20 



The order of crystallization is obscure. There are traces 

 of graphic intergrowth of quartz with plagioclase and with 

 microcline. Quartz is mostly wrapped round by both the 

 felspars; occasionally plagioclase is idiomorphic against it but 

 never microcline. The latter appears to have been the last 

 mineral to cease crystallizing. 



The chemical composition of the rock is given in column i. 

 below 





I. 



II. 



SiO, 



• 75-48 



75-57 



A1363 



12-99 



13-14 



Fe,03 



0-25 ■ 



0-63 



FeO 



0-85 



1-04 



MgO 



0-13 



0-14 



CaO 



0-74 



abs. 



Na^O 



2-31 



2-96 



K,0 



6-06 



6-06 



h;o + 



0-60 



0-46 



h;o- 



0-17 



0-12 



Tib, 



0-12 



0-08 



CO2 



— 



abs. 



P2O. 



— 



0-06 



Other 



constit. — 



0-10 



Total ... 99-70 



Sp. gr. atl8°C 2-615 



Even -grained granite, Port Elliot. 



100-36 



II. 



Anal. W. R. 

 Browne. 

 Granite, Midgee, Hundred of Charleston, Eyre Pen. 

 Anal. W. S. Chapman. Geol. Surv. S. Austr., 

 Bull. 3, p. 14. 

 An approximate mode was calculated from this analysis, 

 giving for the main constituents the following weight 

 percentages : — 



Quartz 36*8 



Microcline 

 Plagioclase (Aby^) 

 Biotite ... 

 Ilmenite 

 Kaolin ... 



47-6 



8-8 



5-1 



-2 



1-5 



The discrepancy between this and the result of the 

 Rosiwal measurement is clue doubtless to the uneven distribu- 

 tion of the minerals through the rock. 



It is interesting to note that this mode calculated from 

 the analysis justifies the assumption of a sensibly constant 

 composition for the microcline. The transfer to plagioclase 



