I/O HOLLAND: CHARNOCKITE SERIES. 



which they cut a relation precisely similar to that existing between 

 the pyroxenites and norites of Pallavaram (see plate IX). 



The hornblende in these rocks is of two kinds, a blue-green 

 acti nolitic variety occurring as patches in the pyroxenes and evidently 

 secondary, and a dark, brownish-green, basaltic variety occurring 

 both in large crystals and as patches in the pyroxenes. This also in 

 part, at least, appears to be of secondary origin. It has a very deep 

 pleochroism and exhibits a very narrow extinction angle (c : c), 

 which on account of the great absorption, it is impossible to measure 

 with precision. The crystals are often twinned parallel to the ortho- 

 pinacoid (ioo), cross-sections showing both the trace of the twinning 

 plane and the prismatic cleavage very distinctly. 



So far as my experience goes, whenever hornblende becomes 

 a prominent constituent of these ultra-basic representatives of the 

 charnockite series, the associated felspar-bearing types — the norites, 

 that is — have also a conspicuous amount of hornblende amongst their 

 constituents. Besides the instances at Tirrupur in the Coimbatore 

 District, there are good examples illustrating this fact in the South 

 Arcot District. At the south-west base of Vitlapuram hill, for in- 

 stance, we have a good example of a rock composed of pale pyroxene 

 and dark-green hornblende (No. 9*809), whilst a hornblende-augite 

 norite forms the summit of the same mass (No. g'Sio). 1 This fact 

 also supports the conclusion that the ultra-basic rocks are true 

 relatives of those which contain felspar, and that they only differ from 

 the latter merely in the local suppression of the white minerals. 

 Sometimes they appear as basic schlieren ; at other times they form 

 distinct and ramifying veins, which must be regarded as intrusive. 

 But the absence of chilled edges, and the similarity of peculiarities 

 amongst the ferro-magnesian silicates, show that the pyroxenites and 

 amphibolites were intruded whilst the norites were still hot, and also 

 that they have been derived from the same magma as that which gave 

 rise to the norites. 



1 A somewhat similar arrangement of ultra-basic forms with regard to the felspathic and 

 quartz-bearing types is seen in the Shevaroys, the rocks composed of pure ferro-magnesian 

 silicates occur near the foot of the ghat leading to Yercaud, whilst the upper parts of the 

 hill are often as acid as pure charnockite. 



( 52 ) 



