I98 HOLLAND: SIVAMALAI SERIES. 



i (*). Basic lenses with basaltic hornblende and calcite. 



At several places in Sivamalai and especially near the summit, 

 black, fine-grained patches occur, generally with a roughly lenticular 

 shape and presenting an approach to foliation by linear disposal of 

 the constituents. By microscopic examination these are found to 

 be composed principally of a brown, barkevikitic, hornblende, with 

 subordinate quantities of elaeolite, orthoclase, plagioclase, calcite, 

 biotite, purple augite, graphite, iron-ores and sphene. The structure 

 is generally granulitic, which is probably due, as usual, to the break- 

 ing up of complicated intergrowths by movement during the process 

 of consolidation ; sometimes in fact the hornblende has been pre- 

 served in its original form, showing a crystallographic parallelism in 

 several isolated sections, and producing a micropoikilitic structure, 

 whilst at other times the hornblende is gathered into groups of 

 variously orientated granules, which is probably due to the mere 

 breaking up of the delicate poikilitic or ophitic systems. I believe 

 this to be very often, if not constantly, the cause of the granulitic 

 structure, as well as the tl blotchy " character, of these rocks, which 

 form slightly foliated lenses in the crystalline schists. In the 

 pyroxene-granulites (charnockite series), for instance, of South India 

 the pyroxenes are frequently so found to form groups of granules 

 of one species, whilst less often the optical continuity of isolated 

 neighbouring sections has escaped destruction. 



The hornblende is deep-brown in colour and intensely pleo- 

 chroic: a= straw-yellow; b and c = deep-brown, almost black, 

 through strong absorption. The biotite is brown and often forms 

 large plates stretching across three or four of the ordinary granules 

 of its fellow constituents. The purple augite, which is so often 

 found in the elseolite-syenites, occurs in some specimens of this rock 

 in very small quantities, but in one specimen a pale-green, slightly 

 pleochroic augite occurs abundantly. The presence of sphene, too, 



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