100 Ernest W. Skeats : 



The plagioclase is assumed to liave the composition Abg Auj. 

 Distiibutiiig the 39.87 per cent, among these two molecules we 

 obtain Albite 25.63 per cent., Anorthite 14.24 per cent. 



Knowing the chemical composition of all the minerals and the 

 percentage of each mineral present we arrive at the ultimate 

 chemical composition of the rock. 



Total - 99.72 



In this mineralogical analysis, ajpart from the small experi- 

 mental error in the traverses and estimation of the volume com- 

 position of the minerals of the rock, there are two sources of 

 ambiguity. The Biotite analysis from Chebarkul, chosen for 

 comparison on account of simiUur specific gravity, is that of a 

 variety in which ferrous and magnesia oxides are present in 

 almost equal amounts. The biotite in thi« grano-diorite may not 

 have these oxides present in similar proportions. The other 

 ambiguity rises from the fact that no allowamce has been made 

 for the Ismail quantity of albite irregularly intergrown with the 

 orthoclase. If this could be allowed for, potai^h would be slightly 

 diminished and soda correspondingly increased in amount. 

 Apart from these possible sources of error, it is believed that 

 the figures fairly represent the chemical composition of the 

 rock. Although tlie indicated silica percentage is higher 

 than in some of the Victorian gi-ano-diorites, the high total of 

 the alkaline earths and the lack of preponderance of the potash 



