OF FOSSILS. 251 



hold a fniall proportion of gold united with a 

 large proportion of iron ; among the galena?, 

 that arc far richer in lead than in filvcr; among 

 the copper pyrites, always producing more iron 

 than copper; and fo on of many others. Ac- 

 cording to fyftcmatic 'rules, the more valuable 

 and (career metal, although it defray the ex- 

 pence of cliqtmtion, mould yet be referred to 

 the genus of the more abundant, though of lefs 

 eftimation. But if the ufe and aim of any fyftcm 

 is confidercd, there can be no doubt that the 

 preference mould be niligncd to the metal of 

 the llighcft value. In fomc degree, however, 

 the determination of this point may be a mat- 

 ter of indifference, provided no dilVmcl genus 

 is thereby dcftroyed ; a circumilancc that would 

 probably aficcl the fiderite, in cafe it were de- 

 cided in favour of fuperiority in weight, as that 

 metal has never yet been found fcpnratc from 

 iron ores, to which it always bears the fmallcft 

 proportion. 



L x x x 1 1 1 . Genera of Pbkgifllc Bodies. 



THE fourth clafs contains the fcwcft genera, 

 fulphur, petroleum, amber, and perhaps dia- 

 mond. 



$ LXXXIV. 



