

, ... 



INTRODUCTION". XVU 



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aid*. Hence have arisen the chief errors in his system, ably 

 exposed by Chenevix, who has shown that the different 

 species of Werner are often vague and indeterminate ; and 

 the order of his arrangement not seldom capricious and 

 imaginary, and far from being founded on his own principle 

 of chemical composition. The calcareous spars are united 

 under several groups, according to the acid which predomi- 

 nates. Those he has marked A, B, C, D, are truly German, 

 distinctions. Dr. Thomson has justly observed, that by his 

 use of groups and families, Werner is struggling against his 

 own system. 



But the mode admitted in the place of the species, obviates 

 these difficulties. It presents a real chief distinction between 

 the species, that founded on chemical analysis, as it refers to 

 the mode of combination, the ruling principle in the difference 

 between one mineral and another, considered even in the 

 most abstracted point of view, and with regard to the purest 

 substances, as crystals, gems, &c. ; as even a variation in the 

 water of crystallisation sometimes distinguishes one mode 

 from another. But though what are admitted as distinct 

 modes, will perhaps always be found to differ in chemical 

 analysis j yet as the science does not admit of too much pre- 

 cision, while the substances themselves are always variable, 

 as partaking of a mutual nature, and only portions of that 

 vast mixture the shell of the globe j the mode may also more 

 laxly be understood to include some modifications of external 

 characters, under what is called aggregation in particular. 

 Thus the aggregated stones may become modes, as well as 

 the combined. But in passing to the Structure and Aspect, 

 the chemical characteristics may in general be considered as 

 abandoned, or exchanged for the physical or external. 



This unavoidable uncertainty has been well illustrated by 

 the greatest of petralogists. 



* Bergman, the father of the system, derives the species from chemistry. 

 See also Brochant, i. 47. Jameson, i. xxv. 



VOL. I. b 



