THE INTRUSIVE DOLERITES AND ALLIED ROCKS 275 



coarse dolerites and absent from the fine-grained and 

 glassy rock, points to the conclusion that the molten 

 rock which forms the latter has been squeezed out of a 

 partly consolidated dolerite in which the large olivine 

 crystals were retained by the partly formed plagioclase 

 and augite. 



In the Eastern Province there are certain basic intru- 

 sions which cut the dolerite sheets and dykes, as for 

 example the two long parallel dykes which pass through 

 the town of Lady Frere. These intrusions do not ex- 

 hibit an ophitic structure, the rocks being usually por- 

 phyritic. In Victoria West and Carnarvon the same is 

 also the case. 



It is not uncommon to find more acid varieties of the 

 dolerite, distinguished from the typical dolerite by their 

 lighter colour, which is due to their containing a larger 

 portion of quartz and potash felspar, less augite and no 

 olivine. In some cases they pass insensibly into the 

 normal basic rock ; for example, in a sheet between 

 Sterkstroom and Tarkastad. A section of the rock from 

 this locality shows an intimate intergrowth of pale 

 brownish augite, pale green hornblende, large crystals 

 of plagioclase, much magnetite, and a very large amount 

 of quartz usually forming a micropegmatitic intergrowth 

 with felspar. 



Generally, however, these acid varieties form veins or 

 dykes which cut through the dolerite sheets, and in 

 many cases the rocks can be termed granophyres. Not- 

 withstanding this, there are strong reasons for believing 

 that they are a late product of the same molten rock- 

 magma that supplied the dolerites ; the more basic por- 



18* 



