400 W. R. BROWNE AND A. B. WALKOM. 



It shows albite twinning and sections in zone perpendicular 

 to (010) give symmetrical extinctions up to 21° showing 

 that it is a labradorite of composition about Ab 4 An 3 . It 

 shows a variety of decomposition products — some of it is 

 decomposed to kaolin, some shows a development of sericitic 

 aggregates along twinning planes, and still other pieces 

 are almost completely pseudomorphed by calcite ; in the 

 latter case there is considerable development of hematite 

 along the cleavage and twinning planes. Biotite is present 

 as small long ragged flakes, some of which are considerably 

 bent. Magnetite and apatite are also present and there 

 is a small amount of chlorite. 



Dacite (dyke rock) New Matthews' Gap Road. 



This is a dyke related to the dacite. It is porphyritic 

 with a glassy base and showing well marked flow-structure. 

 The phenocrysts are generally coarse, with an average 

 diameter of 2 to 3 mm. They consist chiefly of quartz and 

 felspar. The quartz is in some cases beautifully corroded 

 and contains inclusions of the ground mass. The felspars 

 are much decomposed and there is a heavy deposit of 

 hematite along the cleavage and twinning planes. Flakes 

 of biotite are present and also ilmenite. 



Trachyte, Drake's Hill. 



This is a brown, fine-grained rock. It is slightly drusy 

 and secondary minerals are developed in the druses. 

 Crystals of a pinkish-coloured felspar can be recognised in 

 hand specimen. Under the microscope it is hypocrystalline 

 and porphyritic. The phenocrysts' average size is medium. 

 There are two generations of felspar. Those of the larger 

 generation are medium-sized subidiomorphic crystals. They 

 consist of both' orthoclase and plagioclase, the former being 

 the more abundant. The plagioclase is twinned after the 

 albite law. The smaller ones consist of very fine needles 



