THE INTRUSIVE DOLERITES AND ALLIED ROCKS 279 



along the southern dyke is that of the Kabakazi and 

 the lower part of the Kogha, in all about ten miles long. 

 The width of the dykes is at the most 400 feet. 



The intrusion of these dykes w T as certainly later than 

 that of the dolerites, for they cut through the latter. In 

 its nature and composition the augite-mica-diorite form- 

 ing them is intermediate between the ordinary olivine- 

 dolerite and the granophyres mentioned previously. 

 None of the minerals or structures in the diorites are 

 entirely foreign to the dolerites, and the diorites contain 

 much less quartz and micropegmatite than the grano- 

 phyres. Olivine is the only constituent of the dolerites 

 that is absent from the diorites and granophyres. 



This bears out the opinion that all these rocks can be 

 regarded as having been derived from the same molten 

 rock-magma ; it is not assuming too much to suppose 

 that the whole of the dolerite sheets and dykes of the 

 Karroo region belong to one period of igneous activity. 



While dykes and sheets are abundant in the Molteno 

 beds in the Ked beds they are not so numerous, in the 

 Cave sandstone sheets are almost absent, and in the 

 volcanic beds the dolerites are, at least chiefly, represented 

 by narrow vertical dykes. These dolerites which cut 

 both the lavas and the tuff-filled pipes from which they 

 once issued can be definitely traced to the large sheets 

 of the lower-lying country. How much later than the 

 Stormberg lavas they are is not quite clear. Although 

 the dolerites differ somewhat from the Stormberg lavas, 

 in containing biotite mica and sometimes brown horn- 

 blende, and in having a higher specific gravity, and 

 although some of them were intruded subsequently to 



