8 



tive of great inconvenience, as they form so im- 

 portant a part of the history of the earth, and are, 

 moreover, the natural repositories of the greater 

 number of mineral substances. In this manner 

 indeed they have already been separated by miner- 

 alogical writers; among whom it has been the prac- 

 tice to enumerate the simple rocks in their cata- 

 logues of minerals. In those works, no particular 

 inconvenience follows this proceeding : the want 

 of consistency which, in some instances, results, 

 is not without its countervailing advantages. 



But if this plan were extended to all rocks, as 

 it must be, unless a better system of mineral clas- 

 sification is adopted, even the same rock must be 

 sought for in two catalogues, perhaps in the works 

 of two different authors ; if, as frequently hap- 

 pens, it is simple in one part and compound in 

 another. Compact felspar passing into porphyry, 

 basalt passing into greenstone, limestone under 

 many of its modifications, and many other rocks, 

 offer examples illustrative of this inconvenience. 



Although a classification of minerals could be 

 rendered perfect, this objection would of itself 

 form an insuperable bar to the adoption of such 



