178 HOLLAND: SIVAMALAl SERIES. 



west-north-west of it the prevalent form of elaeolite-syenite 

 weathers into tors of pale yellowish -brown blocks. The superficial 

 removal of the easily decomposed elaeolite has resulted in the forma- 

 tion of pitted surfaces, which form a characteristic feature in the 

 weathering of these rocks. The discolouration produced by atmos- 

 pheric agents is found to be absolutely superficial, a new fracture of 

 even small stones showing the grey colour of the fresh rock. The 

 rock is remarkably even in grain, with crystals measuring up to 2 or 

 3 mm. in diameter. The foliation is always apparent even in hand- 

 specimens, but is due merely to a linear arrangement of the con- 

 stituents, especially of the noticeably inequiaxed crystals like the 

 biotite and graphite. With a lens it is easy to distinguish the grey 

 felspar from the oily-yellow elaeolite grains, and the graphite, though 

 never in large quantities in this variety, can be distinguished from 

 the biotite, and may be removed by a needle for special examination 

 (Specimens of the rock, No: 11,460). 



The average specific gravity of the rock is 2'5g3. 



Excepting the graphite, which occurs in little six-sided plates, 

 and possibly the opaque iron-ores, it is impossible under the micro- 

 scope to decide the relative ages of the constituent minerals : in 

 common with most of the rocks which form these curious lenses in 

 the crystalline schists, the prevalent type of elaeolite-syenite in 

 Sivamalai is granulitic in structure, and the crystals have so inter- 

 fered with one another that none of them can be said to show idio- 

 morphic outlines. There is occasionally a sign of hypidiomorphic 

 structure, but generally the crystals are panidiomorphic. 



The chief constituents are elaeolite and felspar with biotite 

 as the principal, generally the only, ferromagnesian silicate, graphite 

 and opaque iron-ores. These are the usual constituents of the most 

 prevalent grey gneissose form on Sivamalai, but in some sections 

 there occur zircon, calcite and, very rarely, hornblende (Cf. ic and 

 id). Generally the biotite distinctly exceeds the graphite in quantity. 



The elseolite crystals always show numerous fracture cracks 

 ( 10 ) 



