ft4% Professor Netker on Minerahg^y considered 



§ 53. Having thus limited our notions of mineral species to 

 crystals of the same actual form and composition, and that of 

 varieties to modifications purely accidental, and not referable to 

 any law, we proceed to the notion of genera. The genus will 

 be a collection of species, having the same fundamental or pri- 

 mitive form, and the same chemical composition. Such a de- 

 finition, which holds good for the greater number of genera, 

 must be modified in those cases in which, by the substitution 

 of isomorphous bases for one another, the general formula re- 

 maining the same (Vide Wachmeister's Analyses and I^'ormulae 

 of Garnets), the particular formula3 are different, and at the 

 same time the fundamental form remaining the same in kind ex- 

 periences some variations in the angles. For such cases the par- 

 ticular formulae, when the isomorphous bases manifest their 

 presence by some mineralogical character (colour, specific gra- 

 vity, efl'ect of blowpipe, &c.), or only by a difference in the 

 angles of the primitive form, will give rise to as many sub-genera^ 

 included in a great genus characterized by the general formula 

 or the corresponding mineralogical characters, and by the gene- 

 ral nature of the primary form. Thus the tremolite^ actinolite, 

 and hornblende will be subgenera of the great genus AmpJdbole ; 

 and all the rhomboidal, simple and compound, carbonates of 

 lime, of magnesia, of iron, of zinc, and of manganese, will be 

 -subgenera of the great genus Spar. As for Arragonite, in 

 compliance with the general rules before proposed, it will consti- 

 tute a genus independent of the calcareous spar, as the indivi- 

 duals of that genus are very different in secondary and primitive 

 forms, as well as in other physical characters. 



§ 54. The families will be collections of genera, having, be- 

 side a common electro-negative element, some remarkable com- 

 mon physical or chemical characters, varying according to the 

 classes and the orders to which they belong, but which give to 

 the members of each family striking features of hkeness. The 

 subdivision of orders into families does not take place in such 

 orders as are composed of genera in so small a number, and so 

 like each other that the characters of the family would be the 

 same as those of the order ; but this subdivision may, at future 

 times, by the increase of the genera of these orders, be made 

 convenient and even necessary. 



