256 AUSTRALASIAN ANTARCTIC EXPEDITION. 



apparently of a felspar (? orthoclase) crowded with confused radiating aggregates of 

 very tiny brown rods of some intermediate substance, possibly rutile'. The last phase 

 of consolidation of the rock has been the introduction of pyrites, often along cracks 

 in the felspar. 



A very fine-grained type is represented by Specimens Nos. 449 and 457, which 

 are really duplicates. The rock is hard, compact, and aphanitic, with a subconchoidal 

 fracture in places, and showing tiny dendritic patches of pyrites on joint planes. It is 

 holocrystalline, intergranular in fabric, and of notably uniform grainsize. 



Felspar (acid labradorite) is arranged in little bundles of parallel laths up to 

 3 mm. long, the bundles being oriented in all directions. The pyroxene is very plentiful 

 in tiny light-green prisms and granules. It is predominantly if not solely monoclinic, 

 but though the presence of enstatite-augite is suspected the small size prevents con- 

 clusive proof. Quartz is in fairly abundant interstitial patches, and an interstitial 

 felspar of low R.I., probably orthoclase, also occurs, but never in intergrowth with 

 quartz. 



Magnetite is quite plentiful, and minute shreds of biotite and needles of apatite 

 are fairly common. Infrequent vesicles are filled with chlorite. (Plate XXXIX, Fig. 3.) 



A different type of rock is No. 459, which is close-grained and slightly vesicular 

 in hand-specimen. It contains two generations of felspar prisms, both about Ab 35 An 65 , 

 somewhat zoned, and often showing only Carlsbad twinning. Enstatite-augite, the 

 dominant mineral, is moulded on the felspar, but typical ophitic fabric is not developed, 

 the pyroxene being in small grains. Iron ores include both ilmenite and magnetite, 

 and a little pyrites has been introduced subsequently to consolidation. Frequent 

 patches of a fox-red pleochroic mineral (? iddingsite) may represent pseudomorphs 

 after olivine, and a good deal of greenish and brownish chloritic material is present, 

 sometimes forming cores to the felspar crystals. (Plate XXXIX, Fig. 4.) The abundant 

 small interstitial spaces of the rock are filled with material of a light brownish-green 

 colour, thickly charged with microlitic magnetite. This has a feeble polarization 

 and may represent the devitrification of a glassy base. 



All these rocks possess certain characteristics which link them with each other 

 and with the quartz-dolerites, the most important being the presence of enstatite-augite 

 and of quartz and orthoclase. Textural and mineralogical variations are such as might 

 be expected in a differentiation series, and such indeed as have been described as resulting 

 from the crystallization of quartz-dolerite magmas. 



It is doubtful whether the rock labelled No. 208 should be grouped along with 

 the dolerites just described. It is a dominantly felspathic rock, with prisms of labradorite 

 averaging about 1-75 mm. in length, another felspar, probably orthoclase, being present 

 in minor amount. Pyroxene is for the most part moulded on the plagioclase, and plays 



1 cf. J. V. Lewis, Annual Report of State Geologist of New Jersey, 1907, p. 117. 



