DESCRIPTION OF THE CHIEF TYPES. 159 



hypersthene, and in crystallizing at such an early stage has apparent- 

 ly taken up most of the iron ; for both the rhombic and the monoclinic 

 pyroxene are most unusually pale, almost colourless in fact, in 

 section. Another point which marks off this rock as an abnormal 

 form is the presence of a considerable quantity of sphene, the titanic 

 acid being in normal members of the charnockite series characteris- 

 tically confined to ilmenite. The quantity of opaque iron-ore in 

 this rock is consequently much smaller than in the normal norites, 

 and most of it is in the form of the sulphides, pyrite and 

 pyrrhotite. 



A distinct mass of this rock occurs about one mile east of the 

 railway station near Pallavaram (No. 9*671). Specimens have an 

 average specific gravity of 296, and are composed of a pale enstatite 

 with very faint pleochroism, colourless augite, plagioclase, a little 

 quartz, magnetite, pyrite, pyrrhotite, apatite in numerous short prisms, 

 brown pleochroic sphene and a deep-brown biotite with strong 

 pleochroism and very narrow optic axial angle. The pyroxenes show 

 a marked tendency to ophitic development around the other minerals 

 which is another feature quite exceptional in the charnockite series. 

 Although the association of this rock with the normal members of 

 the charnockite series, and the presence in it of considerable quanti- 

 ties of rhombic pyroxene, necessitate its inclusion in the series, I am 

 unable at present to recognise any peculiar conditions in its occur- 

 rence which would account for the distinct departure from the nor- 

 mal characters which it shows under the microscope. 



A rock on the western margin of Pammal hill, west of Palla- 

 varam (No. 9*675), shows by the large quantity of biotite in it an ap- 

 proach to this strange variety ; but it agrees nevertheless more nearly 

 with the normal type in the absence of sphene, and in the more 

 marked pleochroism of its rhombic pyroxene. This rock also con- 

 tains a large proportion of quartz and is situated, apparently forming 

 a passage, between normal augite-norite, which forms the central 

 mass of the hill, and a garnetiferous, more acid, form, also with bio- 

 tite, forming the north-western margin of the same mass.~ 



( 4i ) 



