GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 541 



not only the separation and analyses of all the ferric and 

 alferric minerals present, but also the measurement of the 

 rock by the Rosiwal method. When all the necessary data 

 are not available it is possible to calculate the mineral 

 composition approximately by making reasonable assump- 

 tions as to the composition of one or two minerals, or their 

 quantity. This is the course we have followed with regard 

 to the Prospect rocks. The composition of the augite we 

 know by analysis. We assumed a composition for the 

 biotite agreeing closely with a number of analyses of 

 biotite from igneous rocks and left the composition of the 

 olivine to be determined by the remainder. A check was 

 obtained, however, by the necessity of assuming the olivine 

 to have the same composition in both specimens. This led 

 to a slight re-adjustment of the proportion of FeO and MgO 

 molecules in the biotite ; and a closely approximate solution 

 of the compositions of the rocks was finally obtained with 

 the assumption of the following compositions for biotite 

 and olivine: — 



Biotite. Olivine. 





Molecular. 



By Weight. 



Molecular. 



By weight. 



Si0 2 



40 



37*2 



33*4 



35*2 



A1 2 3 



11 



17*3 



... 





Fe,0 3 



4 



9*9 







FeO 



10 



111 



32*1 



40-1 



MgO 



20 



12*4 



34*2 



24'0 



K 2 



6 



8*7 



. 





H 2 



8 



2*2 







TiO, 



1 



1*2 







100 lOO'O 100*0 lOO'O 



It will be seen that, whilst the biotite has quite a normal 

 composition, the olivine is unusually rich in iron, and is a 

 hyalosiderite, approaching fayalite. Its composition as 

 here found is not dependent on the composition assumed 



I 1— Dec. 6, 1911. 



