516 A. G. MacGregor mid F. R. Ennos — 



Sodalite and Analcite. — There was also noticed under the micro- 

 scope the abundant isotropic material previously referred to as 

 analcite in descriptions of the rock. Its characteristics were : — 



1. Isotropic. 



2. Refractive Index much lower than that of the surrounding alkali felspar 

 laths of the groundmass, and very much lower than that of the balsam of the 

 slide. 



3. Cleavage, when present, very ill-defined. 



4. Decomposition. — Among the groundmass felspar laths, with their brown 

 powder of decomposition products, the mineral stood out clear and colourless. 



5. Habit. — Crystal outlines entirely absent. The isotropic material occurred 

 typically wedged in between the felspar laths of the groundmass as abundant 

 small patches, often of roughly triangular shape, owing to the straight bounding 

 edges of the felspars : sometimes in larger roundish patches with ophitic 

 relations to the felspars : exceptionally as a small vein cutting across the rock, 

 moulded on the idiomorphic terminations of felspar laths. The general mode 

 of occurrence did not suggest derivation from nepheline. 



6. Size of patches. — The smaller were about 0-18 mm. by 0-08 mm. The 

 larger roundish patches ranged up to about 0*5 mm. by 0'4mm. The veins 

 were about 0"4 mm. broad. 



From the above characteristics the isotropic material might be 

 analcite or sodalite, or it might be made up of both. 



Olivine. — Besides the green pleochroic augite described by 

 Dr. Hatch in his account of the rock (5), a mineral was noticed with 

 the followiag characteristics : — - 



1. High polarization colours. — Blues, greens, and reds of the second order 

 and higher. 



2. Refractive Index. — High. 



3. Cleavage. — One very pronounced somewhat irregular cleavage or parting, 

 and sometimes another much less well defined but more regular and at right 

 angles to the general direction of the first. Also irregular cracks. Extinction 

 almost always parallel to the cleavages. The optical character of the more 

 regular cleavage zone was positive. 



4. Decomposition. — Often clear and undecomposed, colourless to very pale 

 yellow in parallel polarized light, and not pleochroic. In other cases wholly 

 or partly altered to a pale or ohve -green serpentinous substance with 

 very low polarization colours. Ohvine-shaped pseudomorphs common, 

 occasionally in a more decomposed part of the rock replaced by a red-brown 

 or yellowish transparent mineral. Serpentinization started along the cracks 

 and cleavages with some formation of iron ores. (The pleochroic green augite 

 of the rock was almost invariably fresh and undecomposed.) 



5. Habit. — The mineral occurred in two ways — (a) As small crystals and 

 granules in the groundmass, always with slightly ophitic relations to the felspar 

 laths, the majority with quite irregular outlines, but many showing the 

 characteristic shape of oUvine. (6) As larger scattered patches with ophitic 

 relations to the felspars. The components of each large scattered patch were 

 shown to be in optical continuity by the cleavages and extinction. The 

 ophitic relations were as striking as those commonly shown by augite in 

 dolerites. 



6. Size of crystals. — Irregular grains up to about 1 mm. in length. Shaped 

 olivines up to about 0'3 mm. by 0"18 mm. Ophitic patches extending through 

 an area as large as I'l mm. by 0*9 mm. 



7. Interference figures were biaxial and showed a very large optic axial 

 angle, but in some cases it seemed possible to determine the optical character 

 as definitely negative. 



8. In the crystals with the characteristic shape of olivine there was a very 

 well-defined but irregular parting more or less parallel to (001), and sometimes 



