46 Transactions of the South African Philosophical Society. 



agree with augite. A number of small crystals and grains of zircon 

 also occur in the mass. 



In this base there are large crystals of sanidine, hornblende, and 

 sphene. 



The sanidine is in straight laths, very fresh internally, with dusty 

 alteration products round the edges and along the cleavage cracks. 



The hornblende is very interesting ; it is surrounded by a halo of 

 alteration products, principally bright yellow augite and magnetite. 

 In Slide No. 1147 there are similar sections, in which all original 

 mineral substance has been changed to a mixture of magnetite, augite 

 and felspar. What we see in the present slide is probably an early 

 stage of what we see in No. 1147, in which the ferro-magnesian 

 mineral, caught in the molten magma, has been wholly meta- 

 morphosed. 



The sphene is in large crystals, greyish yellow, with characteristic 

 shagreened surface, marginal reflexion, and high interference colours. 

 The mineral is also curiously abundant in the rock (Slide No. 1152) 

 from Nightingale Island, and is also characteristic of one of the 

 Challenger rock specimens from the Falkland Islands. This last is 

 so interesting that it may be well to set down the Eev. Benard's 

 description of it, especially as attention has primarily been directed 

 to the non-volcanic series in these islands. It is a fine-grained 

 felspathic sandstone, formed of an aggregate of quartz and felspar 

 grains, containing fragments of mica-schist, glassy vesicular lavas and 

 red porphyries, plagioclase felspar, quartz, and sphene, with scales of 

 chlorite. 



Besides these comparatively unaltered elements, there are patches 

 composed of aggregates of magnetite and augite granules, evidently 

 the altered remnants of ferro-magnesian minerals. 



The rock seems to be similar in microscopical characters to the 

 amphibolic andesite containing sanidine of the Challenger rocks, but 

 I believe the rock is an originally basic rock which has picked up and, 

 to a certain extent, absorbed fragments of a porphyritic acid type. 



Slide A red vesicular bomb ; matrix hard and stony. Open 



T?" it s P aces filled with powdery white growths turning yellow 

 on the exterior. There is no rim of anything of the 

 nature of a cooled outer surface. Plenty of large augite crystals. 

 Under the microscope : — 



An opaque mass with minute laths of felspar and granules of 

 augite and olivine forming the matrix. 



Porphyritic crystals of augite, enstatite and olivine, all with many 

 enclosures. 



