94 DISCOVERY REPORTS 



intergranular ground-mass consisting of minute microlites of feldspar and grains of augite and 

 magnetite. Tlie two remaining pellets consisted of basalt glass of a deep olive-buff colour. In one 

 of these microlites were absent, but in the other microlites of plagioclase and augite were abundant, 

 and a little olivine was probably present. 



The Discovery II material submitted to me was dredged at St. 363 from depths between 278 and 

 329 fm. at a locality 2| miles S. 80° E. of the south-eastern point of Zavodovski Island. It consisted 

 of two bags, one containing grey scoria or lapilli, very rough and angular, the largest being about 

 I in. in greatest diameter; the other contained a few of the larger stones picked out from the scoria. 

 Five thin sections were prepared from this material. 



The scoria and lapilli consist of a highly vesicular, opaque, pumiceous glassy basalt. The glass 

 varies in colour from black, even in thin section, to pale brown, and carries minute microlites of 

 plagioclase and pyroxene, the latter being noticeably more abundant in the pale brown glass. A few 

 large crystals are entangled in the glassy sponge; these include plagioclase (bytownite, Angj), pale 

 brownish green diopsidic augite, and olivine, all perfectly fresh. In one specimen the glass is much 

 haematitized, and carries much larger and more numerous feldspar microlites which can be identified 

 as labradorite (Anjo). These rocks are on the borderline between andesites and basalts. Their content 

 of olivine is small and sporadic; and as the glassy ground-mass probably contains much free 

 silica it may be presumed that if the magma had not been so rapidly quenched the olivine would 

 have been made over into pyroxene by reaction, and the rock would then have been revealed as 

 a basic andesite. This description agrees with that of the dredged material off Zavodovski given by 

 Douglas (p. 93). 



One of the dredged stones, however, the largest, is undoubtedly a sedimentary rock. It is a very 

 dense, dark grey material which looks like cementstone. In thin section it shows a carbonate mineral 

 intermingled with argillaceous matter. The rock effervesces only when powdered and treated with hot 

 concentrated acid, and may therefore be identified as a dolomitic mudstone. 



Leskov Island {South Smidwich Islands Memoir, pp. 161-2). This island, the smallest of the South 

 Sandwich Group, lies some distance to the west of the arc on which all the other islands are situated. 

 Its circumference measures only about i| miles. There is no record of any landing on this island, but 

 it was observed at close range by Capt. Larsen ((i), p. 166), Lt. Filchner,^ and by members of the 

 Discovery II party. The last-named state that the island is crescentic in outline and is doubtless 

 a fragment of a volcanic cone. Material dredged by Larsen at a depth of 75 fm. proved to consist of 

 basaltic rocks ((i), p. 167). At the south-eastern corner of the island a conspicuous conical rock 

 consists of columnar basalt; the cliffs round the southern and western sides are formed of rugged 

 flows of basaltic lava inclined towards the sea on the south side at an angle of 45°, but gradually 

 becoming vertical towards the west. The rock walls of Crater Bay are reddish and yellowish in colour 

 and apparently consist of tuff which shows no definite bedding and is much contorted {South Sandwich 

 Islands Memoir, p. 162). 



Visokoi Island {South Sandzvich Islands Memoir, pp. 162-5). This island is one of those that show 

 definite volcanic activity. There is no known record of a landing and most of the information regarding 

 Visokoi was obtained during the visit of 'Discovery II'. The only geological information available is 

 that provided by a sketch of rock exposures on the north coast by Mr F. C. Fraser {South Sandwich 

 Islands Memoir, fig. 8, p. 164), which shows columnar basalt, dark grey rock intersected by dikes and 

 surmounted by light grey stratified rock [tuff?], reddish and grey rocks cut by dikes, and an exposure 

 of stratified rocks [tuffs?] in alternate layers of grey and red tints. The general impression was that 

 the rocks were basaltic lavas and tuffs similar to those seen on Zavodovski. 



^ Zum Sechsten Erdteil, pp. 1 14-15, figs. 32-6 (Berlin, 1923). 



