The Bocks of Tristan d'Acunha. 43 



form ; they are for the most part very minute, '005 millimetres or 

 so, but a few larger ones occur. 



About equal in numbers with the augite, and of the same sizes, 

 there are specks of iron ore. There is no residual glass. 



Small rounded patches crowded with iron ore indicate portions of 

 the magma that have cooled rapidly, the felspar laths wrap round 

 such areas, and thus show that the inclusion of these little specks is 

 due to motion during the flow of the molten material. 



In the matrix there are abundant large crystals of iron ore and 

 often enclosing portions of the microcrystalline base. They fre- 

 quently seem to have gathered round them the augite and iron ore 

 granules to the exclusion of the felspar ; the laths of the last mineral 

 wrap round the whole. 



Very rarely porphyritic crystals of the augite and large laths of 

 the felspar occur. The augite is strongly refractive and sometimes 

 pleochroic from bright yellow to colourless ; round the edges there is 

 a thin zone of more ferruginous material, indicating fusion at the 

 periphery. The felspar laths are about *1 millimetre in breadth and 

 show fusion at the ends, giving the laths a spindle shape ; minute 

 augite granules have grown up on the fused portion, showing a 

 certain amount of transfusion. 



Slide A puce-coloured, stony matrix, crowded with large 



Basalt or black augite crystals, about -J inch in diameter, and 



andesite. white flecks of felspar. The augite is tarnished with 

 iridescent colours. 



Under the microscope : — 



Matrix the same as the last, only the laths are more irregularly 

 placed, though they still show flow structure, and there is a sprink- 

 ling of larger augite crystals. 



Porphyritic crystals of large size include magnetite, augite and 

 labradorite. A few small granules of olivine. 



The augite is in large crystals, enclosing felspar, magnetite, and 

 sometimes smaller grains of augite, with which the larger crystal has 

 not grown up in sympathy. 



The labradorite is in square, stumpy forms, often showing the 

 albite as well as the pericline twinning. Sometimes the stumpy 

 laths form star-shaped aggregates, but, as a rule, the crystals are 

 not lath-shaped. The large untwinned crystals frequently show 

 zoning and shadowy extinction, with more basic interiors. They 

 enclose granules of augite. 



Olivines, mostly in small grains, but sometimes larger ones occur, 

 showing crystalline faces. 



