36 



HOI-LAND : MICA DEPOSITS OF INDIA. 



rocks of the so-called upper division of the Archaean group. Into 

 these schists the pegmatites have been intruded, generally along, but 

 sometimes across, the folia, in the form of thin sheets, lenticular 

 bodies, or large, thick, bosses (Plate VIII and figs. 8 and 9). The 

 common disposition of the mica-bearing pegmatites in sheets seems 

 to have. been entirely overlooked by the miners in India, and ignorance 



Fig. 9. Section of pegmatite vein in mica schists, Sakri river, HazdribAgh 

 district {after Mallet). 



of this fact is the principal cause of the exceedingly wasteful and 

 primitive system of mining now being practised under European as 

 well as Native management (see Chap. VI). 



The general tendency of the pegmatite sheets 1 to follow the 

 planes of foliation is probably due to the fact that this is the 

 direction in which schistose rocks are more easily disrupted. Occa- 

 sionally the pegmatite sheet changes its direction by following a fault 

 plane before resuming its direction parallel to the schist folia. Where 



Fig. 10. Thickening of pegmatite vein at a fault in the schists, Gdvian, 

 HazAribdgh district. 



1 The term "sheet " is more expressive than "vein," which gives one the 

 idea of a more cylindrically shaped body. This appears to be the idea in the 

 minds of the miners in the Behar area, who, by following the mica from " book " 

 to " book," have made worm-like, tortuous excavations dignified in common tain 

 by the name " mines ". Reyer uses the term Blatt (pi. Blatter), leaf or sheet, 

 instead of the commoner term Gang (pi. GSnge), vein (Theoretische Geol., l838). 

 ( 26 ) 



