EL/EOLITE-SYENITE. I 83 



safe application of the data obtained by the analysis of the large 

 crystals of microperthite in the coarse-grained veins. But even by 

 assuming that the microperthite has a constant composition through- 

 out, a calculation based on its potash gives a result which is not 

 far removed from an estimate made by microscopic examination of 

 sections. Deducting, for instance, the amount of potash from 

 the rock analysis due to 34 per cent, of elaeolite, we have 

 5* 16 — 1*88 = 3*28 due to the microperthite, which would contain 

 nearly all the remaining potash of the rock. Assuming the analysed 

 felspar to be the average type of microperthite in the rock, this 

 amount of potash would indicate 3*28 x ioo-r 5*86 = 56 per cent, 

 microperthite. As there is always a certain amount of ordinary 

 plagioclase in the rock, the soda and lime determinations cannot be 

 used to check this result ; the total given for soda and lime, however, 

 exceeds that which would be contained in this amount of micro- 

 perthite, which is consistent with the fact that a small quantity of 

 plagioclase is known to replace some of the microperthite amongst 

 the felspars. 



The estimates thus obtained for the mineral composition of 

 the ordinary, medium-grained, grey form of elaeolite-syenite are as 

 follows :— 



Per cent. 

 Elaeolite 34 



Micrcperthite • . • ■ • • • . 56 

 Magnetite ........ 3*6 



Other constituents, biotite, plagioclase, graphite, etc. • 6*4 



IOO'O 



So far as can be judged by microscopic examination of sections, 

 these results are not far removed from the truth ; so the remarks 

 made below as to variations in the mineral proportions of the five 

 separate varieties may be taken to apply to departures from this 

 estimated composition of the ordinary and most common variety in 

 Sivamalaii 



( 15 ) 



