GEOLOGY OF ERUPTIVE AND ASSOCIATED ROCKS, POKOLBIN. 405 



dant in small grains. A light-coloured ferro-magnesian 

 mineral is present in small grains, probably augite. Chlorite 

 is sparingly present. A good deal of secondary calcite is 

 present. 



Olivine basalt, Portion 78, Parish Milfleld. 



Dark, rather heavy rock. Felspar and a dark mineral 

 can be seen in hand specimen. Microscopically it is hypo- 

 crystalline, the amount of glass being very small. Radiate 

 fabric is fairly well developed and ophitic structure is 

 present. The minerals are: — Plagioclase, olivine, augite, 

 magnetite and serpentine. The plagioclase is quite fresh 

 in idiomorphic lath-shaped crystals whose maximum extinc- 

 tions in sections perpendicular to (010) is 39°, indicating a 

 bytownite of composition near Ab :3 An 7 . The augite is 

 titaniferous, violet in colour and with fairly marked pleo- 

 chroism. It is abundant, but only in small pieces, and 

 ophitically encloses plagioclase. Olivine is present in 

 fair quantity and is partly altered to serpentine, the alter- 

 ation having taken place round the periphery and along the 

 numerous cross cracks present. Magnetite is present in 

 numerous small grains mostly included in other minerals. 



Basalt, Portion 55, Parish Milfleld. 



Under the microscope it is hypocrystalline. The fabric 

 is somewhat fluidal. The minerals are: — Plagioclase, 

 augite, biotite, magnetite and a pale green zeolite. The 

 plagioclase is in lath-shaped crystals and a good deal 

 decomposed to kaolin. The augite is titaniferous and 

 slightly pleochroic. Biotite is dark reddish-brown in colour, 

 idiomorphic, often in hexagonal sections, with parallel 

 extinction and pleochroic. Sometimes the central part is 

 altered to a greenish aggregate. There is also a fairly 

 large amount of pale green zeolitic mineral present. 



