MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 539 
in the entire rock) usually contains 3 to 4, up to 5.14 per cent MgO 
(the last analysis equivalent to 0.78 FeO:0.22 MgO). In the 
dacite No. 53, with 0.53 FeO:0.47 MgO in the whole rock, the 
hypersthene as well as the biotite carries almost exactly 0.50 
FeO:0.50 MgO and the ilmenite 0.96 FeO:0.04 MgO. 
The proportions of FeO and MgO in the ilmenite, from observa- 
tion of the three rocks just mentioned, must be a function of the 
FeO: MgO proportion in the entire rock or in the original magma, 
but in such a manner that the ilmenite throughout shows a very 
extensive relative enrichment of FeO (as FeTiO,) and consequently 
vice versa an extensive, relative decrease of MgO (as MgTiO,). 
[To be continued] 
