PRE-CAPE ROCKS OF THE NORTH OF THE COLONY 73 



lavas, and the matrix in many cases was glassy, but in 

 some there is much quartz and quartz-felspar micropeg- 

 matite in the ground mass. The rocks have generally 

 been much altered, the augite being replaced by uralitic 

 hornblende and the felspar by quartz, calcite and epi- 

 dote. The ilmenite of the original rock has been 

 changed into leucoxene. Porphyritic lavas with rather 

 large felspar crystals sometimes occur in the higher 

 part of the series. Two analyses of the diabase from 

 the Vaal River and the Kimberley mine, with 49'5 and 

 45*6 per cent, of silica respectively, are given by De 

 Launay. 1 In many places the rock seems to have re- 

 ceived an addition of silica, now in the form of either 

 opal or chalcedony. 



More acid lavas, andesites and quartz-porphyries, are 

 found in the higher parts of the series along the Vaal 

 River below Barkly West. Coarse breccias and fine- 

 grained tuffs are often interbedded with the lavas ; in 

 Hopetown and Herbert a band, from 50 to 100 feet thick, 

 of coarse volcanic breccia, with many fragments of a 

 more acid variety of lava than the usual diabase, covers 

 a wide area and lies immediately above the quartzites 

 of the lower part of the series. 



The largest area occupied by the Pniel series, though 

 over parts of it they are thinly covered by later rocks, 

 is that of the Dry Harts, Harts and Vaal River Valleys. 2 

 West of Vryburg they are met with at Takoon, where 

 they form a narrow belt consisting of a thin layer of 

 lavas lying between the Black Reef and the granite ; 



1 Les Diamants du Cap, Paris, 1897, p. 109. 



2 See Sheets 41, 42, 46, 50 of the Geol. Map of Cape Colony. 



