PETR0GRAPH1CAL EVIDENCE IN SOUTH INDIA. 227 



dark patches of micaceous mineral which in the biotite-gneiss 

 indicates its foliation. The charnockite veins look, therefore, as if 

 they were pseudomorphs very imperfectly retaining the structures 

 of the rock they have corroded and replaced. Although, therefore, 

 there is little doubt about the charnockite having trespassed on the 

 gneiss after its consolidation and crushing, the precise nature of this 

 trespass is less easily defined : the interlocking of constituents at the 

 junction-line, the absence of chilled selvages in the charnockite, the 

 imperfect pseudomorphism of the banded structures are points which 

 distinguish this "trespass" of the charnockite from the more usual 

 kind of simple intrusion of one rock along fissures in an older neigh- 

 bour. The tongues of charnockite, however, radiate from a large 

 mass of the same rock and enter the gneiss without regard to the 

 direction of the foliation planes in the latter As I can recall no 

 other case precisely similar to the one now described, we have no 

 previous experience to guide us to a safe interpretation of the facts ; 

 and the only explanation I can offer is that the intrusion occurred at 

 great depths when the biotite-gneiss, being also at a high temper- 

 ature, became corroded by the molten charnockite. Another explan- 

 ation might be offered by those who hold u!tra-metamorphic ideas, 

 namely, local alteration and refusion of the biotite-gneiss. In view 

 however, of the fact that the two rocks are totally different in 

 mineralogical and chemical composition, this alternative theory seems 

 to be unnecessary and less likely ; besides, the way in which the 

 tongues proceed from the large adjoining mass of charnockite, and 

 the clear marks of contact action impressed on the old biotite 

 gneiss 1 indicate that there has been a distinct trespass by the 

 charnockite, although the action is not precisely of the kind we are 



1 A gneiss extremely like this biotite-gneiss occurs as pebbles in the Dharwar conglome- 

 rates of the Kolar gold-field. This indicates that the gneiss is older than that stage of the 

 Dharwars. When we know more about these conglomerates it is likely that some valuable 

 negative evidence will be obtained ; for I feel very much inclined to believe that, among 

 the rocks which we have grouped together as Archaean gneisses, there are formations very 

 widely differing in age, and careful work should enable us to discriminate between older 

 and younger, between sedimentary and igneous. 



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