I S J HOLLAND: CHARNOCKITE SERIES, 



and a local N.W. foliation. These garnetiferous types, of which 

 Nagaramalai is a good example, are always accompanied by 

 marginal lenses of pyroxenite (pyroxene-rocks) which often 

 contain small quantities of olivine and hercynite. The minerals 

 of these rocks in the immediate neighbourhood of the Chalk hills are 

 always well schillerized and the garnets contain strongly bi-refringent 

 needles whose characters and regular crystallographic disposition 

 have been described in a separate paper. 1 



Other masses of the basic varieties ( norites ) without garnets 

 occur as lenses of all sizes in the " leaf gneisses " of the Salem- Ahtur 

 valley. They resemble very closely in mineral composition and struc- 

 ture the basic schlieren in the Shevaroys (11-918, 11-923). 



The members of the charnockite series cropping up between 



the two main magnesite-producinar areas of the 

 * Chalk Hills." , . 



Chalk hills form interesting examples of altera- 

 tion by the action of the peridotites intruded into them. The fresh 

 appearance and the blue colour of the normal rock has been changed 

 to a dirty white. This change of colour is seen under the 

 microscope to be due to the development in the felspars of an 

 innumerable number of minute black dots arranged in rows parallel 

 to the twin planes (plate VIII, fig.5). The action has, however, been 

 apparently confined to the felspars ; for the quartz, hypersthene and 

 iron ores appear to be unaltered (No. '9*689) . The change in the 

 felspars is not unlike that which in the neighbourhood of the Giridih 

 coalfield of Bengal appears to be the preliminary form of static 

 metamorphism which on dynamic action results in the production of 

 scapolite. 2 



Tongues of charnockite corroding the older biotite gneiss are 

 exposed 3I miles south of Salem on the Namakal road; these are 

 described in another section of this memoir (p. 226). The dislocation 

 breccias or so-called " trap shotten " bands are also described in a 



1 Holland, " On the acicular inclusions in Indian garnets." Rec. Geol. Surv. Ind., Vol. 



XXIX, i8o6,p. 16. 



2 Cf. Holland and Saise, " On the igneous rocks of the Giridih (Karharbari) coalfield 

 and their contact effects." Rec. Geol. Surv. Ind., Vol. XXVII (1895), p. 123. 



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