CLA SSIFICA TION OF IGNEO US RO CKS 583 



component of these silicates, it is quantitatively the most 

 important chemical character of the salic minerals. Moreover 

 the femic minerals in most rocks are silicates, and in them also 

 silica is quantitatively the largest factor. Less abundant femic 

 minerals are ferrates, titanates, and silicotitanates, besides still 

 less frequent minerals of other kinds. 



For these reasons the chemical characters of the salic and 

 femic minerals of first importance quantitatively are the acid com- 

 ponents, Si0 3 , Ti0 3 , Fe 3 O s , which might be expressed uncom- 

 bined with bases, or by mineral molecules. In order to express 

 as far as possible the mineral composition together with the 

 chemical it is advisable to express the relative amounts of these 

 acids by means of mineral subgroups. 



Orders in Classes with preponderant salic minerals are conse- 

 quently based on the proportions of quartz to the feldspars, and 

 of feldspars to feldspathoids, since these proportions correspond 

 to differences of SiO s in the salic minerals. In Classes with 

 preponderant femic minerals Orders are based on the proportions 

 between silicate and non-silicate minerals in the first instance, 

 and the recognition of the other acid components occurs in sub- 

 divisions of Orders. 



Suborder. — In those Orders in which there are preponderant 

 amounts of acid components other than silica, the most frequent 

 of these being Fe 3 3 and Ti0 3 , we may distinguish the relative 

 proportions of these in Suborders. They occur in those parts 

 of the system where minerals of the M subgroup preponderate. 



Rang. — Having thus recognized the acid components, with 

 the exception of the radicals CI and S0 3 , which can only be 

 present to a subordinate extent, we have now to take up the 

 recognition and expression of the bases in the magma. Conse- 

 quently, Rangs are formed on the chemical characters of the 

 bases in the minerals of the preponderant salic or femic group, 

 according to the Class under consideration. And since there 

 are several of these bases generally present it is necessary to 

 treat them in successive groups. The first and most general 

 division constitutes Rangs. 



