NOMENCLATURE. 1 20. 



petrographical classification would be separated from one another ; 

 and at the same time we avoid the vexed questions which the terms 

 " pyroxene granulite " and " pyroxene gneiss " would naturally pro- 

 voke. We are far more certain that the different members of the 

 charnockite series are related to one another than we are that they 

 are related to the European " pyroxene granulites, " and for the 

 purpose of mapping it is safer, at least as a temporary measure, to 

 adopt a local term, such as a stratigraphist, for instance, would do 

 under like difficulties. 



Besides the advantages which a new and local term offers from 

 a purely survey point of view, there are certain theoretical ob- 

 jections to the use of the alternative foreign terms for rocks which 

 are probably, though not certainly, the equivalents of our charnock- 

 ite series. Against the use of the term M pyroxene granulite" for 

 instance, there is (1) the fact that the rocks herein referred to are 

 not all granulitic in structure, and (2) the specific meaning attached 

 to the word "granulite" by the French petrographers, who apply it 

 to an eruptive muscovite-granite poor in mica and approaching the 

 so-called aplites in composition. 1 Against the use of the term 

 "pyroxene-gneiss " there is (1) the fact that the rocks under consi- 

 deration are not always " gneissose ", (2) the circumstance that to 

 many this term would imply a definite geological age and origin 

 which are not proved, and (3) there are many gneisses in India con- 

 taining pyroxene which are not genetic relatives of the charnockite 

 series, and should not therefore be grouped with them. 



Unfortunately our modern systems of nomenclature make no 



provision for indicating petrographical pro- 



Classification by petro- vinces, because no system of rock-classification 



graphical provinces, > J 



makes any pretence towards an expression of 

 genetic relationship between the so-called families. From the purely 

 petrological and hand-specimen point of view, it may be convenient 

 to adopt Rosenbusch's system of dividing rocks into three classes 

 according to the purely accidental circumstances attending their 



1 Michel-Levy. Bull, de la soc. geol., 3rd series, Vol. II (1874), p. 180. 



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