SOUTH SHETLAND ISLANDS 47 



phenocrysts of plagioclase (Ab55An4g) with both chemical and mechanical zoning, embedded in an 

 intergranular ground-mass consisting of feldspar microlites mingled with granules of augite and iron 

 ores. In other specimens the ground-mass contains some glassy matter usually blackened with 

 iron-ore dust, and is of intersertal or cryptocrystalline texture. 



In one of the rocks olivine is serpentinized and much reduced in amount, but its place is taken 

 by a small quantity of pleochroic hypersthene, illustrating the affinities of these olivine-basalts with 

 the more common hypersthene-augite-andesite lava-type. This association suggests that the olivine- 

 basalts are possibly due to some accumulative process operating in the early stages of the crystallization 

 of a pyroxene-andesite magma from which olivine began to separate. 



A closely comparable olivine-basalt has been described from Edinburgh Hill, a volcanic vent in 

 Livingston Island on the M'^Farlane Strait coast (Ferguson, op. cit. p. 44; (i), p. 66). Mr Ferguson's 

 fine photograph {op. cit. pi. i, fig. i) illustrates the magnificent columnar structure of this plug. An 

 olivine-basalt also occurs in the Desolation Islands, off the northern coast of Livingston Island (this 

 paper, p. ^i). Olivine-basalts of very similar characters have been described by H. H. Thomas 

 from Roberts Island ((2), p. 86). Basalts have also been described from the volcanoes of Deception 

 Island and Bridgeman Island. 



The mainland coast opposite Penguin Island, according to Mr Marr, consists of cliffs of lava, 

 fronted by extensive raised shingle beaches. Only one specimen was collected from this locality. 

 This is a typical augite-andesite with a beautiful pilotaxitic texture. The few phenocrysts are small 

 and consist mainly of a colourless augite which is, however, occasionally zoned with cores and bands 

 of a yellowish variety. The remaining phenocrysts are of andesine feldspar (Ab5An4). This lava is 

 quite fresh and no doubt belongs to the younger lava series. 



Many specimens of the coarse rounded shingle on the beaches of Penguin Island and the adjacent 

 mainland were collected. These consist of the older andesite lavas, together with many of the typical 

 plutonic rocks of the region — granite, adamellite, tonalite, quartz-monzonite, quartz-pyroxene- 

 diorite, etc., and two highly metamorphic types, quartz-chlorite-biotite-schist and hornblende- 

 granite-gneiss. 



Martin s Head and The Lions Rump. These are conspicuous adjacent headlands on the western 

 side of King George Bay. Mr Marr's report states that the basal portion of both headlands consists 

 of a dark grey columnar 'basalt' about 100 ft. in thickness, and with the columns inclined at a steep 

 angle towards the south. At Martin's Head the ' basalt ' is overlain by a massive rock with a ' twisted 

 appearance' (? confused columns), and from 50 to 60 ft. in thickness. This in its turn is covered by 

 what appeared to be a tuff (Fig. 3). Behind the headlands are tuff slopes characterized by an 

 abundance of angular rock fragments of many different kinds (.'' agglomerate). About 200 ft. above 

 the Lion's Rump there is what appears to be an old volcanic crater, now almost completely filled with 

 dirty stagnant ice (Fig. 4). A little to the north of the headland is a conspicuous lava flow reaching 

 the sea. Near by, perched on the beach, are several gigantic erratics of conglomerate, one of which 

 must weigh more than 200 tons. The conglomerate is exceedingly coarse, containing rounded water- 

 worn stones from a few inches in diameter to some 2 ft. across. 



The columnar lava of Martin's Head is a fresh hypersthene-augite-andesite of the type common 

 among the younger lava series. An andesite of similar type, but much richer in feldspar phenocrysts, 

 poorer in augite, and apparently devoid of hypersthene, was collected i mile east of the Lion's Rump. 

 From the same locality comes a green mudstone, consisting of finely divided quartz and vermicular 

 chlorite, much of the latter being aggregated into small rounded or ellipsoidal pellets. It is difficult 

 to diagnose this rock in the absence of data regarding its field occurrence, but it may be a muddy 

 sediment made up of decomposed wash from a surface composed of the older andesite lavas. 



