41 



PART I. PETROGRAPHY OF THE SOUTH SHETLAND ISLANDS 



INTRODUCTION 



TH I s work is based on two collections of rocks, made during the third and fourth commissions of 

 the 'Discovery II' in 1934 and 1937 respectively. The specimens were accompanied by excellent 

 geological and geographical notes, those of 1934 by Dr N. A. Mackintosh, and those of 1937 by 

 J. W. S. Marr, M.A., B.Sc. Relevant points from these notes have been incorporated, with appropriate 

 acknowledgement, in the following descriptions. 



Bibliography. A full bibliography of the earlier literature relating to the geology and petrography 

 of the South Shetland Islands (and adjacent lands) is given in my paper listed as (i) below. Only 

 papers which have been published since 1920 are given in the following list: 



(i) G. W. Tyrrell. 'A Contribution to the Petrography of the South Shetland Islands, the Palmer 

 Archipelago, and the Danco Land Coast, Graham Land, Antarctica.' Travis. Roy. Soc. Edinb. Liii, 

 pt. I, 1921, pp. 57-79. 



(2) H. H. Thomas. 'On the Innes Wilson Collection of Rocks and Minerals from the South 

 Shetland Islands and Trinity Island.' Ibid. pp. 81-9. 



(3) O. Holtedahl. 'The Geology and Physiography of Some Antarctic and Sub-Antarctic Islands.' 

 Scientific Results of the Norwegian Antarctic Expeditions, 1927-28 and 1928-29, instituted and financed 

 by Consul Lars Christensen, No. 3, Norske Vidensk.-Akad., Oslo, 1929, 172 pp. 



(4) T. W. F. Barth and P. Holmsen. ' Rocks from the Antarctandes and the Southern Antilles 

 (Being a Description of Rock Samples collected by O. Holtedahl, 1927 28, and a Discussion of their 

 Mode of Origin).' Ibid. no. 18, 1939, 64 pp. 



General. The South Shetlands comprise a group of ten large and small islands extended in a 

 north-east to south-west direction parallel to, and at a distance of from 60 to 70 miles from, the 

 coast of the Graham Land peninsula, from which they are separated by Bransfield Strait. From 

 north-east to south-west the islands are Bridgeman Island, King George Island, Nelson Island, 

 Roberts Island, Greenwich Island, Livingston Island, Deception Island, Snow Island, Smith Island, 

 and Low Island. Of these, practically nothing is known of the two last-named. Deception Island, 

 a sea-flooded Recent crater, is the best known. Bridgeman Island, too, is a Recent volcano and may, 

 like Deception Island, have been in comparatively recent eruption. Mr Marr's notes make it clear 

 that Penguin Island, off the eastern horn of King George Bay in King George Island, is also a Recent 

 volcano comparable with Deception Island and Bridgeman Island. 



The rock specimens collected during the recent Discovery II expeditions number in all 141, of 

 which 81 come from King George Island, 19 from Deception Island, 17 from Roberts Island, 16 from 

 Livingston Island, 4 from Nelson Island, and 4 from Snow Island. 



The plan of the present paper is to describe the collections from each of these islands in turn, 

 incorporating as much of the geology as can be gleaned from the field notes made by Dr Mackintosh 

 and Mr Marr. The chemistry of the igneous suite will then be studied with the aid of previously 

 published and two new analyses, and finally a conspectus of the geology of the South Shetlands will 

 be attempted from the material now available. 



