336 N. E. Junner: 



pass out to the edges. In bright, reflected sunlight, the olivine is 

 seen to be mostly altered to red iron oxide, probably hematite. 

 An occasional porphyritic crystal of augite occurs, and has sui!ered 

 like the olivine. The bulk of -the augite, however, occurs scattered 

 through the ground mass of the rock as yellow and colourless 

 anhedral grains and prisms. The ground mass of the rock consists 

 of microscopic laths of felspar, often in fluidal arrangement, and 

 abundant dust and fine grains of black iron oxide. Zeolites occur 

 in all sections examined, and they are frequently associated with 

 apatite needles, and contain prisms of augite and grains of 

 magnetite as inclusions. The phenocrysts of olivine and plagio- 

 clase had probably crystallised out from the magma before ex- 

 trusion, and rapid chilling caused the separation of microlites 

 of felspar and dust of iron oxide. Section A.32, Kangaroo Groimds, 

 quarry, south-east of Garden Hill, shows i^ilotaxitic structui'e. 

 Zeolites of very low birefringence occur filling vesicles. Radiating 

 natrolite with birefringence, considerably above the felsj^ar also 

 occurs, filling steam cavities. Section A28 shows olivine frequently 

 clear and colourless. Occasionally the outlines of felspar now 

 replaced hy zeolites are seen. 



2. Medium to coarse grained basalt. — In hand specimen, with the 

 aid of a lens, crystals of weathered olivine and felspar laths can lie 

 sometimes identified. 



Microscopically this type differs from the first in the presence 

 of numerous phenocrysts of titaniferous augite, and in the iiatu>'e 

 of the giound mass. Section A15, Kangaroo Grounds, is a holo- 

 crystalline fairly even grained hypidiomorphic rock with oj^hitic 

 texture. The minerals present ai'e plagioclase, augite, olivine, 

 magnetite, ilmenite and apatite. Zeolites and chlorite occur as 

 secondary constituents. Plagioclase is present in long prismatic 

 lath.«!, frequently zoned, and having a maximum extinction of about 

 42 deg. The augite is a titaniferous variety, and it is pleochroic 

 from jiurple to brown or yellow, and occurs in anhedral forms. 

 Extinction angle of the augite is 50 deg. from 100. The augite is 

 ophitically penetrated by the felspar laths. Numerous inclusions 

 of olivine occur in the augite. Olivine is present chiefly as allotrio- 

 morphic grains. Alteration has taken place along ci'acks to 

 greenish chloritic material, and occasionally to red iron oxide. 

 Magnetite commonly occurs idinmorphic as octahedra. Numerous 

 irregular grains, purple in reflected light, are probably ilmenite. 

 Low polarising zeolites frequently fill interstices in the rock. 

 The rock may bo described as an ophitic olivine dolerite. 



