248 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Elsewhere the intergrowth consists of tiny squares and rectangles and gnomons 

 of felspar in optical continuity with the outer margins of the plagioclase laths, forming 

 a pattern on a larger surface of orthoclase. Other patches of the mesostasis consist 

 of graphically intergrown quartz and ortboclase, the former being recognised by the 

 familiar triangular forms of its skeleton crystals, while again, extensions of the plagio- 

 clase laths may be graphically intergrown with quartz. This latter mineral is also 

 found as independent interstitial grains quite evidently primary in many cases, though 

 at times possibly secondary. On the whole, however, quartz plays a very minor part 

 in the mesostasis. 



What appears to be minutely granular pale chlorite functions occasionally as 

 an interstitial filling in the two finest grained rocks. 



Iron ores, including apparently both magnetite and ilmenite, are not very 

 abundant. They occur mostly embedded either in the peripheral parts of the pyroxenes 

 or else in the mesostasis. 



Tiny apatite needles are pretty abundant in the mesostasis of all the rocks, but 

 practically nowhere else, so that the total proportion of apatite in the rocks is insig- 

 nificant. 



Of the two specimens labelled No. 732, the finer-grained, which we may call 

 No. 732B, was collected " 6 feet above the contact of the volcanic rock and the sand- 

 stone." The hand specimen shows very marked prismatic jointing on a. small scale. 

 The main constituents of the rock are, in volume percentages : 



Plagioclase 41 



Pyroxene 44 



Iron Ore 1 



Altered Olivine (?) 3 



Mesostasis ... ... ... ... 11 



The plagioclase is mostly in thin to stout laths averaging perhaps -3 mm. ; a few 

 larger individuals apparently slightly more basic than the rest are aggregated in 

 glomeroporphyritic fashion. Enstatite-augite is probably the dominant pyroxene, but 

 an interesting feature is the presence of a very little rhombic pyroxene, very pale- 

 coloured and non-pleochroic, but having negative birefringence. It generally occurs 

 completely enclosed in augite, with which it is in parallel orientation. A very few 

 minute flakes of biotite were detected attached to grains of pyroxene. 



A peculiar feature of this rock is the presence of scattered patches of a substance, 

 green to brownish in colour, pleochroic, with a fibrous structure and strong birefringence. 

 The patches are streaked with carbonates and crossed with cracks containing magnetite 

 dust. These characters suggest aggregates of talc, or perhaps iddingsite, after olivine, 

 but this is by no means certain, especially as enstatite or hypersthene might alter in 

 similar fashion. The outlines of the aggregates do not help much in the determination 

 of the original mineral. 



