap into wacken, 

 and the like. 11. The Decomposed Rocks, which gradually 

 decay into sand, clay, or productive soil. 12. The Volcanic, 

 which require no other description. 



Having thus established the Domains, or Great Divisions, Modes t 

 of Petralogy, the smaller distinctions can be derived only 

 from the objects themselves, as we now arrive at what are 

 by most mineralogic authors denominated Species, though in 



