:oo DISCOVERY REPORTS 



Cook Island at depths between 155 and 322 m., and a few small fragments of rock at St. 368 in 

 Douglas Strait, i mile north of the Twitcher Rock, dredged from a depth of 653 m. near the bottom 

 of the great submerged crater. These pebbles, which consist mainly of slaggy and vesicular lavas, 

 one or two being well rounded, range in size from about 2 in. in greatest diameter down to about | in. 



Sixteen of these stones were sectioned for microscopical examination. All of them were found to 

 be textural variants of an olivine-basalt lava. Nearly holocrystalline varieties are grey and compact, 

 and the glassy types black, vesicular, and slaggy, in hand specimens. In thin section these rocks are 

 found to be highly porphyritic, carrying very numerous small phenocrysts of plagioclase, augite, and 

 olivine, in a ground-mass consisting, when holocrystalline, of minute crystals of plagioclase, augite, 

 and magnetite. In the more slaggy varieties the ground-mass becomes richer in dark glass and the 

 number of microlites diminishes. In fact a complete passage from holocrystalline to a purely glassy 

 ground-mass can be traced. 



The plagioclase phenocrysts are generally most numerous, with augite and olivine following in 

 that order; but in a few rocks the olivine almost rivals the feldspar in abundance. The plagioclase is 

 both chemically and mechanically zoned and shows complex twinning ; its composition ranges between 

 An7o and Augs . The pyroxene is again the yellowish, slightly pleochroic variety of the enstatite-augite 

 series. The olivine is perfectly fresh and often euhedral, especially in the more glassy varieties of the 

 rock. It gives a dead straight isogyre and therefore contains about 13 per cent of the fayalite molecule. 



This type is an olivine-basalt which compares closely with that from Bristol Island (p. 98), and 

 with the younger basalts of the South Shetland Islands (e.g. Penguin Island, p. 46). A chemical 

 analysis of one of the more holocrystalline types is recorded in Table 8 (p. 10 1). 



Thule Island {South Sandwich Islands Memoir, pp. 187-9). The south-eastern plateau of Thule 

 Island appears to be composed of the usual black columnar basalt. Near Cape Flannery on the 

 west coast are beds apparently composed of yellowish tuff and ash, and farther north the rocks are 

 definitely stratified, three layers of ash separated by red tuff overlying black basalt. 



A landing was made by the Discovery II party on Beach Point at the north-eastern corner of the 

 island. The ridge at Beach Point is composed of hard grey rock with outcrops of red tuff and a soft, 

 crumbling, black rock, perhaps volcanic ash, at its summit. The steep cliffs facing Douglas Strait 

 show contorted masses of red, yellow, and dark brown rocks with intrusive dikes. 



Rock specimens collected here were described by the writer in an appendix to the South Sandwich 

 Islands Memoir (pp. 191-7). Of the fifteen specimens, eight were obtained from exposures and seven 

 were cobbles from the beach. Six rocks were obtained from an escarpment at 50 ft. above sea-level. 

 Four of these were acid lavas (dacite) with good flow structures, and two were pyroxene-andesites 

 containing both augite and hypersthene. As a black slaggy andesitic lava with red crusts was collected 

 at 100 ft. it is inferred that the upper part of the cliff probably consisted of andesite while the dacites 

 came from an underlying flow. At the top of the cliff, 150 ft. above sea-level, a true andesite-tuff was 

 collected, which may represent the final explosive discharge of this volcanic episode. The beach 

 cobbles and pebbles consisted mainly of dacites and andesites similar to those collected m situ. In 

 addition there was a specimen of olivine-andesite (or andesitic basalt) and one of andesitic pumice. 



Thule Island is therefore notable as providing the only acid lavas so far known in the South 

 Sandwich Islands. The hypersthene-bearing andesites are also distinctive as they have hitherto only 

 been recorded from Candlemas Island (p. 95). Analyses of dacite and hypersthene-andesite from 

 Thule Island were published in the above Appendix, and are restated in Table 8 below. 



I 



