48 Transactions of the South African Philosophical Society. 



composed of the same augite and magnetite as the general 

 ground-mass, but the latter mineral is in far greater proportion ; 

 some felspar substance is also present. One of them shows a 

 residual portion of augite ; therefore it is probable that these 

 patches represent crystals that have got into the magma from 

 previously consolidated lava. From the similarity in Slide No. 1145, 

 where the hornblende is altered, I have thought that perhaps these 

 patches represent melted up portions of extraneous rocks, and the 

 unusual presence of brown mica in great quantities in the matrix 

 lends some sort of support to this view. 



The magnetite is in large crystals, and has grown up with it 

 crystals of augite, olivine and plagioclase. The felspar laths of the 

 matrix show a streaming round the crystals, stronger than those for 

 any of the other porphyritic crystals. 



Slide A slaggy, ropy, vesicular red lava, with crystals of 



No. 1148. ., 



Vesicular au g lte - 

 Basalt. Under the microscope : — 



An opaque matrix with laths of felspar blown into a network, with 

 rounded vesicles. 



Large porphyritic crystals of augite, olivine and felspar, with 

 occasional black stony patches, representing inclusions of dirt. 



The augite is in large, sharply-bounded crystals with zoned edges ; 

 sometimes it occurs also in little aggregates of stumpy columns. 



The olivine is quite fresh, with glass inclusions ) it sometimes is 

 in irregular forms grown up with magnetite. 



The plagioclase is sometimes in very large individuals ; it shows 

 inclusions of glass which form a sort of reticulation. There is a 

 certain amount of alteration to zoisite. The end portions of the 

 larger crystals are sometimes bent. 



Slide An inky black vesicular bomb, in which the vesicles 



No 1149. . 



are far larger in the periphery, where they are coated 



with a brownish-red deposit. Towards the centre the rock becomes 



less vesicular, and in the centre it becomes a ropy fibrous glass, 



the strands, as it were, separated from each other. 



Under the microscope : — 



The matrix is a brown granular glass crowded with minute 

 granules of magnetite, through which the felspar laths show 

 somewhat shadowy. Only one porphyritic crystal is apparent ; 

 it is an augite, one part of which has been changed to, or has 

 had included in it, brown hornblende. 



