EI./E0LITK-SYEN1TE. TQI 



• 

 "reaction rims" which are so often found separating olivine and 



plagioclase in the olivine-norites. In the present instance, however, 

 the reaction borders are more irregular and consist of one mineral 

 only, which, nevertheless, is probably the result of the orthoclase 

 having stolen all the potash from its immediate borderland and 

 having left a u crystal-court " residue which has consolidated as 

 pure plagioclase. These strips of plagioclase, though very common, 

 are by no means constantly found separating the elaeolite and ortho- 

 clase crystals. They are more constantly seen, for instance, when 

 orthopinacoidal sections of the orthoclase adjoin elaeolite crystals, 

 and are often not discoverable when the orthoclase section shows its 

 microperthitic structure. But this of course may be due to twinning 

 of the plagioclase strips according to the albite law, in which case 

 such sections would not always show the lamellar twinning. 



The coloured minerals, which occur in relatievly small quantities 

 in the contemporaneous veins, do not differ essentially from those 

 which occur more abundantly in the fine-grained, grey rock — large 

 flakes of graphite, dark-green biotite and lumps of magnetic iron- 

 ore being the usual accessories. 



The lumps of the common fine- grained type occasionally found 

 in the contemporaneous veins are merely pieces of the ordinary rock, 

 separated from the general mass and caught in the mother-liquor 

 which formed these coarse-grained veins. If the contemporaneous 

 veins were regarded as subsequent and distinct intrusions of a 

 different composition, these " horses " of fine-grained rock would be 

 regarded as xenoliths ; but it seems unnecessary to regard them as 

 "foreigners " when it is so very evident that the coarse and the fine- 

 grained elaeolite-syenites are derived from the same magma and 

 represent merely two phases in the general consolidation of one great 

 eruptive mass. They are consequently regarded as true autoliths. 



At the summit of Sivamalai another form of the rock occurs as 

 veins. These consist principally of granular grey microperthite and 

 pink elaeolite, but also with crystals of the biotite, often the green 



( 33 > 



