CLA SSIFICA TION OF IGNEO US RO CKS 585 



evident that there may be genetic groups of several degrees of 

 consanguinity, for which more specific designations will be 

 necessary. It is suggested that for these groups terms analog- 

 ous to Family be used. 



The term Series may be used, in an extension of Brogger's 

 sense, 1 for groups of rocks characterized by similar ratios of cer- 

 tain constituents, as alkalies, but with varying amounts of other 

 constituents, as silica. These series will be of very different 

 characters dependent on the petrographic province, and will 

 have no place in the system proper, since they will traverse 

 it in all directions. They may be designated, as at present, bv 

 the use of the names of the extremes, with or without that 

 of an intermediate member. 



Classes. — It is proposed to divide igneous rocks into five 

 Classes, according to the calculated proportions of the standard 

 mineral groups above defined. As previously stated, these 

 groups are designated as sal and/em in formulae, and the descrip- 

 tive adjectives derived from these are given below. 



Class I : - — > - , persalic. 

 fern 1 



This Class contains rocks extremely rich in the salic minerals 

 — quartz, feldspars, feldspathoids, and corundum. 



^7 rr sa I 7 5 

 Class II: - — < - > - , dosahc. 

 rem 1 3 



In the rocks of this Class the salic minerals are domi?ia?it and 

 the femic minerals subordijiate . 



Class III : ^— < - > - , salfemic. 

 fern 3 5 



In this Class fall the rocks in which the salic and femic min- 

 erals are equal or nearly equal in amount. 



Class IV: -^- < - > - , dofemic. 



fern 5 7 



1 Die Eruptivgesteine des Kristianiagebieles, Vol, I (1894), p. 169. 



