272 Report S.A.A. Advancement of Science. 



of this rock type may profitably be deferred for the present. Under- 

 lying the basalt are 250 feet of black, carbonaceous shale (202), which 

 is certainly of Karroo age, and probably represents the Upper Dwyka 

 Shale (b) of Cape Colony. Below it comes a thin representative of 

 the well-known fossil boulder-bed, the Dwyka Conglomerate (203), 

 which has here dwindled to a thickness of only 10 feet. These rocks 

 need no further reference here. 



Below the conglomerate we come to the next great development of 

 igneous rocks, which is about 400 feet thick. This is the rock 

 referred to by Gardner Williams and others as melaphyre (204, 205. 

 205a). It is an amygdaloidal, non-porphyritic rock, which is con- 

 siderably decomposed, so much so that determination of the ferro- 

 magnesian minerals is difficult. The rock varies a good deal in 

 texture and structure, since it occurs in very thick masses, and the 

 inner parts appear to be somewhat coarser than the margin. The 

 structure, apart from the amygdaloids, is essentially that of a coarse- 

 grained volcanic rock, and it is apparently not holocrystalline, 

 although decomposition has proceeded so far that it is difficult or 

 impossible to determine the original nature of the interstitial matter. 



The dominant minerals are a plagioclase feldspar and green 

 chloritic pseudomorphs, representing some member of the ferromag- 

 nesian group. The feldspar occurs in idiomorphic, somewhat 

 elongated prisms. The majority show twinning on the albite law, 

 and often Carlsbad twinning also. The extinction angle, measured 

 on the albite twinlamellae, rarely exceeds 5°, so that the feldspar may 

 be regarded as oligoclase. In a few cases a somewhat higher angle 

 indicates andesine, a few sections show Carlsbad twinning only, and 

 these prisms are rather shorter and stouter than the others, so that 

 a little orthoclase may be present. This point is somewhat doubtful. 



The ferromagnesian minerals are unfortunately very much decom- 

 posed, being chiefly represented by green chloritic pseudomorphs. 

 Many of these pseudomorphs enclose, however, a few fibres of a pale 

 or colourless actinolitic hornblende, of the character usual in this 

 mineral when of secondary origin, from the uralitisation of augite, 

 and a few crystals of comparatively unaltered augite may also be 

 seen. It it clear, therefore, that the original mineral was augite, 

 which has undergone the usual cycle of changes, being first converted 

 by uralitisation to actinolitic hornblende, and this in its turn to 

 chlorite. 



Some specimens of the rock show a certain amount of interstitial 

 quartz, and in places it is clear that the last substance to crystallise 

 was a eutectic of quartz and feldspar, as is so common in the more 

 acid members of the dolerite group. 



There is a good deal of iron ore in small scattered grains, includ- 

 ing both magnetite (or ilmenite) and pyrites, and apatite is very 

 abundant in minute needles. There are also a certain number of 

 idiomorphic crystals of a pleochroic yellowish green epidote, which 

 may be an original constituent, but is more probably secondary. As 



(b) Rogers. Geology of Cape Colony, p. 147. 



