600 MINERALS AND VEINS OF jEGIRINE-SYENITE, 



In cases of extreme attack, as met with in class (b), subclass 2, 

 type /3, semi-opaque rusty-brown areas shading into secondary 

 chalcedonic depositions alone remain. A comparative petro- 

 logical examination of the various types of veins shows that the 

 conditions active in uralitising the segirine and decomposing 

 the arf vedsonite have likewise radically affected the sanidine. 



In several of the veins, in association with much secondarily 

 deposited silica, the sanidines, first slightly corroded, have 

 received a peripheral addition in the form of a very narrow film 

 of apparently clear glassy felspar; the width of the bands is only 

 about 0-02 mm., too narrow for obtaining an interference figure 

 or to apply microchemical tests. It shows twinning, and is pro- 

 bably a soda felspar. 



Accessory Minerals. — Quartz developed in small idiomorphic 

 dirhombohedral pyramids subordinate to the arfvedsonite and 

 segirine, and evidently the last crystallisation of the primary 

 cycle. Frequently it shows evidence of corrosive and penetrative 

 attack in common with all the original minerals. Primary quartz 

 was noted only in class (b), subclass 2 veins, and then not in every 

 slide. 



Perofskite (?) — Very tiny idiomorphic crystals of a light honey- 

 yellow mineral are rather numerous in the larger types of class 

 (6) veins. Their outlines are always quite sharp, usually in very 

 perfect isometric octahedrons, and are found embedded in any of 

 the minerals succeeding the sanidine in period of crystallisation. 

 They are little affected by alteration, though leucoxene-like 

 products are sometimes to be noted developing from without, 

 inwards. 



In all cases the mineral is isotropic, not showing any of the 

 optical anomalies usual with perofskite; it may, therefore, be 

 one of several somewhat similar minerals as pyrochlore or 

 dysanalyte. 



Ilmenite in large crystals is occasionally noted, and then 

 usually far advanced in leucoxenisation. Small crj'stals of 

 ilmeniferous magnetite are abundant in the parent mass. 



