74 Transactions of the SoutJi African Pliilosopliical Society. 



also occurs, sometimes forming part of a crystal which is chiefly 

 made up of the pale kind. Occasionally small crystals showing the 

 prism faces are met with, hut the larger plates seen in the slices are 

 always irregularly hounded by contact with other minerals, notably 

 plagioclase. This last remark applies also to the augite, which is 

 colourless in section and appears identical in character with the 

 augite of the olivine-dolerite. The hornblende and augite usually 

 occur together, intergrown with their orthopinacoidal faces parallel. 

 The augite often forms the inner part of a section of the two 

 minerals and is surrounded by a zone of a micropegmatitic inter- 

 growth of the two minerals, outside this area hornblende encases 

 the whole. The structure is easily seen by ordinary light under the 

 microscope, as the augite is colourless and the hornblende pale 

 greenish brown ; l)ut l^etween crossed nicols the two minerals are 

 still more clearly distinguished owing to their appearing dark at 

 different positions of the nicols. The intergrowths of the two 

 minerals are sometimes twinned, the twin-plane being the ortho- 

 pinacoid, common to both minerals. 



Hornblende is rarely found in the ohvine-dolerites, but it does 

 occur in them ; e.g., in the coarse olivine-dolerite of the sheet seen 

 on the shore between the Gxagha and Kologha Elvers, and in the 

 Kologha sheet ; in a slice from the dolerite sheet exposed along the 

 Kei Eiver at Mimosa Hill (the Kologha sheet) there is much horn- 

 blende of the same variety as that in the Gap-rock, and it is also 

 intergrown with the augite. 



The mica is a red strongly pleochroic variety, frequently altered to 

 a very pale greenish mineral with weak double refraction. The 

 mica appears to be uniaxial when examined in convergent light. It 

 is sometimes intergrown with the hornblende, but generally seems to 

 have crystallised later than that mineral. It frequently encloses 

 small zircons, round which there is always a " pleochroic halo " ; 

 jzircon occurs similarly in the hornblende. This mica occurs 

 frequently, and is an important constituent of the Gap dykes ; a 

 precisely similar variety of mica is found in almost all slices of the 

 Transkei olivine-dolerites, but in very small quantity. 



Quartz is alDundant in some parts of the Gap dykes, and present in 

 .all slides examined. It was the latest constituent to crystallise out 

 from the liquid magma ; it frequently forms a micropegmatitic inter- 

 ^rowth with a cloudy untwinned felspar, which is probably ortho- 

 clase. Both micropegmatite and quartz are occasionally seen in the 

 slices of the dolerites, but they are generally very subordinate con- 

 stituents of that rock, 



The ii'on ores are magnetite, titaniferous magnetite or ilmenite with 



