MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 667 



between biotite and hypersthene stands in any relation to the com- 

 position of the rocks determined by the quantitative chemical 

 analysis. The formation of biotite thus is not dependent on 

 the content of K2O in the rocks nor on the proportion of K2O to 

 MgO (or MgO+FeO). In some norites with 0.5-0.7 per cent K2O 

 the entire quantity of K2O enters into the feldspar (as ICAlSijOg 

 in the plagioclase) . In other norites with the same percentage 

 of K2O, as much as ca. f of the content of K2O of the rock may 

 enter into the biotite and only | into KAlSijOg of the plagio- 

 clase. When we consider this in the light of all the other obser- 

 vations here treated, the conclusion clearly is justified that the 

 cause of the greatly varying quantity of biotite in these rocks 

 must be due to the variations in the quantity of H2O in the magma. 

 But even in those norites that carry as much as ca. ^ of biotite in 

 proportion to the sum of hypersthene and biotite, the H2O per- 

 centage in the magma must have been rather small. 



As earlier mentioned, many Norwegian norites and gabbros 

 do not contain any primary hornblende at all, and where this 

 mineral occurs it is somewhat younger than the pyroxene (p. 521). 

 We may here apply the same considerations as those regarding the 

 relation between the hypersthene and biotite in the quartz norite 

 from Romsaas. 



The anorthosites constantly carry, as is well known, a small 

 admixture of hypersthene, or sometimes of augite (diallage) and 

 olivine, while primary hornblende seems to be entirely lacking. 



As we shall show in a later paper, the peridotite series in the 

 first stage of concentration — carrying about 35-50 per cent olivine 

 and with a chemical composition 41-49 per cent SiOa, 5-10 AI2O3, 

 6-10 CaO, 0.25-2 alkalies, 10-15 FeO, and 15-25 MgO, thus about 

 0.75 MgO:o.25 FeO — in almost every case is characterized by some 

 primary hornblende, at times also by some primary biotite. In the 

 progressive concentration of the olivine — and simultaneously with 

 diminishing percentage of AI2O3, CaO and alkalies and increasing 

 Mg2Si04 in proportion to Fe2Si04— the hornblende is, on an aver- 

 age, diminishing in quantity and in peridotite rocks with at least 

 85-90 per cent olivine hornblende as a primary formation is entirely 

 or almost entirely lacking. 



