AUC1TE-SYENITT. 199 



is noteworthy on account of its general absence from the other varie- 

 ties of this area. 



The white constituents are about evenly divided in (jiiantity. 

 The orthoclasc is without the microperthitic structure so commonly 

 seen in these rocks ; the plagioclase agrees in character with that 

 already described, and appears to be albite. The elxolite is per- 

 fectly fresh and wholly resembles that already observed in the 

 ordinary types. Calcite forms isolated granules, generally well 

 twinned into lamellae. 



The iron-ores and graphite exist only in very small propor- 

 tions. 



2. — Augite-syenite. 



As is so commonly the case with elaeoliteTsyenite, the rock 

 of Sivamalai is accompanied by an augite-syenite, which resembles 

 Brogger's laurvikite of the well-known elaeolite-bearing area of 

 South Norway in containing olivine. 



This augite-syenite forms a hill composed of red, rounded 

 masses of rock lying to the north-east of Sivamalai, and in a more 

 coarsely crystallized form at the north-east foot of Sivamalai, that 

 is, between the main mass of elaeolite-syenite and the hill composed 

 of the finer-grained form of augite-syenite. Both the coarse and 

 the fine-grained varieties are dark-green in colour when freshly 

 fractured, and dark-red on the surfaces exposed to the weather. 

 They are essentially the same in mineral composition. 



The constituents are, in approximate order of abundance, felspar, 

 chiefly microperthite, green non-pleochroic augite, opaque black 

 iron-ores, olivine, zircon, greenish-brown hornblende, hypersthene, 

 apatite and biotite. No trace of elaeolite has been found. These 

 rocks, in common with the elaeolite-syenite, show a very imperfect 

 foliation, and as usual the ferromagnesian silicates display a ten- 

 dency to gather into glomero-plasmatic groups. The hornblende 

 apatite and iron-ores occasionally show idiomorphic outlines, but 



l 3» ) 



