16a HOLLAND: CHARNOCKITE SERIES. 



These needles, he says, exhibit a positive double refraction and give 

 straight extinctions. It is now practically certain, however, that 

 the particular rocks referred to in this connection came from 

 the immediate neighbourhood of Salem town and are very similar in 

 composition to those occurring at Nagaramalai, which, as already 

 stated, contain inclusions with quite different physical properties. I 

 have found very similar acicular inclusions in the garnets of the char- 

 nockite series in other parts of the Madras Presidency, particularly on 

 the southern flanks of the Nilgiri Hills, but in all cases the needles 

 show a wide extinction angle. 



It is impossible to determine with certainty the mineralogical 

 nature of needles so exceedingly minute. Harker referred those in 

 the Port Tana eclogite to kyanite, a mineral which has frequently been 

 found in eclogites, and the narrow extinction angles which kyanite dis- 

 plays in brachypinacoidal sections would make it a difficult matter to 

 distinguish small needles of the mineral from monoclinic crystals, as 

 they have been considered to be by Diller and by myself. Having 

 obtained extinction angles up to as much as 39, ° I favoured the idea 

 that the needles might be sphene. The fact that they are often black 

 and opaque for a portion of their whole length might then be due to 

 replacement by ilmenite. 



It is certain, however, that they are not rutile, although 

 the hair-like inclusions in the blue quartz or the charnockite 

 series may be so nevertheless, as they invariably show straight extinc- 

 tions in horizontal sections of the quartz {vide supra, p. 138). Titanic 

 acid introduced into quartz might remain pure and might crystallize 

 as rutile needles ; but the same substance introduced into a garnet 

 might be changed to a titanate of some protoxide. It is not without 

 interest that the plates and rods which give the schiller appearance to 

 hypersthene, and which are very abundant in the hypersthenes of the 

 Nagaramalai rock, have also been referred to titanic acid by Kos- 

 mann, Tornebohm, Brogger and others, although the different authors 

 are not agreed as to the precise origin of the inclusions. Whatever 

 they are in composition— and there is little doubt about the fact that 



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