556 SAMUEL WEIDMAN 



the rock. It does not occur in veins, segregations, or cavities, but as 

 a common constituent distributed through the rock, hke the quartz, 

 feldspar, nepheline, and other common rock-forming minerals with 

 which it is associated. 



Like the other abundant minerals associated with it, it does not 

 occur in idiomorphic crystals, but assumes shapes in its development 

 due to the mutual interference of surrounding minerals. Where 

 present with an abundance of dark-colored silicates, it is usually in 

 direct contact with them, but in quartz syenite it occurs in direct 

 contact with quartz, and in certain phases of the nepheline syenite 

 it is entirely surrounded by nepheline, or nepheline and feldspar. 



The fayalite has not been observed to occcur with the alkali- 

 iron pyroxene, aegirite, which is a common constituent of several 

 phases of the nepheline-bearing rocks. While it may be stated with 

 certainty that these two iron-rich minerals do not occur in the same 

 thin sections of rock examined — and this includes a considerable 

 number — with this special relation in mind, yet a final conclusion 

 upon this point, it is believed, should be held in abeyance. 



The pyroxene, asgirite, it will be recalled, contains a high per- 

 centage of ferric oxide and but a small proportion of ferrous oxide. 

 On the other hand, the fayalite contains ferrous oxide only, and the 

 pyroxenes, amphiboles, and micas abundantly associated with the 

 fayalite, as shown by the analyses, contain a marked excess of ferrous 

 oxide over ferric oxide. 



It may be, therefore, that in those magmas, or portions of magmas, 

 very poor in magnesia, which contain a large excess of ferrous oxide 

 over ferric oxide, the chemical conditions for the development of 

 fayalite exists, and in those containing an abundance of ferric oxide 

 the conditions for the development of aegirite are present and that 

 necessarily these two minerals will not be expressed in the same 

 phase of rock. A deficiency of oxygen as well as of magnesia may 

 therefore be necessary for the development of fayalite. 



Fluorite is a persistent, though not an abundant, constituent of 

 the various phases of this series. The fluorite is colorless, has low 

 birefringence and perfect octahedral cleavage, and on account of 

 its extremely low index of refraction, produces the apparent anoma- 

 lous appearance of having a very high index of refraction. The 



