l86 HOLLAND : SIVAMALAI SERIES. 



most acid phase, being the consolidation product of the more silice- 

 ous mother-liquor; in this elaeolite-syenite the processes of differ- 

 entiation have tended to a concentration of the alkalies, and so the 

 contemporaneous veins now under consideration represent the 

 highly alkaline mother-liquor, containing not less than 16 percent, 

 of soda and potash. 



The weathered surfaces of these coarse veins in the elaeolite- 

 syenite very strikingly show the difference between the elaeolite and 

 the felspar in their resistance to the action of the atmosphere : the 

 surfaces of the former are depressed, often quite a quarter of an 

 inch below the latter, and the decomposition products which fill 

 in cracks in the elaeolite stand out like a network of minute strings. 

 Rounded boulders of pure elaeolite weighing 2 to 4 lbs. are frequently 

 found lying in the debris at the foot of the hills. 



Besides the elaeolite, microperthite, biotite, graphite and iron- 

 ores, a crystal of white mica was found in one of these coarse veins ; 

 it was hexagonal in shape, measured half an inch across the basal 

 plane, and showed the wide angle and correct position of the optic 

 axial plane for muscovite. This was the only specimen of muscovite 

 found in these rocks, but the rarity of this mineral in the elseolite- 

 syenites makes it worthy of record. 



The large size of the crystals of el-geolite facilitates the ex- 

 amination of their properties. The mineral shows a very imperfect 

 basal cleavage which, however, is sufficiently pronounced to obtain 

 flakes showing the negative character of the uniaxial figure in con- 

 vergent polarized light. In the small hills lying west-north-west of 

 Sivamalai the elaeolite is of a yellowish-brown colour ; but in Siva- 

 malai itself it generally has a pink tinge. The differences in chemical 

 composition between these two varieties has not been tested, but to 

 all other tests they present similar reactions. 



The specific gravity is 2*62, all determinations being in very 

 close agreement, which might be expected from the freshness of the 

 material. 



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