34; DOMAIN I. SIDEROUS. 



the particles are earthy instead of being crystal- 

 lised, as in granite, where the silicious part super- 

 abounds ; a wide difference has always been al- 

 lowed. A variation of the same ingredients will 

 indeed ever form one of the chief distinctions in 

 mineralogy; for it must be repeated, that it is 

 not the ingredients, but the mode of their com- 

 bination, which forms the chief distinction : dia- 

 mond being akin to coal ; sapphire only con- 

 sisting of clay and rust ; and, among the argil- 

 laceous and magnesian rocks, silex is commonly 

 the predominating ingredient, but still the argil 

 and magnesia give the character and name. 



But let us listen to the great master of petra- 

 logy on this interesting topic. 



" ^ ca ^ t ra P> a roc k composed of small grains 

 of different qualities, confusedly crystallised, in- 

 closed in a cement, and sometimes also united 

 together without any distinct cement ; and with 

 no perceptible regular crystals, except rareljr 

 and accidentally. 



" This definition connects traps with granites, 

 and porphyries ; but M. Dolomieu has made it 

 very evident that this approximation already 

 exists in nature. He observed at Rome, in the 

 masses of granite and porphyry selected and 

 worked by the ancients, as we observe it in ou| 

 Alps, and in the blocks that are detached from 



