44 Transactions of the South African Philosophical Society. 



Slide A dense grey, stony rock, with lighter spots on the 



No 1141. , & J J 



Basalt. outer surface. 



Under the microscope : — 



The same as 1139, with larger granules, and without the flow 



structure. The section is very transparent, and the boundaries of 



the minutest augite granule can be distinctly seen. There is a 



great amount of residual glass, which is a half-formed felspathic 



mass. 



Slide A dense black lava, with small vesicles; rounded 



Basaltic glass. Patches, containing somewhat larger vesicles, occur 

 throughout it. 

 Under the microscope : — 

 The rock consists of two portions. The one or more normal type 

 is a glass full of black granules, in which long laths of felspar are 

 confusedly set. There are a number of small rounded vesicles, the 

 rims containing a brownish limonitic glass, less dense than the main 

 mass. The other kind, which penetrates the first with a definite 

 black line between the two, is composed of the same glassy matrix, 

 more discoloured by weathering. In this last there are large 

 irregular vesicles, bordered by a wide zone of clear, colourless glass. 

 In the interior of the vesicles there is a brownish opaque deposit. 



Slide A friable, light and dark grey mottled, stony rock with 



No 1143 

 Basalt or crystals of augite. 



andesite. Under the microscope : — 



The same as Slide No. 1140, with larger granules, and a good deal 

 of residual glass. 



The augites show twinning, with re-entrant angles and zonal 

 structure, also inclusions of olivine and magnetite. 



The felspars show Carlsbad twinning and zoning, also rims of 

 growth or alteration. 



The olivine is very fresh, usually associated with magnetite ; it 

 does not show crystalline boundaries, but is not corroded. 



The matrix is composed of felspar laths, enstatite, and magnetite. 

 The enstatite is granular only, some of the larger individuals show 

 the lath-form giving straight extinction and the rhombic section, 

 with the little glass tubule in the centre. There is a certain amount 

 of flow structure shown in the laths wrapping round the larger 

 magnetite crystals and the other porphyritic minerals. 



Slide A stony, vesicular rock with many crystals of augite. 



N -2* 11 1 x Under the microscope : — 



Basalt. r 



Large crystals of augite, felspar, olivine, and magnetite, 

 set in a microcrystalline base. 



