GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 547 



aegyrine-augite. The quantity of the latter in terms of 

 pure aegyrite (acmite) is quite insignificant, probably much 

 less than one per cent, of the rock. It seemed needless 

 therefore to attempt to determine the quantity of the 

 aegyrite molecule separately, and it was regarded as a 

 component of the augite, which would thus be somewhat 

 richer in soda than that of the main rock. There remained 

 but one ferro-magnesian mineral of which both the quantity 

 and the composition are thus approximately determinable, 

 Quartz and lenads being proved absent microscopically, 

 constituents were first assigned to apatite, ilmenite, and 

 orthoclase. Albite was formed with all the soda except 

 a reasonable amount held for the augite. The anorthite 

 was reduced to the minimum compatible with not giving 

 the augite a very unusually high percentage of alumina, 

 the difficulty being that increasing the anorthite would 

 have left the augite almost impossibly poor in lime. Per- 

 haps the rock has lost some of its lime by decomposition. 

 The anorthite thus determined, the remainder, after a 

 reasonable quantity of iron oxides has been assigned to 

 magnetite, is treated as augite. Its composition has been 

 calculated and is stated in the first column of the following 

 table. It is seen to be very poor in lime and rich in ferrous- 

 oxide and alumina : — 



Composition 





Compos- 



Compos- 







a 







<n 





of Augite 





ition of 



ition of 



<D 





<j 



a> 



+J 





CD 



by Weight. 





Rock by 



Eock by 



ci 



o 



q 



,a 



2 



O 



&D 



6C 







Weight. 



Molecules 



< 





O 



< 



< 



1 



<J 



50-3 



Si0 o 



58-82 



980 







193(630 



28 





130 



9-8 



Al 2 O a 



16 -it] 



166 







32 ! 195 



14 





15 



3-1 



Fe,0 3 



2-40 



15 







1 





12 



3 



21-7 



PeO } 

 MnO / 



4-72 



66 





14 



! 





12 



40 



56 



M^O 



0-88 



22 













22 



7-9 



CaO 



2'42 



43 



7 









14 





22 



1-6 



Na,0 

 K.,0 



6-74 

 2-96 



109 

 32 







32 



105 







4 







ioo-o 



TiO„ 



p 2 o; 



1-14 



0-34 



14 

 2 



2 



14 













Perce nta« 



r e compos it 



ion of the 



! ruck by 

















weigl 



it 







0-7 



22 



18-256-3 



4-0 



2-8 



1-58 



