276 ACID INTRUSIONS 



tion represented by the dolerites was got rid of, and a 

 part of the more siliceous residual matter was extruded 

 after the dolerites had solidified. These acid dykes 

 appear to be more abundant and larger in the Transkei 

 than elsewhere. 



On Plate XIX. is shown a thin dyke of light colour 

 traversing a sheet on the shore near the Kobonqaba 

 mouth, Kentani. The sheet itself is a rather coarse 

 ophitic olivine-dolerite ; a small amount of green horn- 

 blende is intergrown with the augite, and red biotite, 

 magnetite, and apatite are present in fair quantities and 

 quartz in very small amount ; the olivine is partly con- 

 verted into serpentine. The light-coloured dyke has no 

 olivine or augite in it and very little hornblende ; red 

 mica is abundant ; the plagioclase forms zoned crystals. 

 In addition there is much orthoclase in the rock, inter- 

 grown with quartz to form micropegmatite. Apatite, 

 magnetite, and zircon are found in the acid rock as well 

 as in the dolerite. 



Near Gqunqi there is a short dyke-like mass of grano- 

 phyre about a mile long from north to south and several 

 hundred yards wide ; it is rather like the acid dykes of 

 the Kobonqaba sheet. It traverses both the sedimentary 

 rocks and the Manubi sheet. 



A large mass of very acid rock later than the dolerite 

 sheets forms a considerable part of Gonubie Hill in 

 Komgha; it is a microgranite consisting of quartz, 

 orthoclase, and black and white mica. 



The east and west dykes that traverse the Kentani 

 Division are very remarkable ones ; they extend from 

 the Kogha mouth to the Gcua Biver and can be followed 



