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



are slightly pleochroic but too small to be accurately 

 determined — they are probably pyroxenes. The base forms 

 about 20 per cent, of the rock and contains a moderate 

 proportion of light-coloured glass. 



Trachy-andesite, Matthews' Gap Road. 



Pine-grained rock, appearing rather tuffaceous in parts. 

 A pink secondary mineral is present. Under the micro- 

 scope it is hypocrystalline and slightly porphyritic; the 

 fabric is trachytic. The phenocrysts are of felspar and 

 magnetite ; the felspar is in idiomorphic tabular crystals, 

 somewhat decomposed, and is mostly orthoclase. The base 

 is composed of minute felspar microlites and numerous 

 small magnetite grains. There are a few grains of pale 

 green mineral with low D.R., probably chlorite. 



Andesite, Old Road. 



Dark blue rock, slightly porphyritic with fine-grained 

 base. Very hard and fresh and weathers into spheroidal 

 lumps. Minerals visible in hand specimen are lath-shaped 

 glassy felspars and a small amount of dark ferro-magnesian 

 mineral. Under the microscope the texture is porphyritic. 

 The phenocrysts average about 1 by 1*5 mm., the base is 

 hypocrystalline. Plagioclase constitutes the great majority 

 of the phenocrysts and a section parallel to (010) gives an 

 extinction of - 20° measured from the cleavage parallel to 

 (001) and the plagioclase is therefore labradorite (Ab 3 An 4 ). 

 There are a few large grains of magnetite. The base is 

 made up of small lath-shaped felspars, very numerous grains 

 of magetite, minute apatite prisms, small dark grains, 

 almost opaque, too small to identify but probably pyroxene, 

 and a fair amount of light coloured-glass. This has been 

 analysed, with the following result : — 



