PRE-CAPE ROCKS OF THE NORTH OF THE COLONY 113 



mica-augite-diorite. The felspars consist both of ortho- 

 clase and plagioclase ; hornblende is more abundant than 

 augite. This rock may perhaps be one of the later 

 groups as it resembles in some respects the rocks of the 

 Transkei-gap dykes. 



A long dyke of diorite containing augite, hornblende, 

 enstatite, biotite and plagioclase felspar cuts the granite 

 area at T'Kuip near Omdraai's Vlei. 



Diabases of various types are met with in all the 

 formations from the Kheis to the Matsap series. In 

 some varieties there is quartz often intergrown with 

 felspar to form micropegmatite. The augite has usually 

 been partially or wholly converted into pale green fibrous 

 hornblende (uralite), but in some dykes there is an 

 original green or brown hornblende ; enstatite is not 

 uncommon. These intrusions are usually fine or medium 

 grained, but occasionally they may have porphyritic 

 felspar crystals as much as an inch across. Several 

 large intrusions of diabase in the Lower Griqua Town 

 beds in Hay must, from their petrological character, be 

 regarded either as sheets formed during the pouring out 

 of the Middle Griqua Town (Ongeluk) lavas or as feeders 

 by which these volcanic rocks reached the surface. 



In the Prieska district there are several masses of 

 gabbro and hornblende-pyroxene-picrites, the ages of 

 which are unknown. One of the picrite dykes near Kheis 

 has been taken to be a basic member of the Karroo 

 dolerites, but this is doubtful. Another mass, with the 

 character of olivine-gabbro at Brakbosch Poort, has been 

 invaded by granite and grades into hornblende-schist. 



The picrites contain both augite and hypersthene. 



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