86 



of biotite. This accounts for the iron, potash and mag- 

 nesia found in the analysis. Neglecting these the compo- 

 sition corresponds rather closely to that of an andesine 

 with the formula AbsAn 2 , with 0-96 per cent too little of 

 silica and 1-68 per cent too little of lime. The specific 

 gravity for such a mixture should be 2-68, while that of 

 the andesine separated from the rock was 2-668, this 

 slight decrease in weight being, no doubt, due to the 

 unusually low lime. For purposes of comparison, the 

 theoretical composition of andesine corresponding with 

 the generally accepted formula for this species of plagio- 

 clase with the ratio of the soda to the lime of 1 : 1 (Ab 2 Ani) . 

 is given under II, while under III is quoted the composition 

 of andesine, made up of a mixture of albite and anorthite 

 in the ratio of 3:2. 



— 



I. 



II. 



III. 



Si0 2 



57-15 

 26-74 



} 0-25 



6-66 



o-59 

 trace 

 0-38 

 6-83 

 0-90 



59-84 

 25-46 



6-97 



58-11 

 26-6 



AU03 



Fe 2 3 





FeO 



CaO 



MgO 



8-34 



MnO 





K 2 







7-73 





NaO. . . 



6-93 



H2O 









99-50 

 2-668 



100 -00 



2-671 



100-00 

 2- 680 







An analysis ofjthe blue corundum which occurs associ- 

 ated with this rock was made by Mr. M. F. Connor, with 

 the following results: — 



Si0 2 none. 



A1 2 3 (diff.) 96-90 



Fe 2 3 + FeO 0-76 



CaO 0-46 



MgO 1 -oo 



H 2 o-88 



