96 DISCOVERY REPORTS 



olivine, and a little analcite was found in one of the thin sections. A chemical analysis of the principal 

 type, free from analcite, is published, which is set out with others from the South Sandwich Islands 

 in Table 8 (p. loi) of this memoir. Biickstrom calls the rock a basalt. 



Owing to unfavourable conditions the Discovery II party was unable to land on Saunders Island, 

 but on 28 November 1937, Mr G. Rayner was able to get ashore for a few hours from the 'William 

 Scoresby'. He made some geological observations and collected a small number of rock specimens 

 which are described below. The observations that follow are condensed from his MS. report. 



Mr Rayner landed near the penguin rookery on the south side of Cordelia Bay (see Chart in the 

 South Sandwich Islands Memoir, pi. xix). The beach material consisted of a loose black volcanic ash, 

 the size of coarse sand or grit. Behind a low cliff of compressed snow heavily loaded with the same 

 ash was a level area extending back to the hills. This platform consisted of a loose ash-like material 

 to a depth of some inches, with occasional small boulders up to 18 in. in diameter of a heavy dark 

 basaltic rock resting upon it. 



From this point Mr Rayner walked along the shore eastward until he reached the first outcrop of 

 hard rock which forms the basement of the Nattriss peninsula. Here he ascended the hill to the south 

 near the point marked 800 on the Chart. On its northern slopes there were several outcrops of a soft 

 volcanic mudstone with a sub-horizontal stratification, standing up as buttresses and ridges between 

 steep-sided ravines. Mr Rayner thus gained a ridge which sloped eastward to Nattriss Point. The 

 higher parts of this ridge still consisted of the stratified mudstone, which was undergoing extremely 

 rapid atmospheric erosion. At one place he encountered a remarkable pillar 15-20 ft. high carved 

 out of the soft material (' The Beacon '). Elsewhere along the ridge a light, vesicular, reddish, scoriaceous 

 rock was found. 



Descending eastward towards Nattriss Point Mr Rayner found that the rock became coarser in 

 texture, and took on the appearance of volcanic tuff, light buff in colour, in which many large fragments 

 of rock were embedded. This series of coarse tuffs rested on the roughly horizontal platform of dark, 

 vesicular, basaltic rock of which Nattriss Point is composed. This rock falls in sheer cliffs to the sea 

 and has a columnar appearance owing to wave erosion along vertical joints. 



With, as the writer thinks, considerable probability, Mr Rayner concludes that ' a volcanic explosion 

 has occurred at no very distant date, possibly from the crater to be seen to the south-west of our 

 landing-place, and near the junction of the ice-covered main part of the island and the earthy region 

 explored. This explosion has thrown up the clastic material forming the hill now resting on a hori- 

 zontal table of rock of which Nattriss Point is the visible part. The finest material would be the last 

 to settle, and this has formed the upper strata of soft mudstones seen in the fast dwindling ridges and 

 buttresses along the hillside and in the pillar at the summit.' 



Six thin sections were made from the specimens collected by Mr Rayner. The lava which forms 

 the basement of the Nattriss peninsula is a black, highly vesicular rock which, in thin section, shows 

 an abundant ground-mass of minute microlites of plagioclase with granules of augite and magnetite, 

 within which is set a generation of somewhat larger feldspar laths, and finally a few micro-phenocrysts 

 of feldspar and yellowish augite. Owing to their small size it is difficult to make out the composition 

 of the plagioclase microlites of the ground-mass, but they give extinctions up to about 15'' indicating 

 a composition Aug,,. The larger microlites and micro-phenocrysts are highly zonal, and their com- 

 position ranges about Augs, which is the composition ascertained by Backstrom. The pyroxene, too, 

 is zonal, as shown by an undulatory extinction. It is a pale yellow variety of moderate double refraction, 

 and is probably, as Backstrom surmises, a member of the enstatite-augite series. The larger feldspars 

 and pyroxenes, while occurring independently, are often aggregated into clots of which the feldspar 

 forms the greater part, and the microlites of the ground-mass are stream-lined around these clots. 



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