CLA SSIFICA TION OF IGNEO US RO CKS 5 8 1 



being commonly used in classification. But Rank and Grade are 

 so frequently employed in a general sense that it is advisable to 

 substitute archaic forms in their stead. 



These divisions, which are successively smaller, are based on 

 characters of the magma of less and less importance. In other 

 words, the highest divisions express the broadest and quantita- 

 tively most important magmatic characters, those next to them 

 less important ones, and so on. It has been our aim to select 

 the sequence of characters in accordance with this plan, and also 

 to have homologous divisions throughout the system based on 

 the same kinds of characters. 



The broadest distinguishing chemical characteristics are 

 expressed by the aluminous non-ferromagnesian minerals and 

 their associates, quartz and zircon, the salic minerals, on the one 

 hand; and by the ferromagnesian non-aluminous minerals and 

 their associates, titanite, apatite, etc., the femic minerals, on the 

 other. 



Consideration of the salic and femic mineral groups shows, 

 however, that the former is more simple in composition than the 

 latter, so that certain modifications, not of the principles, but of 

 their application, will be necessary in places. 



Thus the salic minerals are composed chiefly of Si0 2 as 

 representing the acid radical, with small amounts of CI and S0 3 

 in the sodalite group, and of K 2 0, Na 2 and CaO as represent- 

 ing the bases, these last being always accompanied by an equal 

 amount of A1 2 3 , except in the sodalite group, where this is 

 slightly less than the soda. The alumina, in most cases, 

 reckoned among the bases, may play the role of acid to some 

 extent in certain cases. On the other hand in the femic min- 

 erals, leaving apatite, fluorite, sulphides, etc., out of account, 

 Si0 2 , Fe 2 3 and Ti0 2 represent the acid radicals (the last two 

 possibly uncombined with a base as hematite and rutile), and 

 K 3 0, Na 3 0, CaO, MgO and FeO represent the bases. 



Consequently, to keep to our system of two factors, already 

 described, a more numerous subdivision of the divisions in which 

 the femic minerals preponderate is necessary. But, as will be 



