43 



PETROGRAPHY 



KING GEORGE ISLAND 

 Admiralty Bay. The Ullmann Range, a ridge trending north and south, projects into Martel Inlet 

 (north-east arm of Admiralty Bay) and forms the eastern side of Visca Anchorage. Specimens were 

 collected from the western side of this ridge. In his notes, Dr Mackintosh has given an excellent 

 sketch of the Ullmann Range as seen from Visca Bay, and has called attention to a prominent dike 

 which climbs the scarp and culminates in a sharp pinnacle near the central point of the ridge. This 

 view is undoubtedly the subject of Mr Ferguson's fine photograph (Ferguson, op. cit. pi. iii, fig. 1),^ 

 which clearly shows the dike and a series of lava scarps to the left (north) of it. 



The dike consists of a highly porphyritic pyroxene-andesite with phenocrysts of plagioclase (basic 

 andesine, Ab^j), yellow augite, and chloritic pseudomorphs after orthorhombic pyroxene, in order 

 of abundance. There are also some large irregular masses of magnetite. The ground-mass is fine- 

 grained, but apparently holocrystalline, although somewhat altered. It contains a little quartz. 



The lavas of which the Ullmann Range is composed are represented by several specimens mainly 

 collected from screes. Alongside the dike occurs a trachytic lava with a very dense fluxional ground- 

 mass, consisting of minute feldspar microlites, apparently orthoclase, in a cryptocrystalline base. 

 There are numerous small phenocrysts of soda-orthoclase and a plagioclase which is now mostly 

 albite, but the presence of epidote suggests that it may originally have been a more calcic variety. 

 The rock also carries numerous euhedral crystals of ilmenite rimmed with a leucoxenic alteration 

 product. Traces of ferromagnesian minerals are present, but are altered beyond recognition. This rock 

 is notable in containing a few crystals of pale blue pleochroic apatite. 



A coarser textured specimen provides further data. The ground-mass is seen to consist of laths of 

 orthoclase mingled with oligoclase, and contains visible quartz. Still another specimen consists of 

 an angular breccia of fragments similar to the above. Many of the fragments are rich in quartz. The 

 shapes of some pseudomorphs outlined in iron ores suggest that the ferromagnesian mineral in these 

 rocks may have been hornblende. 



These lavas may be provisionally classed as dacite or quartz-latite according to the amount of 

 quartz or orthoclase present. Similar types have been described from Admiralty Bay by the author 

 ((i), p. 71). They also occur in the Fildes Strait area (p. 44). 



Near the beach on the western side of the Ullmann Range was collected a lava which maybe described 

 as an altered quartz-andesite. It contains phenocrysts of plagioclase badly carbonated, and chloritized 

 pseudomorphs after pyroxenes. Quartz is comparatively abundant, but is partly of secondary origin. 

 Bluish apatite crystals are abundant, and the lava is therefore regarded as belonging to the same 

 series as those described above. From the screes to the south of this point a silicified and pyritized 

 volcanic tuff was collected. 



Mr Marr collected three specimens from the western side of the Keller Range along the eastern 

 shore of Mackellar Inlet. He describes this coast as consisting of slopes of reddish brown tuff with 

 frequent outcrops of lava which are also prominent at sea level. While two of his specimens are so 

 highly carbonated and silicified that they can only be described as altered andesites, the third, which 

 is stated to have come from a fan-shaped columnar outcrop, is less altered, and can be described as 

 pyroxene-andesite. Feldspar phenocrysts are numerous and, although badly carbonated, can be 

 identified as plagioclase of composition about k\,An^,. The ferromagnesian constituent consists 

 of chloritized pseudomorphs after pyroxenes, usually found in crystal clots along with feldspar, 

 ilmenite, and large crystals of apatite. The ground-mass is dense, brown, and cryptocrystalline, the 

 only identifiable constituent being feldspar microlites showing straight extinction (.? oligoclase). 



1 For full reference see p. 76. 



