The Bocks of Tristan d'Acuriha. 49 



Slide A white mica and biotite gneiss, showing well-marked 



Gneiss partings caused by the aggregation of mica plates. The 

 whole boulder is rounded as if water-worn. 

 Under the microscope : — 



Biotite in normal pleochroic brown plates, sometimes with a 

 non-pleochroic feebly doubly refractive green alteration product. 

 The muscovite is also quite normal and very fresh. 



Quartz in large proportion, showing hackly borders and a 

 certain amount of dusty, matter between the contiguous grains. 

 No large fluidal cavities, though dusty inclusions occur, which may 

 be minute cavities or merely dust ; slender apatite needles and 

 trichite-like crystals also are present. Some of the grains show 

 strain effects, as if there had been an attempt to granulate the 

 whole, this appearance under crossed nicols. 



The felspars are microcline, with streaks of albite, showing almost 

 invariably the microperthite structure. 



There are some patches of isotropic substance, clear and colourless, 

 with strong single refraction ; the substance falls out in grinding the 

 section, so that it is difficult to say what the mineral is. The most 

 likely suggestion is garnet. 



Slide A red stony lava with irregular- rounded vesicles, com- 



pressed and flattened 



Under the microscope : — 

 An opaque glass with dusty granules and well-formed laths of 

 felspar. The small, rare porphyritic crystals are augite. 



Nightingale Island. 



Slide A fine-grained, semi-crystalline, blackish rock, with 



a °j ■+• scattered little laths of felspar ; sheared, and weathers 

 Andesitic , *• ' 



glass. with a pitted surface. 



Under the microscope : — 

 A fine, transparent glass containing microscopic rods of augite and 

 ill-formed laths of felspar. 



The augite is in very minute lath-shaped crystals ; the felspar laths, 

 as is often the case where the augite is crystalline and not granular, 

 seems hardly to have crystallised out of the glassy base, which is an 

 indefinite felspathic mass. Magnetite is wholly subordinate and 

 exists in granules. Long apatite needles sharply terminated, by the 

 simple pyramid, sometimes grown together in parallel position, at 

 others showing the characteristic transverse cracks, the separate 

 portions of the needle being sometimes quite far apart ; they are of 



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