480 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



IV. 



As measured by Rosiwal's method, and uncorrected. 



Augite 



34*2 



Felspar 



31*7 



Iron Ores 



17*2 



Serpentine 



7'1 



Chloritoid and 



chlorite 5*0 



Analcite 



4*0 



Biotite 



O'l 



Apatite 



0*7 



100*0 



The relative proportions of the minerals present in the 

 rock have been determined by two methods : by calculation 

 from the bulk analysis of the rock 1 (column I), and by 

 measurement according to Rosival's method. The existing 

 composition of the rock, including the quantities of 

 secondary minerals, as determined by the latter method is 

 given above in column IV; and the original composition as 

 deduced from this by making corrections for the decompo- 

 sition is found to agree very closely with the composition 

 as calculated from the analysis, as may be seen by compar- 

 ing columns II and III. The method of correcting for 

 decomposition is fully explained, with this specimen as an 

 example, in Appendix II. 



Inspection of the slides of this specimen seemed to show 

 that the lines measured had been unlucky in missing biotite 

 crystals. Probably there is nearly two per cent, present ; 

 hence for the purpose of calculation 1*7 per cent, was the 

 quantity assumed. 



The augite is titaniferous and shows hour-glass structure 

 faintly. After separation from the rock the augite of this 

 specimen was sent to Mr. Mingaye and kindly analysed by 



1 For details of method see Appendix I. 



