126 HOLLAND: CHARNOCKITE SERIES. 



Plagtoclase. — In the basic types generally approaches labra- 



dorite or labradorite-andesine in composition. 

 Augite* — A pale green, feebly pleochroic variety, frequently 



exhibiting lamellar twinning. 1 

 Hornblende, — A brown-green, highly pleochroic variety with 



an extinction angle of about n°, 

 Btotzte.— -Very variable in quantity, appears in most expo- 

 sures. 

 Graphite. — Evenly distributed like the other constituents, 

 though smaller in quantity, and found in two expo- 

 sures only. 

 Zircon and apatite are nearly always present in small 



quantities. 

 Iron-ores, magnetite and titanoferrite are invariably pre- 

 sent and generally in large quantities ; pyrite and 

 pyrrhotite are often developed near the junctions 

 of two distinct varieties. 

 Except in one aberrant form presenting several peculiar and un- 

 usual features sphene is absent. The occurrence of the titanic acid 

 in the form of ilmenite instead of as sphene is a feature which dis- 

 tinguishes these rocks from some associated gneisses, and also separ- 

 ates the normal from the altered forms. 



1 Lacroix (Rec. Geol. Surv., India, XXIV, 173) mentions the occurrence, in the " pyroxene 

 gneisses" of Salem, of a monoclinic pyroxene with a pleochroism similar to that of 

 hypersthene, namely, *=sea green, b= bright pink and » = yellowish green, with an extinc- 

 tion angle of 45° on the clinopinacoid (010). It may be stated at once that, although I have 

 examined hundreds of cases from the Salem district and from all parts of the Madras Presidency, 

 I have never yet found a pyroxene in these rocks giving the pleochroism of hypersthene 

 without at the same time, when definite cleavage lines are exhibited, showing a straight 

 extinction . At the same time the commonest of all the types of these rocks is one in which both 

 pyroxenes occur together; the one strikingly pleochroic. and unmistakeably rhombic in its 

 crystallization, whilst the other is very feebly pleochroic in greens only, giving wide 

 extinction angles. 



The green colour of the hypersthene, however, so nearly resembles that of the monoclinic 

 pyroxene, that without moving the polariser the similarity of refractive index and crystal 

 habit might, In a hasty examination, result in a confusion of the two forms. As the rocks des- 

 cribed by Lacroix so remarkably resemble in other respects those which I include in the 

 series now under description, I have very carefully searched every specimen in the extensive 

 collection made by my colleagues and myself j and have to confess my inability to discover a 

 single instance of such a pleochroic monoclinic pyroxene, 



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