DESCRIPTION OF THE CHIEF TYPES. l6l 



polariser one might very well confuse the pink of the hypersthene 

 with that of the garnet The garnets are often very irregular in their 

 shape and spongy in structure on account of the inclusion of numerous 

 vermiform or otherwise-shaped pieces of white mineral (quartz or 

 el spar) which I regard as the acid bye-product separated during the 

 break-up of the pyroxene to produce garnet. Sometimes the garnets 

 form a sort of corona to the hypersthenes, and all stages are found 

 from a narrow ring around the pyroxene to a complete broad ring of 

 garnet surrounding a core of granular quartz (Plate VIII, fig. 6). 



In common with all the rocks in the neighbourhood of the Chalk 



Hills the rocks of Nagaramalai are schillerized. 



Schiller phenomena and The most interesting form of schillerization is 



acicular inclusions. 



displayed by the garnets, which contain numerous 

 needles possessing a high double refraction with a wide extinction 

 angle up to as much as 39 . These needles are arranged with a 

 remarkable regularity of crystallographic disposition within the 

 garnets, as described in a previous paper.' Needles apparently 

 similar in crystallographic disposition and somewhat similar in optical 

 characters were noticed by Marker in the garnets of an eclogite from 

 Port Tana, N. Norway, and were referred to as kyanite on account 

 of the wide extinction angles which they exhibited. 3 In the garnets 

 of a pyroxene-granulite found near the peridotite of Elliott County, 

 Kentucky, Diller found what appears to be similar inclusions, which 

 he says are arranged at angles of 45 to one another, and are dis- 

 tinctly monoclinic with a maximum extinction angle of 30 . 8 



Lacroix, on the other hand, has referred to rutile regularly 

 arranged needles in the garnet of rocks which he has described as 

 basic pyroxenic and hornblendic gneiss from Salem and Ceylon, 4 



1 Rec. Geol. Surv. Ind., Vol. XXIX (1896), p. 16. 



2 Geol. Mag,, 3rd decade, Vol. VIII (1891), pp. 170, 171. In his " Petrology for Students " 

 (p. 300), Harker refers to these needles as rutile, although in his original paper he says they 

 exhibit extinction angles up to about 31 , whilst rod-like, reddish-brown crystals of rutile 

 were found in the same rock, which I find to be the case also with the rocks of Nagaramalai. 



3 Bull. U. S. Geol. Surv., No. 38 (1887), p. 27. 



4 Rec. Geol. Surv. ind.,\o\. XXIV (189O, p. 176 ; translation from Bull, de la Soc 

 Min. de Fr. t Vol. XII (1889), p. 3". 



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