40 HOLLAND: MICA DEPOSITS OF INDIA. 



pegmatites were formed by the direct alteration of the schists ; more 

 probably the circumstances under which the pegmatite magma was 

 injected permitted the permeation of the surrounding rocks with 

 vapours which favoured their metamorphism with the formation of 

 mica, and possibly also led to a modification locally in the composition 

 of the pegmatite. 



" Country " rock. 



Pegmatites carrying valuable mica are not as a rule found travers- 

 ing massive gneisses and granite. In India the only occurrences of 

 value are found in the very composite group of schists, which are 

 generally referred to as the upper division of the Archaean crystalline 

 rocks, and are thought by some to be younger than the massive fels- 

 pathic gneisses. The following types have been definitely determined 

 in the " country " of Indian mica-bearing pegmatites : — 



Compact quartzite. 



Coarse granular quartz-rock with muscovite scales. 



Quartz-biotite schists. 



Quartz-schists with mica and kyanite. 

 Ditto with fibrolite. 



Ditto with iron- ore. 



Fibrolite-gneiss. 



Chiastolite-mica schists. 



Quartz-epidote gneiss. 



Ouartz-biotite-hornblende rock with large lumps of magnetite. 



Epidiorite. 



Hornblende-schist with garnets. 



Ditto with scapolite. 



Pyroxene-granulite. 

 Granulite (leptynite). 

 Diopside-gneiss with sphene. 

 Anthophyllite-rock. 



Garnetiferous biotite-gneiss (biotite-granulite). 

 Ditto with octahedral magnetite. 



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