44 DISCOVERY REPORTS 



Two specimens were collected by Mr Marr from near Point Thomas, Admiralty Bay. One, from 

 the coast a little south of the Point, is a fresh hypersthene-andesite. This rock appears to be identical 

 with the rock called hypersthene-augite-bandaite of the volcanic vent of Three Brothers Hill, Potter's 

 Cove, Fildes Strait, described by the author ((i), p. 68) from Mr Ferguson's collection, and the 

 reader is referred to this full description for petrographical details. In fact, Mr Ferguson actually 

 collected material from the same area ((i), p. 69). The extreme freshness of this rock, as compared 

 with the extensive alteration suffered by the lavas from the interior of Admiralty Bay, suggests that 

 it belongs to the later of the two volcanic episodes on the mainland of King George Island. 



On the other hand, the rock collected by Mr Marr from the coast of Ezcurra Inlet, one mile west 

 of Point Thomas, is an altered pyroxene-andesite which clearly belongs to the older series of lavas. 

 This occurrence suggests that the boundary between the older and newer series of lavas should be 

 drawn a little farther south than is shown on Mr Ferguson's map (D. Ferguson, op. cit. supra, 



fig- 2, p. 38). 



Fildes Strait. Fildes Strait separates King George Island from Nelson Island to the west. 

 Dr Mackintosh collected several specimens from a harbour (St. 1482) near the south end of the 

 strait, which may be identical with the ' Potter's Cove ' of Mr Ferguson, or it may be the ' Marian 

 Cove ' of the same author which is a little farther north. Dr Mackintosh describes the rocks as much 

 weathered, breaking down into screes through which solid rock appears here and there. 



Three of the specimens from this locality are dark, very compact rocks of basaltic type. They 

 consist mainly of a very fine-grained ground-mass of intersertal type with numerous microlites of 

 a striated feldspar giving extinctions up to 20° (andesine), scattered patches of chlorite and obscure 

 brownish material probably representing pyroxenes, and particles of haematitized iron ore embedded 

 in a reddish cr>'ptocrystalline or glassy base. The few small phenocrysts consist of epidotized plagioclase 

 (originally labradorite), and, in one section, fresh, euhedral, colourless augites of small optic axial 

 angle (.? pigeonite). A chemical analysis (p. 59) shows that these rocks must be regarded as of 

 tholeiitic composition. 



One specimen from this locality, however, is much more acid than the above, and must be classed 

 as soda-rhyolite or quartz-keratophyre. It is a whitish felsitic rock much reddened by haematitic 

 staining. In thin section it is seen to consist of a dense quartzo-feldspathic ground-mass with an 

 obscure hint of spherulitic structure, which carries numerous large phenocrysts of turbid albite and 

 haematitized biotite. 



A single specimen was collected from another locality on Fildes Strait near the narrow northern 

 entrance (St. 1483). Dr Mackintosh states that the rock formation here appeared to be quite different 

 from that of St. 1482, an observation which is confirmed by examination of the specimen. One 

 adjacent islet consisted of a dome-shaped mass of rock, ' probably basalt ', with a pronounced columnar 

 structure, but the outcrop from which this specimen was collected was not columnar. 



This rock turns out to be a feldspathic olivine-basah or olivine-andesite. Large phenocrysts of 

 fresh basic labradorite (Ab^Ang) are very abundant. Calcified and serpentinized olivines are numerous, 

 but a fresh pale augite is quite subordinate in amount. These are embedded in an intergranular 

 ground-mass consisting of plagioclase laths, augite and iron-ore granules, and a dark crypto- 

 crystalline base. 



North Foreland District. The North Foreland is the tip of a long narrow peninsula springing from 

 the north-eastern corner of King George Island. A shorter peninsula ending in a steep blulT headland 

 called Brimstone Peak occurs a mile or two to the west, and the two peninsulas enclose a deep bay. 

 Still farther west comes the well-known Esther Harbour, which was apparently not entered on this 

 occasion. This district (St. 1949) was visited by Mr Marr. 



