CLASSIFICATION OF IGNEOUS ROCKS 607 



cases present in quantities which are not negligible, so that the 

 rock cannot be classified directly from the minerals actually 

 present without consideration of the mutual changes involved 

 in the diverse crystallization of the various minerals possible. 

 Such modes may be called ab?wrmative. 



The abnormative modes may be referred to two kinds, of 

 which the second is by far the most important and common, 

 though the first naturally precedes it in the discussion. 



a) The minerals present may be all standard, but differ from 

 those of the norm either in kind or relative proportions, or both. 

 This case is generally due to shifting of the SiO s and Ti0 2 , and 

 may be illustrated by these instances : quartz and leucite crys- 

 tallize instead of orthoclase, albite instead of nephelite with 

 leucite, as in leucite-basanites and tephrites, quartz with olivine 

 as in quartz-basalts, hypersthene with nephelite, titanite instead 

 of perofskite or ilmenite, the silica in this case coming from 

 normative feldspar molecules. The normative corundum may 

 also form muscovite with molecules of orthoclase and water. 



These cases, it will be seen, affect the mineral molecules of 

 oxides capable of combining in more than one proportion with 

 Si0 2 , namely K 3 O t Na 2 0, (Fe,Mg)0. All of these modifications 

 are within the range of the standard salic and femic minerals, 

 and if not accompanied by notable amounts of non-standard 

 minerals (as augite or hornblende), may be called respectively 

 salic, femic, or salfemic abnormative modes, according as the 

 abnormative minerals belong to one or the other or both groups. 



b) The mode may differ from the norm through the presence 

 of alferric (aluminous ferromagnesian) minerals, augite, horn- 

 blende, biotite, garnet, etc. 



As is explained in the Part on Calculation, and as is evident 

 on consideration of the subject, the formation of any of these 

 minerals involves changes in some, if not all, of the standard 

 minerals calculated as present in the norm. While minerals of 

 both the standard salic and femic, and non-standard alferric 

 groups are involved, yet it will be sufficient to characterize these 

 modes as alferric. 



