15 8 HOLLAND: CHARNOCK1TE SERIES. 



sthene, are the ferromagnesian constituents. Rocks of this kind are 

 represented especially well in the hill-mass east of the railway 

 station at Pallavaram (Nos. 8752, 9*661, 9*667) and also in the 

 Pammal hill, west of the railway (No. 9*395). 



Thehypersthene, augite, felspar and accessory minerals are appa- 

 rently of the same kind as those already described as constituents of 

 the augite-norite obtained in the neighbourhood of St. Thomas' 

 Mount, 



The hornblende is the peculiar deep green-brown, highly 

 pleochroic variety which seems to be characteristic of these rocks 

 and of their ultra-basic associates. Its crystals are often larger than 

 those of the augite and hypersthene, sometimes giving the rock in 

 consequence a feeble porphyritic aspect on weathered surfaces. 

 The crystals are generally elongated parallel to the vertical axis and 

 in cross-sections the characteristic cleavage is extremely well marked. 

 The pleochroism is — 



a==pale yellow to bright yellow. 



b=brown. 



c=brownish green. 



The absorptions for fe and c are about equal, both being much 

 greater than for rays vibrating parallel to a. The extinction angle 

 {c : c ) is very narrow, and, on account of the great absorption, 

 difficult to measure precisely, 



Biotite- Augite Norite. 

 The basic rock described under this name forms an excep- 

 tional type amongst the charnockite series and appears to be very 

 limited in its distribution. Biotite is extremely common as an ac- 

 cessory amongst the intermediate and basic members of the series, 

 but it is generally small in quantity compared with the hypersthene, 

 augite and hornblende, and from its intimate 



Aberrant form of norite. 



association with the hornblende appears to be, 

 in part at least, of secondary origin. In this particular rock, however, 

 it presents its crystal 'Outlines against both the augite and the 

 t 4o ) 



