210 HOLLAND: CHARNOCKITE SERIES. 



CHAPTER VIII. 

 Petrographical Evidence in South India. 



The attempt to settle the origin of the charnockite series by cor- 

 relation with its foreign equivalents leads, therefore, to results which 

 are not conclusive. In the first place, they present certain similarities 

 to rocks whose eruptive origin is now considered to be established, 

 such, for instance, as the norites and related rocks of Scandinavia, the 

 anorthosites of Canada and the norites of some parts of the United 

 States. In the second place, they present a much more perfect 

 resemblance to the pyroxene-granulites of Saxony, concerning the 

 origin of which very diverse opinions have been published, whilst 

 Lacroix has correctly indicated the resemblance they bear to the 

 French pyroxene-gneisses. 



The origin of the charnockite series must consequently be settled 

 purely by evidence obtainable on the spot, and without regard to the 

 results obtained for mineralogically similar foreign rocks. In dealing 

 with a comparatively young and only slightly altered formation, one 

 would naturally look to the local and direct evidence first as the least 

 complicated and most straightforward order of procedure ; but with 

 the ancient crystalline rocks, conclusions based on analogy are 

 generally our best, sometimes unfortunately our only, evidence as 

 to origin. The processes of metamorphism tend to reduce the 

 points of difference between rocks of diverse origin, and the conse- 

 quent tendency for them to become similar to one another is real as 

 well as apparent. Some rocks, like the limestones and sandstones, 

 may successfully resist any changes but those of a purely structural 

 character, and consequently are not difficult to distinguish from 

 recks of igneous origin ; but others, like the shales, and especially the 

 impure shales, may, after metamorphism, imitate igneous types too 

 perfectly to permit safe identification ; consequently a large number 



c 92 ) 



