

MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 195 



The term pilotaxitic is applied to a holo-crystalline structure, 

 especially characteristic of certain porphyrites and basalts in 

 which the ground-mass consists essentially of slender laths and 

 microlites of felspar in felted aggregation. The lath-shaped 

 felspars often exhibit a streaming fluxion-structure. 



The addition of films of glass to a pilotaxitic rock produces a 

 hyalopilitic structure. 



Dolerite, basalt, and anamesite are terms of pre-microscopic 

 days, and may be of occasional use still. The modern petrographers 

 generally follow Professor Rosenbusch and refer the basalts to 

 one or other of these structural varieties : — 1. hypidiomorphic 

 granular, 2. intersertal, 3. holo-crystalline-porphyritic, 

 4. hypo-crystalline, and vitropyric* 



Slice 1. — Portion of a lava flow; Tumberumba, New South 

 Wales. A grey to black rock, showing greenish crystals of olivine; 

 breaks with a conchoidal fracture. Under the microscope it shows 

 an ophitic structure, prisms of felspar penetrating the pyroxene in 

 a perfect manner. The rock is microscopically holo-crystalline 

 approaching to a pilotaxitic structure. In plane light pyroxenes 

 show a rich brownish-red. The felspars are colourless and the 

 magnetite, of course, black and opaque. In shape the magnetite 

 is seen to occur as cubical grains, long prism-like forms, and also 

 in aggregations. The largest magnetites measure about the one- 

 hundredth part of an inch. There is a considerable amount of 

 green colouring matter following the cracks and lining a few 

 cavities in the slice. This colouring matter has its origin in the 

 decomposition of the olivines as can easily be understood by an 

 inspection of the slice. In polarized light, with crossed nicols, 

 the rock has a particularly rich and bright appearance, interstitial 

 matter not being too abundant. The felspars show no disposition 

 towards a flow structure ; on the contrary they present the appear- 

 ance of a felted aggregate. The minerals in this slice are, plagio- 



* Rosenbusch, Mikroskopische Physiographie der Massigen Gesteine, 

 Second Edition, pp. 723 - 729. 



