SOUTH SHETLAND ISLANDS 45 



Mr Marr writes that ' the cUffs forming the west side of the Foreland ... are composed of a massive 

 grey rock much traversed by cracks and joints, giving it a very shattered appearance '. This is borne 

 out by the three specimens collected here, which are all parts of a plutonic igneous rock of variable 

 grain size. This may be described as quartz-hornblende-pyroxene-diorite, and represents a ver)' 

 abundant type in West Antarctica ((i), p. 6i). Its three principal minerals are plagioclase (core 

 andcsine; outer shell oligoclase) ; pale green hornblende, sometimes with a pale brown tint; colourless 

 diopsidic pyroxene which is altering into a pale green amphibole. The accessory minerals are quartz, 

 filling the interstices between the main constituents; some large flakes of reddish biotite; abundant 

 ilmenite altering to leucoxene ; and a considerable amount of apatite. The amphibole and pyroxene 

 tend to form well-shaped crystals, and to enter into clots with biotite and ilmenite. One of the 

 specimens is a true plutonic type with allotriomorphic texture and comparatively coarse grain. Another 

 is a fine-grained aplitic type poorer in the mafic minerals, which may be styled quartz-microdiorite ; 

 and the third is a porphyritic type in which the feldspars, hornblendes and pyroxenes (including both 

 augite and hypersthene) occur as phenocrysts in a fine-grained granulose ground-mass. A few large 

 crystals of bluish apatite occur in this rock. This type may represent a chilled marginal phase of 

 the intrusion. 



It is clear that the vicinity of North Foreland is occupied by a large plutonic intrusion of the same 

 type as occurs at Noel Hill, Marian Cove ((i), p. 6i), and at Le Poing on the west side of Admiralty 

 Bay ((i), p. 62). This mass may occupy the whole of the eastern side of King George Island, as 

 Mr Marr states that the cliflFs to the east and south of the Foreland, and probably as far as Cape 

 Melville, are high and sheer, and seem to consist of the same grey massive rock. 



Brimstone Peak is said to be composed of perpendicular 'basalt' cliffs rising sheer out of the sea 

 to a height of 150 ft. The single specimen obtained from this locality shows, however, that the rock 

 is a fresh hypersthene-augite-andesite of the Recent type so common elsewhere in King George Island. 

 The hypersthene is mostly altered to chlorite or bastite, and often forms the core of an augite crystal. 

 A single crystal of magnetite-rimmed brown hornblende was present in the thin section. 



Bolinder Beach (St. 1953) is situated a few miles west of Esther Harbour and Brimstone Peak. 

 It is described by Dr Ommanney as a bluff peak crowned by three buttresses of dark grey and light 

 brown rock veined by what, on closer examination, proved to be finely crystalline rose and amber 

 quartz. All the rock specimens collected here were lost in a boat accident except a few from a 100 ft. 

 cliff at sea-level on the northern face of the bluff. 



This rock proves to be an enstatite-andesite of micro-porphyritic and intersertal texture, consistmg 

 of very numerous feldspar laths (andesine, AbgAn,), and less abundant pseudomorphs in chlorite 

 after enstatite (typical square prisms with truncated corners), in a dense, brown, cryptocrystallme to 

 glassy ground-mass. It probably belongs to the older series of lavas, as it is intersected by mineral 

 veins which may represent the same group of veins (quartz and pyrites) as that described by Ferguson 

 from the islands of Esther Harbour {op. cit. supra, p. 41). These veins run nearly east and west, and 

 might thus probably intersect the region of Bolinder Beach. 



Pengum Is/and and Adjacent Mainland. Penguin Island is situated off the eastern horn of King 

 George Bay. That Penguin Island is a Recent volcano, one of the line of volcanoes fringing Bransfield 

 Strait, is Mr Marr's important and most interesting discovery. The following is a description of 

 Penguin Island quoted from Mr Marr's report: 



The southern half of Penguin Island is a volcanic cone. The northern half consists of a long, very low plateau, 

 much of it only about 50 ft. high. The western face of the cone is steep and has a deep brick-red tint. On its south- 

 eastern and eastern sides the cone slopes down to a plateau roughly 100 ft. high, which is continuous in a wide 

 sweep with the lower plateau which forms the northern half of the island. On the southern side the cone ends 



