DESCRIPTION OF THE CHIEF TYPES. 157 



a feature which is never shown by the pleochroic pyroxene that 

 I consider to be rhombic (vide supra, p. 126). 



The crystals of plagioclase are always remarkably clear and 

 fresh. They are twinned both after the albiteand the pericline plan. 

 Measurements of the angles between the positions of extinction of 

 adjacent lamellae in sections across the albite twins seldom vary much 

 from 29 ; so the felspar approaches labradorite(Ab, An) in compo- 

 sition. 



The crystals of felspar which show no lamellar twinning appear 

 sometimes as Carlsbad pairs, and thus probably indicate the pre- 

 sence of potash felspar ; but I have never found a trace of micro- 

 cline in the norites whose specific gravity exceeds 3. From the abun- 

 dance of microcline in charnockite one would expect the same struc- 

 ture to appear also in the accessory potash-felspar of the norites, but 

 extended search has so far been unsuccessful, and in view of the fact 

 that plagioclase occasionally shows no twin lamellae, the evidence in 

 favour of orthoclase must be looked upon with suspicion. 



The opaque iron-ores are always abundant in the norites, 

 much more so than in the associated pyroxenites. Sometimes they 

 are pyritous; but magnetite, often titaniferous, is the prevailing 

 variety. I have found no green spinel in the norites near Pallavaram, 

 although it is abundant in the non-felspathic associates. 



Quartz often occurs intergrown with the other constituents. 

 The quartz-bearing veins show no sharp junction lines with the norites 

 amongst which they ramify, the crystals interlocking across the 

 junction after the manner of segregation and contemporaneous veins. 



Apatite^ in short prisms, is always more abundant in the norites 

 than in the associated charnockite. 



An occasional granule of zircon is found in almost every slide. 

 So far as my experience goes, sphene never occurs in the unaltered 

 varieties of norite. 



Hornblende- Augite Norite, 



The most prevalent variety of the basic forms of the charnockite 

 series is one in which hornblende, as well as augite and hyperi 



( 39 ) 



