10 



those compounds ; the most important characters 

 of which are frequently independent of the mine- 

 rals in their composition. Nor is it unusual for a 

 rock to preserve all its most essential characters, 

 even when it has lost one of the substances which, 

 on such a classification, must be esteemed essen- 

 tial, or when it has acquired another which ought 

 to transfer it to a different division. Such a basis 

 of classification is, in fact, too narrow ; independ- 

 ently of the more essential fault of possessing no 

 necessary relation to the other important charac- 

 ters of rocks. 



If rocks are to be arranged according to some 

 mineral, as, for example, mica, it is evident that 

 substances so discordant and multifarious as gra- 

 nite, gneiss, micaceous schist, micaceous sand- 

 stone, micaceous shale, porphyry containing mica, 

 and claystone with mica, will be found associated 

 in one division. 



If again, in thus arranging the compound rocks, 

 the name of some one integrant mineral be made 

 the leading characteristic, it will frequently hap- 

 pen that no sufficient reason can be assigned for 

 selecting one in preference to another ; as, in many 



