SOUTH SANDWICH ISLANDS 97 



Olivine does not occur in this type which, owing to its feldspathic composition, would be better termed 



andesite than basalt. 



Another specimen was taken from what appeared to be an inclusion within the above-described 

 lava It is not so dark in colour, but the thin section shows that it is the same lava with, however, a 

 somewhat finer grain and a few sporadic olivine crystals, most of which are altered to green serpentine. 

 This rock is probably a portion of the same lava, but consolidated slightly earlier than the main mass 

 of the flow, and thus retaining a few of the early crystallized olivines. It may have been carried as 

 a solidified lump of slag on the surface of the moving flow, and have been incorporated in it by 



over-rolling. . . , ■ n 



The coarse agglomeratic tuff which overlies the lava basement of the Nattnss peninsula is a well- 

 consolidated material of light buff colour containing numerous fragments of gravel size. In thin 

 section it proves to be a coarse lithic tuff consisting mainly of large angular fragments of the lavas 

 embedded in a matrix of smaller fragments and broken crystals. The lava fragments are vesicular 

 andesitic basalts of the same type as that described above, but they show every gradation of texture 

 from purely glassy to holocrystalline-micro-granular. The broken crystals include plagioclase, augite, 

 and fresh olivine. Conspicuous among the rock fragments are glasses of a bright green colour. An 

 isotropic or very feebly birefringent zeolite with cubic cleavage forms a scanty cement in some parts 

 of the sUde This may be the analcite recorded by Backstrom ((i), p. 171)- This rock must have 

 been formed by an explosion in or under a fully consolidated lava, and it may be suggested that it 

 was produced by renewed activity in a nearby volcano which had been temporarily sealed by a plug 



of solidified lava. r , m .. • 



The volcanic mudstone which overlies the lithic tuff and forms the higher parts of the Nattnss 

 peninsula, in contrast to the lithic tuff, is a vitric ash consisting almost entirely of small angular 

 fragments of clear brown glass. The only other constituents are a few small fragments of feldspar, 

 aueite and magnetite. This was undoubtedly formed by explosions within a still liquid lava. Hence 

 the sequence of events pictured by Mr Rayner (p. 96) must be slightly amended. The vitric ash does 

 not represent the finer, and the lithic tuff the coarser, material derived from one and the same 

 explosion- but the lithic tuff probably represents the disintegration by explosion ot a solidified plug, 

 and the vitric ash a subsequent explosion within the liquid lava that welled up into the crater 



The coarse black sand at the landing-place in Cordelia Bay consists of angular fragments of brown 

 glass often blackened with separated magnetite, and crystals, in about equal proportions. The crystals 

 fnclude plagioclase, augite, and olivine, the last-named being rather more abundant than usual This 

 material may have been formed by explosion in an olivine-basalt magma within whic4i while still 

 liquid crystallization had advanced to a considerable extent. Examination of a small pebbe enclosed 

 in the sample bears out this diagnosis. It is an olivine-basalt with large phenocrysts of labradorite 

 (An ) abundant fresh yellowish olivine, and some magnetite, in a very dense ground-mass consisting 

 of augite granules and feldspar microlites, in which the augite is decidedly predominant. 



MontcZ Island {South Sandwich Islands Memoir, pp. 174-6). Montagu is the largest island ot he 

 group with a circumference of about 24 miles, and is one of the least well known. It contains wha 

 fs probably the highest summit of the group, Mt Belinda (4500 ft.), almost certainly an ext.nc 

 volcano. Montagu is the most heavily glaciated island of the arc, and has fewest signs of residual 

 warmth in the shape of areas free from snow and ice. • r , • ^u^ 



The Discovery II party did not land on the island, but they had the opportunity of making the 

 following observations on the rock exposures as seen from a distance: 



As on other islands the lowest strata seen in rock exposures are usually of black bas^alt, often columnar in structure, 

 and itTs ol blalt that the outlying rocks are formed. Above it red and yellowish tut^s with some hard grey rock are 



