CLASSIFICATION OF THE ROCKS. 115 



recognised in the hand specimen, also occur intimately inteigrown 



with one another, in consequence of which the hematite is masked 



and the whole grain appears to be magnetic. This accounts for the 



fact that these ores have hitherto been described simply as magnetite. 



Knowing that hematite is transformed into magnetite by 



heat, the discovery of the two minerals together 



Suggested origin of the [ n the well known ore-beds of South India 



magnetite. 



suggests the natural inference that they repre- 

 sent a stage in the thermal metamorphism of hematitic quartzites, 

 similar, for instance, to those which are well known further north as 

 members of the Dharwar system. Indeed, there are so many points 

 of correspondence between the ore-beds of south Salem and those 

 of the Dharwars that one naturally enquires into the possibility of 

 the former being merely highly metamorphosed outliers of the latter. 

 The differences between the two are just those which we should 

 expect to result from an advance in the degree of thermal metamor- 

 phism : the Salem magnetic beds are coarser in grain and generally 

 more perfectly crystallized than those of the Dharwars ; the hema- 

 tite has been more completely changed into magnetite, 1 whilst the 

 griinerite represents in the southern beds the chloritic minerals of 

 the Dharwar schists. Knowing now that hematite exists side-by- 

 side with magnetite in most, if not in all, of the magnetic ores of 

 the south, the differences in the condition of the iron-ores become 

 merely differences of degree, not of kind. 



Having no fossils in any of the Peninsular transition rocks, 

 the correlation of isolated exposures of the Dharwars rests purely on 

 penological similarities. If, then, these outliers of ore-beds differ 

 from the typical Dharwars merely in degree of alteration, the reasons 

 for excluding them from the Dharwar system must rest on the very 

 slender assumption that the degree of metamorphism is a safe 



1 I have recently observed much magnetite also in many of the Dharwar hematitic 

 quartzites. 



( 13 ) 



