644 CROSS, IDDINGS, PIRSSON, WASHINGTON 



3. The constant relation between Fe 2 3 and FeO in magne- 

 tite (Fe 3 O s : FeO : : 1 : 1). 



4. The relation between FeO and TiO s in ilmenite (FeO : 

 Ti0 2 :: 1 : 1). 



5. The relation of TiO s and CaO in titanite and in perofskite 

 (Ti0 3 : CaO :: 1 : 1). 



6. The development of the titano-silicate, titanite, in the 

 more siliceous rocks, and of the non-siliceous perofskite in its 

 place in the less siliceous ones. 



7. The constant relation of P 2 5 to CaO in apatite (P 2 5 : 

 CaO : : 1 : 3.33). 



8. The relation of CaO and (Mg,Fe)0 in monoclinic pyrox- 

 ene, diopside (CaO : (Mg,Fe)0 : : I : 1). 



9. The occurrence of (Mg,Fe)0 both as metasilicate or 

 orthosilicate, in hypersthene and olivine. 



10. The frequent, but not invariable, relation between (Mg,Fe)0 

 and available silica, whereby metasilicate, hypersthene, forms 

 instead of orthosilicate, olivine, with sufficient available silica. 



1 1. The occasional occurrence of a sodium metasilicate mole- 

 cule, Na 2 O.Si0 2 , which enters into arfvedsonite in rocks in 

 which K 2 and Na s O are in excess of Al 2 O s and Fe 2 O s . 



12. The fact that, apart from apatite, the only common non- 

 silico-aluminous, primary rock-making minerals are magnetite 

 and ilmenite; the iron being the only element of the important 

 ones which in rocks can crystallize without Si0 2 or A1 2 3 . 



13. Finally the common occurrence of Si0 2 , CaO and Na 2 

 in minerals of both the salic and femic groups, and their result- 

 ing interdependence. 



CALCULATION OF THE NORM. 



The method adopted by us of calculating the standard min- 

 eral composition is as follows : 



1. Determine the molecular proportions of the chemical 

 components of a rock as expressed by the complete analysis, by 

 dividing the percentage weights of each component by its 

 molecular weight. 



