36 IIASTC INTRUSIONS 



magnesian limestone belonging to the Malmesbury 

 series has been altered by the granite to a cream-coloured 

 marble containing crystals of phlogopite mica and minute 

 specks of a black mineral, probably graphite. Some 

 distance farther to the west the granite has penetrated 

 quartzite, and angular blocks of the latter have been 

 wedged off and enveloped by the intrusive rock. The 

 quartzite has become impregnated with granitic material, 

 and felspar has been developed in it along with a little 



mica. 



Basic In trusions . 



At several places in the south-western districts igneous 

 rocks of more basic composition than granite occur as 

 dykes in the Malmesbury beds and in the granite. The 

 dykes in the Peninsula, consisting of augite, plagioclase, 

 and magnetite, may be regarded as belonging to the 

 group of the Karroo dolerites. Near Somerset West 

 there are intrusions commonly showing prisms of 

 plagioclase felspar ophitically enclosed in augite, but the 

 latter has been partially or wholly changed to pale green 

 hornblende (uralite) and in some cases there has been a 

 further alteration into chlorite ; the ilmenite has been 

 converted into leucoxene. At Gordons Bay there are 

 small diabase dykes of this type, but more highly sheared 

 and altered. 



Some interesting rocks, which may be called diorites 

 and quartz-diorites, form rather limited dyke-like 

 masses in the granites of the Malmesbury district. At 

 Klein Paarde Berg there is a broad dyke about a mile 

 long, composed of hornblende, felspar, mica, quartz, 

 magnetite, apatite and zircon. It is a holocrystalline 



