ADELAIDE ISLAND 75 



andesite), with their tuffs and explosion-breccias, which has been subjected to extensive crushing and 

 shearing. This complex appears to be identical with that described by Quensel {op. cit. supra) from 

 Patagonia and Tierra del Fuego. 



The same or a similar complex of acid igneous rocks has also been noted in at least three localities 

 in the Graham Land peninsula and adjacent islands. Thus, O. Nordenskj6ld,i writing of the loose 

 blocks on the land surface and in the moraines, and of the boulders in the Late Mesozoic and Tertiary 

 conglomerates, found in the northern part of the peninsula, says that they include quartz-porphyries 

 of various types, some showing such a high degree of mechanical metamorphism that they have been 

 transformed into sericite-schists. He remarks the similarity of these rocks to the porphyry formations 

 of Patagonia which he had previously investigated. Again, in 1913, Nordenskjold'^ stated that at 

 Hope Bay, within the eastern ranges of Graham Land, there occurred acid porphyries and porphyry 

 tuffs apparently concordant with the folded and metamorphosed Jurassic sediments of that locality. 

 He further remarked that these rocks are probably the same as those that form part of the South 

 American cordilleras. 



At Hope Bay, on the western side of Antarctic Sound at the northern tip of Graham Land, J. G. 

 Anderson^* described sediments with Jurassic plants overlain, in Mount Flora, by 200 m. of whitish 

 tuffs derived from acid volcanic rocks. 



Finally, E. Gourdon* described an erratic from the north of Hovgaard Island as a 'rhyolite with 

 globular quartz', which he regarded as an 'ancient facies' of porphyry. This rock carries porphyritic 

 orthoclase and bipyramidal quartz, and the crystals are associated with sinuous flow lines. The quartz 

 is much corroded and surrounded by aureoles of ground-mass material. The rock, he says, has suffered 

 severe mechanical deformation. It obviously has a close resemblance to the porphyroids of Adelaide 



Island described above. 



The last remaining group of rocks from the Adelaide Island collection consists of oligoclase-andesite 

 lavas, and coarse tuffs or breccias consisting mainly of fragments of the same type. Eight stones 

 are assigned to this group. Two are normal lava types, two are slaggy and vitreous variants, and the 

 remaining four are coarse tuffs or breccias. The lavas exhibit numerous very small micro-phenocrysts 

 of fresh oligoclase, usually with well-marked parallel flow-orientation, embedded in a fine-grained 

 ground-mass consisting of microlites of oligoclase and orthoclase, with chlorite representing the 

 original ferromagnesian mineral (probably augite). This is peppered with numerous, irregularly shaped 



particles of iron ore. t^i a- j 



Slaggy variants of this lava contain much dark glass and are somewhat haematitized. The tuffs and 



breccias consist of angular fragments of the above-described lava of varying textures, with an occasional 



flake of mudstone or shale. Furthermore, volcanic mud has infiltrated into the breccias and acts as 



a scanty cement. 



These rocks recall the characteristics of some of the older group of andesite lavas which are so 

 conspicuous in the geological make-up of the South Shetland Islands (Tyrrell, op. at. supra and 

 preceding paper, pp. 43 et seq.). 



CONCLUSIONS 



The rocks from Graham Land and adjacent islands described in the foregoing pages strengthen the 

 already abundant evidence that the igneous rocks of the region, down to the latitude of Adelaide 

 Island at least, are identical with those of the Patagonian Andes. Of particular interest is the discovery 



1 'Petrographische Untersuchungen aus dem westantarktischen Gebiete', Bull. Geol. Inst. Upsala, vi, 1900, p. 241. 



2 'Antarctis', Handbuch der Regionalen Geologie, Bd. viii, Abt. 6, 1913, p. 9- 



3 'On the Geology of Graham Land', Bull Geol. Inst. Upsala, vn, 1906, p. 24. or 



* 'Geographie physique, Glaciologie, Petrographie ', Exped. Antarctique Franf.aise, 1903-5, Pans, 190b, p. 103- 



