GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 551 



Relative Volumes. Differences, 

 Chlorite 5*0 -f- 2*7 = 1*83 



Augite destroyed 7*9 -f- 3'4 = 2*32 



| - 0-47 



Serpentine 7*1 -r 2*6 = 2'74)_ u1 .o 9 



Olivine 5'1 -^ 3*6 = 1'42 j "*" x 6Z 



[+ 0*85 



The net increase of volume (0*85) multiplied by the specific 

 gravity of the felspar (2*65) gives us 2*3 as the weight of 

 felspar replaced by serpentine. Adding to this the existing 

 felspar and that replaced by analcite, we have : — 



31*7 + 4*7 + 2*3 = 38*7 = total felspar. 



The other constituents of the rock are unaltered, so that 

 the corrected relative weights are as in column I below. 

 Their sum is greater than 100 because the original minerals 

 are heavier than the decomposition products, and reducing 

 to percentages we obtain the figures in column II. In 

 column III is placed, for the sake of comparison, the pro- 

 portions of the constituents as calculated from the analysis, 

 and in column IV the differences. 



Augite 



42*1 



40*5 



38*2 



+ 2*3 



Felspar 



38-7 



37*2 



38*0 



- 0-8 



Iron Ores 



17*2 



16*6 



16*0 



+ 0'8 



Olivine 



51 



4*9 



5*7 



- 0-8 



Biotite 



0*1 



0*1 



1*7 



Assumed 



Apatite 



0*7 



0*7 



0*4 



+ 0*3 



103*9 100*0 lOO'O 



It will be seen that the proportions by the two methods 

 are in general agreement, but that by the Rosiwal method 

 the percentages of augite, iron ores, and apatite are greater 

 than by calculation, and the quantities of the other minerals 

 less. The disparity is probably due to two causes: partly 

 to some serpentine having been counted as chlorite, and 

 partly to the tendency of the Rosiwal method to overesti- 



