SOUTH SHETLAND ISLANDS 65 



The latest volcanic episode seems to have been the extrusion of olivine-basalt lavas mainly from a series of volcanoes 

 in the north-west side of Bransfield Strait (Deception Island; Edinburgh Hill, Livingston Island; Bridgeman 

 Island). These volcanoes are largely built of basalt tuffs with subordinate basalt lavas and intrusions. Deception 

 Island, however, contains hyalodacites and oligoclase-trachytes, as well as basalts. Nordenskjold (Antarctis, 1913, 

 p. 11) suggests that these volcanoes may have some relation to the subsidences of the Bransfield Strait region.. . .He 

 regards the Bransfield Strait volcanoes also mainly as of early Quaternary age; but Deception Island, and probably 

 Bridgeman Island, continued erupting until recent times.. . . 



The main addition we have been able to make to Nordenskjold's account of the region is the recognition of folded 

 sediments in the South Shetland Islands, similar to those of the Palmer Archipelago and the Danco Land coast, 

 but here interbedded with, and covered by, typical Andean lavas. It seems probable that a tectonic zone parallel 

 to those of the Palmer Archipelago and Graham Land runs through the South Shetland Islands. It is worthy of 

 note that the intensity of plutonic action diminishes towards the outer (north-western) part of the region. Plutonic 

 rocks build up the greater part of the mainland ranges; they are also abundant in the Palmer Archipelago, but folded 

 sediments are here also very conspicuous, while in the South Shetlands plutonic masses are small and isolated, and 

 very subordinate in bulk to the sediments and lavas. Conversely the volcanic rocks are very largely confined to the 

 South Shetlands, and are rare in the Palmer Archipelago and the Danco Land coast. 



The new Discovery II collections described in this memoir make it clear that King George Island, 

 at any rate, and probably all the larger islands, are mainly composed of the older series of andesites, 

 dacites, rhyolites, etc., with their tuffs, volcanic breccias and agglomerates, which are interbedded in 

 places (Admiralty Bay; Marian Cove) with argillaceous and arenaceous sediments, all conjecturally 

 of late Mesozoic age. This series is intersected by a number of tonalite, diorite and gabbro intrusions. 

 Although Ferguson {op. cit. p. 37) has tabulated a thick section of the older andesites, tuffs, agglo- 

 merates and sediments in Admiralty Bay, it seems possible that the importance of the sedimentary 

 intercalations has been exaggerated in previous accounts. Ferguson himself collected only a very 

 few of these sediments, and other collections from many localities in King George Island have not 

 included any. If the sediments had been at all prominent in the field, it seems likely that they would 

 have bulked much more largely in the collections, notwithstanding their inconspicuousness in contrast 

 with the more spectacular igneous rocks. 



On the other hand, the importance of the plutonic intrusions in the make-up of the South Shetland 

 Islands may have been minimized in previous accounts. The Discovery II collections have brought 

 to light the existence of a large mass of diorite on the eastern coast of King George Island ; and diorite 

 seems to form a part of the previously unknown Snow Island. Diorites are also known to occur in 

 Livingston Island, Greenwich Island, and Nelson Island. These rocks are certainly intrusive into 

 the older series of andesites and sediments, as shown by their contact-metamorphic effects. It may 

 be conjectured that these plutonic masses are the underground equivalents of the later and fresher 

 series of andesite lavas which appear to be unconformably banked up against, and superposed upon, 

 the older andesite series. That a long period of erosion succeeded the extrusion of the older series is 

 shown by the occurrence of large erratics of coarse conglomerate at Martin's Head (p. 49), which 

 contain well-rounded boulders of the older andesite, altered tonalite, and comparatively fresh augite- 

 andesite. Since the last-named contains the blue apatites characteristic of the older series of lavas, it 

 is a reasonable assumption that all the boulders and pebbles belong to the older series. 



The latest volcanic episode is represented by a series of Quaternary or Recent volcanoes along 

 Bransfield Strait, the craters of which are still well preserved. It is probable that the Deception Island 

 and Bridgeman Island volcanoes have erupted within historical times (Ferguson, op. cit. pp. 36, 45). 

 A very notable addition to our knowledge has been provided by Mr Marr's discovery of the Penguin 

 Island volcano (p. 45). The lavas of Penguin Island and Bridgeman Island are olivine-basalts. 

 Olivine-basalt was also erupted at Deception Island ; but the main products from this volcano were 

 slaggy and glassy andesites of peculiar composition (p. 54). 



