SOUTH SANDWICH ISLANDS 09 



entangled in it. This seems to represent an extremely vitreous phase of the andesitic basalt lava 

 described by Backstrom, and carries the same lime-rich feldspar. 



Most of the smaller fragments recovered from the piece of floating ice answer to the above de- 

 scription. A larger stone, however, is 2 in. in length and presents a microscopic appearance very 

 similar to that of the ' feldspathic basalt ' described and figured by Backstrom. It shows very numerous 

 micro-phenocrysts of plagioclase (An^j-g,,) with subordinate augite and olivine, in a dark glassy 

 ground-mass carrying microlites of feldspar and augite. All the phenocrysts are perfectly fresh and 

 euhedral. The augite is a yellowish, slightly-pleochroic variety belonging to the enstatite-augite series. 

 In this rock the olivine is much more abundant than in Backstrom's material, and it must be regarded 

 as an olivine-basalt. 



Two other stones are interesting, as they are non-igneous. One is a fragment from a quartz-vein 

 rock, and the other is an epidote-biotite-gneiss. In thin section the latter shows a coarse mosaic of 

 quartz and oithoclase alternating with folia consisting of straggling crystals of bright yellow biotite 

 and epidote (with some clinozoisite). There is also a little ilmenite altering to sphene, and a few 

 fragments of deep green pleochroic hornblende. It is not possible to say whether this is an orthogneiss 

 or a paragneiss. The mineral composition favours the orthogneiss interpretation, but an arkose would 

 yield this type of gneiss on metamorphism. 



The label attached to the material from floating ice does not state on which side of Bristol Island 

 it was recovered. As the metamorphic fragments were closely associated with scoria which indubitably 

 came from Bristol Island, it seems probable that they too were derived from that locality. It is possible 

 that the metamorphic pebbles came from Freezeland Peak which the Discovery II party believed to 

 consist of sedimentary and metamorphic rocks. 



Southern Tlmle Group (South Sandwich Islands Memoir, pp. 178-89). This group consists of three 

 islands, Thule, Cook, and Bellingshausen, in order from west to east. Of these, Cook Island is the 

 largest, having a circumference of 9^ miles; Thule, the next largest, is more embayed than Cook 

 and has a coastline of 10 miles; Bellingshausen, the smallest, is only i^ miles wide. 



Bellingshausen is still an active volcano, as shown by the steam and vapour rising from it, and by 

 the admirable sketches of Lt.-Cmdr. J. Irving (South Sandwich Islands Memoir, fig. 19, p. 184). 

 Cook and Thule, however, are buried beneath thick ice caps and there are no signs of present volcanic 

 activity. Nevertheless, soundings in Douglas Strait between Thule Island and Cook Island have 

 disclosed a steep-sided basin of elliptical shape and more than 400 fm. in depth. At the north and 

 south entrances to Douglas Strait the depths are less than 20 fm. This has been interpreted, correctly 

 in the writer's opinion, as the inundated crater of a volcano of which Thule Island and Cook Island 

 are the remnants. This view is reinforced by the parallelism of the eastern embayment of Thule 

 Island, and the western embayment of Cook Island, with the adjacent contours of the submerged 

 basin (South Sandwich Ishmds Memoir, fig. 16, p. 179), and by the photograph of the eastern side 

 of Thule Island (ibid. pi. xxx, fig. 4), which shows bedded lavas and ashes dipping westward and 

 outward from the Douglas Strait crater. 



Of the geological constitution of the Southern Thule Group little is known. On Bellingshausen 

 the Discovery II party noted, as on other islands, black columnar basalt with overlying agglomerate, 

 tuff, and ashes. 



Cook Island (South Sandwich Islands Memoir, pp. 185-6). Rock faces are exposed in the cliffs 

 bordering Douglas Strait. They are described as of yellow, red, or brown colours, sometimes showing 

 signs of bedding but always much crumpled and contorted, and seamed with dikes of grey rock. 

 Large, apparently intrusive, masses of brown rock showing a vertical striation were also seen. 



Fortunately, however, some stones were dredged by 'Discovery 11' at St. 366, 4 cables south of 



