SOUTH SANDWICH ISLANDS 93 



PETROGRAPHY 



General. The South Sandwich Islands, so far as present observations go, are composed exclusively 

 of Recent volcanic rocks, the products of present-day and recently extinct volcanoes. Five of the 

 islands, Zavodovski, Candlemas, Bellingshausen, Saunders, and Visokoi (map, Fig. 14), show definite 

 signs of volcanic activity and emit vapour and fumes; another three, Leskov, Vindication, and 

 Montagu, show no activity at present, although large areas of ice- and snow-free ground, indicating 

 residual warmth, exist on the islands. The remaining islands, Bristol, Cook, and Thule, are heavily 

 glaciated, and show no signs of volcanic activity or warm ground.^ 



The South Sandwich Islands Memoir (p. 150) states that the rocks consist mainly of 'reddish tuff 

 and black basaltic lava ', and this is supported by the petrological examination of the collected rocks. 

 The 'reddish tuif' may include reddened slags, and the lavas, while mainly basaltic, include basic 

 andesites and even more acid types such as dacite. A possible exception to this generalization is 

 Freezeland Peak, a small islet to the west of Bristol Island, which is referred to later (p. 99). It is 

 proposed to describe the petrography of each of the islands in turn, starting from the northern end 

 of the chain. 



Zavodovski Island (South Sandwich Islands Memoir, pp. 156-60). This island is nearly circular in 

 outline and is 9 miles in circumference. It consists mainly of an active volcanic cone which rises 

 from a lowlying plateau of black basaltic lava most conspicuous on the eastern side of the island. 

 There are subsidiary craters on the slopes of the main cone, and to the south of West Bluff there are 

 fumaroles in reddish ground with some patches and streaks of sulphur. At one point horizontal strata 

 apparently consist of alternate beds of ash and tuff. 



In 1908 the Norwegian, Capt. C. A. Larsen, landed on many of the South Sandwich Islands and 

 collected rock specimens of which, unfortunately, some were lost by accident. The collection was 

 presented to Goteborg Museum, and was later described by O. Backstrom (i). Larsen landed 

 at the north-west end of Zavodovski Island, which was found to consist of a porous lava carrying 

 zeolites in the vesicles. These specimens were lost by the upsetting of the boat. Only a few small 

 pebbles and lapilli were retained, which Backstrom identified as olivine-basalts and their tuflrs. The 

 fragments of which the latter were composed showed fresh phenocrysts [feldspars?] in a ground-mass 

 which had been altered by the action of solfataric gases. Their richness in phenocrysts and in shattered 

 basaltic ground-mass material showed that they represented a common type of 'Aschentuff' which 

 was probably rather glassy. 



During the Quest Expedition of 192 1 G. V. Douglas saw the island at close range although he was 

 unable to land (2). He does not state from which direction the 'Quest' approached the island, but 

 from the fact that he mentions a cliflF 40 ft. high with a long gentle slope inland, it may be assumed 

 that he saw the low plateau on the eastern side. Douglas states that : ' The lava flows seen on the cliff 

 face appeared to consist of a compact columnar basalt at the base. Above, there was a line of red 

 cinder, and above this again what looked to be rough paehoehoe lava.' 



Material obtained by dredging at 19 fm. corresponded with the above-described section. The 

 sample consisted of rounded black pellets of diameters between i and 5 mm. Twenty of these were 

 sectioned, and ten of them were found to consist of dense black glassy basalts free from olivine. Some 

 were crowded with minute laths of plagioclase; others showed a few small phenocrysts of plagioclase 

 and augite. Four of the pellets consisted of dense, dark brown, glassy olivine-basalts, some containing 

 many crystals of plagioclase and only a few of olivine and augite. Four others were paler basalts of 

 holocrystalline-porphyritic texture with small phenocrysts of plagioclase and sometimes augite in an 



1 South Sandmch Islands Memoir, pp. 151-2. 



