MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 197 



is often seen in the slice moulded perfectly round the felspars, 

 and, in one instance, a cruciform felspar is completely enveloped 

 in the opaque magnetite. It is not at all likely in the original 

 separation of the minerals from a molten magma that the more 

 basic mineral should be the last to crystallize. Magnetite was, 

 without doubt, the first to separate and these anomalies must 

 be explained by assuming that a certain quantity of the magnetite 

 is of secondary origin. The largest felspars measure the one- 

 fiftieth of an inch, their average size being about one-seventieth 

 of an inch. There is very little olivine in the slice and when 

 recognised under crossed nicols, it is always seen in allotrio- 

 morphic granules. The pyroxene is of a greenish-yellow shade in 

 ordinary light. To distinguish it from the olivine recourse must 

 be had to crossed nicols under which the olivines are seen to 

 stand out in clear prismatic colours. 



Slice 4. — Basalt, Tingha, New South Wales. This rock forms 

 portion of a Tertiary basaltic flow. On freshly broken surfaces 

 it shows a sparkling blue-black appearance. A few micro-porphy- 

 ritic minerals are just visible to the unaided eye. Under the 

 microscope the rock is seen to consist mainly of plagioclase and 

 augite with rods and patches of magnetite. These minerals form 

 a micro-crystalline ground-mass in which are set micro-porphyritic 

 crystals of fairly fresh olivine. No isotropic matter can be 

 detected. The rock is coloured a light green in places through 

 secondary minerals. Rods of magnetite are very noticeable in 

 this slice and many of them measure one-seventieth of an inch in 

 length, In polarized light, olivine is found to occur as granules 

 in the base. No porphyritic crystals of augite occur, but this 

 mineral fills up spaces in the ground-mass, both as granules and 

 short prisms. 



Slice 5. — Tertiary Basalt, Tingha, New South Wales. Only 

 one of the constituents (olivine) of this rock is visible to the 

 unaided eye in the slice. Magnetite is abundant, and is seen 

 moulded around the ends of the felspars as in the last slice. The 

 magnetite is also often seen in perfect wedges filling up the angles 



