26 



Albite is slightly zoned, but its precise constitution could not 

 be ascertained. Quartz and microcline are at times inter- 

 grown in nide graphic fashion. 



The chemical composition of the rock, as determined by 

 the author, is : — 



SiO, ... 







.. 76'65 



AlA ... 







.. 12-98 



FeA ... 







. 0-25 



FeO ... 







. 0-32 



MgO ... 

 CaO ... 







abs. 

 . 0-40 



Na.O ... 







2-64 



K.O ... 







.. 6-18 



h:o+ 







. 0-29 



h:o- 







.. 0-17 



Tia ... 







.. 0-11 



Total 



. 99-99 



Sp. gr. at 



27i°C . 



.. 2-590 





From this analysis an 

 culated : — 



approximate 



mode ^ 



Quartz 





:: )::. : 



.. 35-34 



Microcline 





.. 51-22 



Albite 





.. 



.. 9-96 



Kaolin 







.. 2-59 



Magnetite 

 Ilmenite 



... 







-46 

 •30 



Water 







-09 



was cal- 



or, distributing the kaolin proportionately between 

 felspars, neglecting the water and recalculating to 100:- 

 Quartz 35*40 



the 



Microcline 

 Albite 

 Magnetite 

 Ilmenite 



53-46 



10-38 



-46 



-30 



In calculating this mode there was found to be insuffi- 

 cient lime available to satisfy the proportion required by 

 Gartrell's analysis; this would indicate that the microcline is 

 of different composition from that in the granite, and that the 

 albite is practically if not absolutely a pure soda felspar. 

 The same conclusion is indicated by the low specific gravity 

 of the rock, wliich points to a strong preponderance of the 

 potash felspar. 



A Rosiwal measurement was made of the rock, but as it 

 was found impossible* to differentiate the two felspars owing 



