I « 



REPORT ON ROCKS FROM THE 

 SOUTH ORKNEY ISLANDS 



By C. E. Tilley, b.Sc, Ph.d. 

 (Text-fig. i) 



ROCK collections from seven stations (1089-1095) were made by the 'Discovery II' 

 .on the islands of the South Orkney group. These collecting points are noted on the 

 accompanying map of the islands (Fig. i), and the following petrological report is based 

 on the rocks collected at the stations indicated below. 



Station 1089: Michelsen Island and Powell Island 



The chief rock types from this station include greywackes and conglomerates. 



The greywackes are all of similar type and are composite rocks consisting of angular 

 fragments of quartz frequently showing strain shadows, twinned plagioclase, sericitized 

 albite and also a more basic felspar (andesine) with bent twin lamellae, fragments of 

 myrmekite (rare), granophyre and shale. Shreds of muscovite, biotite and grains of 

 garnet and epidote are also found. They are grey in many cases, obviously clastic rocks 

 with a grain size averaging |— f mm., and are often veined by quartz. These rocks occur 

 both on Michelsen Island and at a point on the south-east corner of Powell Island. 



The conglomerates, which in one place on Michelsen Island are said to rest on a band 

 of greywacke, consist essentially of fragments of vein quartz often sheared and granu- 

 litized, shale, quartz mica schist or phyllite darkened with carbonaceous material, and 

 abundant pebbles of a quartzo-felspathic greywacke identical with the greywackes 

 described above. 



These fragments range up to i|-2 in. in diameter, but there occur among the pebbles 

 extracted from a conglomerate on Michelsen Island some examples reaching a diameter 

 of 6 in. Some of these pebbles contain felsitic fragments, detrital garnet and shreds of 

 detrital biotite. 



Conglomerates are recorded both from Michelsen Island and from the southern face 

 of the southernmost peak of Powell Island. They are reported to become coarser in type 

 towards the south. 



Station 1090: Fredriksen Island and 

 HoLMEN Gras (Island) 

 Specimens were collected from the west coast of Fredriksen Island at points marked 

 (see map), and also on Holmen Gras. 



Of these rocks those from Holmen Gras are conglomerates containing conspicuous 

 quartz pebbles. The components of the conglomerates consist of quartz showing strain 



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