PETROLOOICAL NOTES ON FURTHER ROCK Sl'Ki I MKNS. 



329 



Tlie usual occurrence of pyroxene in rocks whose nature is definitely not acidic 

 obscmvs this two-fold mode of its alteration. It is only in this so-called " Charnockite 

 Scries " that much pyroxene is found in definitely acidic rocks. It is, then, possible to find 

 the metamorphic derivatives of pyroxene in acid surroundings in this suite only. Even 

 so, the usual slight metamorphism which rocks of this suite from other regions have 

 suffered has permitted Washington (op. tit., p. 335), after citing the occurrence of the 

 cliariKK kite rocks from Norway, Ellesmere Land, New York and West Africa, to say that 

 " biotite is rare or accessory and quite absent in most of the types." That this is not 

 necessarily so in the metamorphic representatives of the suite is seen from the Rosiual 

 analyses of the " Intermediate Charnockites " Nos. 775, 783, where 5-4 per cent, and 

 13-9 per cent, of biotite is present. The amount of pyroxene in each of these rocks is 

 32-4 per cent. The same reaction is shown more powerfully by the following table 

 where the volumetric compositions of two metamorphic derivatives (Nos. 778, 787) 

 of " hypersthene granites " are given : 



I. Rock No. 778. 

 H. Rock No. 787. 



III. Specimen (9-658) of the Indian Survey. Occurs in central part of Magazine 

 Hill, St. Thomas Mount, eight miles south of Madras. (Washington, op. cil.) 



It will be seen that in each of these three rocks no hornblende has developed, 

 but there is an appreciable quantity of biotite, especially when referred to the 

 hypersthene in No. 787 it is nearly twice as abundant. 



It can be concluded then that amphibolization of the pyroxene is the characteristic 

 alteration of this mineral in basic (and related) rocks, but that the formation of biotite 

 is the characteristic metamorphic process in rocks which have original pyroxene and 

 yet are acidic. Such rocks are the so-called hypersthene-granites and diorites and it 

 must be among the comparatively rare metamorphic derivatives of these rocks that 

 we must look to see the effects which have been suggested above. The rocks here 

 described from Madigan Nunatak seem to be among the most interesting from this 

 point of view. 



