172 PETROLOGICAL NOTES ON SOME OF THE ERRATICS 



It is biaxial, optically positive, with 2V fairly large and shows very decided dispersion, 

 p < v. 



These characteristics are somewhat puzzling. At first sight the mineral suggests 

 a member of the epidote group, but the constant high extinction measured from the 

 length and the character of the dispersion seem to put epidote out of court. 



Of a'l the minerals described in Rosenbusch's Mikroskopische Plnjsiographe wohlerite 

 possesses most features in common with the one under discussion. 



It is quite possible that we are dealing with a new mineral; but, unfortunately, 

 only a very small chip of the rock was collected, and there was not sufficient material 

 for analysis or for mineral separation. 



Another feature of this remarkable rock is the habit of the magnetite, which is a 

 fairly abundant constituent. In addition to normal octahedra there are still more 

 abundant distorted crystals possessing a prismatic habit. 



Magnetite and wohlerite occur together, forming intergrowths. 



Apatite is present in exceptionally large crystals up to 1 mm. by 0"2 mm., together 

 with interstitial sodalite and small grains of magnetite and wohlerite. Here and there 

 quite large patches of sodalite occur. 



535. Sodalite Syenite 



Macroscopic Characters. — Light grey rock of even grainsize (0"5 mm.) 

 composed essentially of felspathic and ferromagnesian minerals in about equal propor- 

 tions. The ferromagnesian minerals include black augite and dark-reddish mica. 



Microscopic C haracter s. — Hypidiomorphic granular rock of medium grain 

 consisting essentially of anorthoclase sodalite, augite, olivine, biotite, micaceous 

 haematite, and magnetite; with smaller quantities of hornblende, oligoclase, and 

 apatite. 



Anorthoclase is much the most abundant constituent; it is subidiomorphic and 

 tabular parallel to (010). Twinning on a large scale is after the Carlsbad law or is 

 absent, but the individuals are always multiple twinned on an extremely fine scale 

 so that the extinction is quite cloudy. Only occasionally can the actual twin lamellae 

 be resolved. The refractive index is never higher than that of Canada balsam and is 

 mostly decidedly less. 



Sodalite is not very abundant. It is mostly interstitial but in some instances 

 appears to be perthitically intergrown with anorthoclase. 



Augite, light bluish-grey, subidiomorphic and mostly untwinned. The crystals 

 are crowded with inclusions chiefly of apatite and magnetite. The mineral is not 

 pleochroic. 2V is large. The grains are moulded on olivine, but are older than horn- 

 blende and biotite. 



Olivine in idiomorphic crystals up to 0-63 mm. by - 28 mm. is decidedly yellowish 

 in colour and considerably altered around the outside and along cracks with separation 

 of much magnetite. Two of the cleavages are quite perfect. Double refraction is 

 very strong, while the optic axial angle, though quite large, is nevertheless relatively 

 small for olivine. 



Biotite in irregular flakes and occasional subidiomorphic sections, averaging about 

 22 mm. diameter, is very dark in colour, the pleochroism being from brownish-red 

 to opaque. It is crowded with plates of micaceous haematite lying parallel with the 

 cleavage. 



Micaceous haematite is very abundant in flakes averaging 016 mm. diameter. 



Hornblende is not abundant; it occurs mostly associated with augite, forming 

 a halo round that mineral. The colour is very dark brown, and, when the trace 



