The Bocks of Tristan d'Acunha. 47 



The augite is in large, simple and twinned crystals ; they enclose large 

 irregular spaces filled with the opaque matrix ; in one instance the 

 plane of composition of the twin is the seat of a arge zone of matrix, 

 which contains olivine grains. 



The enstatite is in large crystals with the same shagreened surface 

 as the olivine, but with low interference colours. It is slightly 

 coloured yellow, weakly pleochroic to blue, in contrast to the olivine, 

 which is quite clear and colourless. The crystals show no decom- 

 position along the cleavage cracks, while the olivine has red stains 

 along the cracks. The enstatite has been corroded, and portions of 

 the matrix dip into the substance of the crystals, making pegs and 

 mushroom- shaped indentations ; the olivine is not corroded. The 

 enstatite has rounded enclosures of the opaque glass, the olivine 

 shows no inclusions. 



The olivine is in clearly-defined crystals, showing forms with dome 

 faces without the basal plane. 



Very rarely there is a small porphyritic crystal of plagioclase, in 

 stout columnar forms, with perfect crystalline contours except for a 

 little corrosion along the sides. Inclusions of glass occur in strings 

 along the axis of the prisms. 



Slide A grey rock with lighter rounded spots, stony, and 



Basalt. showing fluidal parting planes, crystals of augite. 

 Under the microscope : — 



Matrix, a fine-grained aggregate of well-formed felspar laths, 

 augite granules, strongly pleochroic biotite and magnetite, with 

 a good deal of clear residual glass showing a certain amount of 

 bi-refringence. There is a distinct flow-structure. 



Porphyritic crystals of augite, olivine felspar and magnetite. 



The augite is in aggregates that seem to have been crushed 

 together, though some are intergrown crosswise. The larger 

 specimens show T zoning and inclusions of felspar laths and magne- 

 tite; their edges have been melted and recrystallised in the less basic 

 variety of augite as shown in the clearer granular forms of the 

 matrix. 



The olivines are in irregular crystals, very clear and fresh with 

 inclusions of magnetite and sometimes forming, as it were, small 

 adjuncts to the larger magnetites. 



The felspars are striated and are sometimes altered with develop- 

 ment of zoisite, otherwise they show a great number of inclusions 

 arranged along the crystalline axis ; the ends are corroded. 



There are darker patches in the matrix showing crystalline 

 boundaries which are fused ferro-magnesian minerals ; they are 



