276 H. IS. Sunn/ners : 



The FeO may be assumed to be present as magnetite, as tlie 

 percentage of FeO also includes the Fe203, and the small amount of 

 alumina is negligible considering the data on which the estimation 

 is made. It would seem then that the acid limit product of the 

 differentiation would be a rock consisting of approximately 38.5 

 per cent, fi-ee quartz and 59.5 per cent, felspar. In the case of the 

 more acid of the liypothetical limits of differentiation in the direc- 

 tion of decrease of silica the composition may be estimated as 

 follows : — ■ 



It will be seen that there is more than sufficient silica left after 

 the production of felspar to form a metasilicate of magnesia, 

 ferrous iron and alumina. This excess of silica would be actually 

 greater, as portion of the iron would go to form magnetite. If we 

 allowed an equal amount of ferrous oxide for magnetite, as was 

 required for ilmenite, viz., 28 parts, this would reduce the amount 

 of ferrous iron by 84 points, and a possible composition of the 

 hypothetical rock would be as under : — 



It will be seen that the rock would consist essentially of plagio- 

 clase and an aluminous amphibole or pyroxene with some quartz 

 and a moderate amount of iron minerals. 



