GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 549 



which must have been displaced by the net expansion of 

 the augite and olivine as they altered to chlorite and ser- 

 pentine. There is little doubt that some lime and silica 

 are completely removed from the rock itself by the 

 percolating water, to be deposited in adjacent cracks and 

 cavities, and perhaps a little soda also goes. Oalcite and 

 stilbite, where they appear in measurements are allotted 

 to both augite and labradorite. The successive steps in 

 the restoration of the original minerals will now be detailed, 

 the figures obtained in the correction of the measurement 

 of specimen I. being quoted as an example. 



The composition by weight of specimen I was found to 

 be as follows : — 



Augite 



34*2 



Analcite 



4*0 



Felspar 



31*7 



Biotite 



0*1 



Iron Ores 



17'2 



Apatite 



0*7 



Serpentine 



7*1 









Chlorite 



5*0 





100*0 



To form olivine out of serpentine, deduct approximately 

 12 per cent of water and then 18 per cent, of silica, for the 

 dehydrated serpentine will contain nearly 50 per cent, of 

 silica, and deducting silica to 18 per cent, of its weight 

 will leave a remainder having about 38 per cent, of silica, 

 which is the proportion in a ferriferous olivine, thus : — 

 7*1 x *88 x '82 = 5*1 = total olivine. 



In forming augite from chlorite regard must be had to 

 the compositions of these minerals as determined by analysis. 

 The principal difference in composition between the two 

 minerals is as regards the water, silica, and lime, the losses 

 or gains of other constituents being comparatively negli- 

 gible. We may assume that a little calcite is usually mixed 

 with the chlorite, say about 3 per cent., and we must then 

 deduct 16 per cent, for water and carbon dioxide, leaving 

 a remainder which has approximately the composition 



