364: Geological S octet//. 



The Lecturer then described the map of Mysore which, on a 

 scale of 8 miles to the inch (1 : 506,880), presented a simplified 

 summary of the work of the Mysore Geological Survey. On 

 lithological grounds the Dharwar System was divided into an 

 Upper and a Lower Division. The former was composed largely 

 of basic flows and sills with their schistose representatives. 

 Whether some of the chloritic schists, slates, phyllites, and argil- 

 lites were of sedimentary origin was still doubtful. In the series 

 as a whole, chlorite predominated and hornblende was subordinate. 

 The presence of carbonate of lime, magnesia, and iron was a 

 strikingly prevalent feature. The Lower Division was composed 

 of dark hornblendic epidiorites and schists, which were distinguish- 

 able from the greenstones of the Upper Division by their dark 

 colour and practical absence of chlorite. Many of the greenstones 

 and schists of the Upper Division appeared to resemble Keewatin 

 rocks of Lake Superior, such as the Ely Greenstone series (save 

 that augite is conspicuously absent in the Mysore rocks), and it 

 had been suggested that the dark epidiorites, which naturally crop 

 out between the rocks of the Upper Division and the intruding 

 gneisses, might be merely metamorphosed portions of the green- 

 stones and chlorite -schists. This might be true in some cases, but 

 the independent existence of the dark hornblendic rocks of the 

 Lower Division was supported by the fact that they do not exist 

 in many places where the gneisses come into contact with the 

 greenstones ; that many of the former retain original igneous 

 structures, which would be unlikely to survive the chloritization 

 and the subsequent change to epidiorite; and, finally, that the 

 amphibolitization of the rocks of the Lower Division appears to 

 have been complete before the intrusion of the earliest of the 

 gneisses which, with its associated pegmatites and quartz-veins, 

 has developed secondary augite in the hornblendic rocks along 

 intrusive contacts. 



The Lecturer referred briefly to the autoclastic conglomerates 

 which were usually associated with intrusions of the Champion 

 Gneiss, to the intrusive character of some of the quartzites or 

 quartz-schists, and to the evidence that the limestones were, partly 

 if not wholly, due to metasomatic replacement of other rocks by 

 carbonates of lime and magnesia. 



The Dharwar schists of Mysore contain a widely extended series 

 of banded quartz iron-ore rocks, very similar to those of the Lake 

 Superior district, the origin of which has been the subject of 

 much discussion, and is still very perplexing. -Some of the earlier 

 American geologists considered them to be directly igneous in 

 origin, but these views are now discredited, and replaced by an 

 interesting and ingenious theory of chemical precipitation from 

 liquids associated with subaqueous lavas. The Lecturer suggested 

 that some of these rocks might be pegmatitic intrusions of quartz 

 and magnetite, and that some might be the metamorphosed relics 

 of igneous rocks composed, largely, of highly ferruginous amphi- 

 boles (such as cummingtonite) or other chemically allied minerals. 



