SOUTH SHETLAND ISLANDS 55 



the rock. Associated with and apparently passing into the tridymite aggregates there are a number 

 of small spherulites giving a perfect extinction cross, of which the constituent fibres have straight 

 extinction and a refractive index much lower than that of canada balsam. While these may be tridymite, 

 it is possible that they represent cristobalite. A. G. MacGregor has described both tridymite and 

 cristobalite from the Recent lavas (pyroxene-bandaite) of Montserrat.' He writes: 'The cristobalite, 

 besides obviously replacing tridymite laths and twins, often occurs as innumerable rounded to 

 irregularly shaped spots up to o-i mm. across', but he does not mention any spherulitic structure. 



Oligoclase-andesite (oligoclase-trachyte — Thomas; santorinite — Barth and Holmsen; oligoclase- 

 dacite (ungaite) — Tyrrell). This rock represents a somewhat more acid development of the magma 

 which gave rise to the hyalo-andesites above described. Its nomenclature presents a rather per- 

 plexing problem, and it has been given various names by different authors as shown above. As 

 indicated by the analyses (p. 58), the free silica works out at between 15 and 20 per cent. The writer 

 has shown that the average andesite contains round about 15 per cent of normative quartz;- and as the 

 principal feldspar in the rocks under discussion is oligoclase, it is thought that oligoclase-andesite is the 

 best name for the type. It is, however, of somewhat unusual composition, as shown by Barth and 

 Holmsen ((4), p. 13), in that the ratio of soda to potash is much higher than in normal andesites. 

 They have marked this distinction by conferring the name santorinite, since the lavas of Santorin are 

 found to be the closest analogues of this rock type. Perhaps the most acid types should be called 

 oligoclase-dacite to mark the presence of as much as 20 per cent of free silica. 



In hand specimens these rocks vary from light grey compact 'stony' to black pitchstone-like 

 material, which carries scattered whitish crystals of feldspars and often shows marked parallel banding 

 due to flow. 



In thin section they are seen to contain very sharply bounded micro-phenocrysts of plagioclase, 

 augite, enstatite, olivine (fayalite) and magnetite, embedded in a ground-mass which varies greatly 

 in its proportion of glass to crj'stals. The glass may form at least 50 per cent of the ground-mass ; at 

 the other extreme the rocks may be almost completely crystalline. The glass is usually yellowish brown 

 in colour, but may be colourless; it contains many minute needle-like crystallites. Numerous 

 microlites of oligoclase-albite (and perhaps a potash-soda feldspar) stream through the glass in fluidal 

 fashion, mingled with minute granules of pyroxenes and iron ores. The feldspar micro-phenocrysts 

 were identified in my earlier memoir as anorthite ((i), p. 71). Dr Thomas also found anorthite in his 

 material ((2), p. 82), but Barth and Holmsen ((4), p. 1 1) apparently noted only andesine of composition 

 AbesAnas . The ferromagnesian phenocrysts include augite (probably diopside) in well-shaped prisms 

 and octagonal basal sections, enstatite and fayalite. The micro-phenocrysts often cluster in groups. 

 Only one of the rocks w^as vesicular, and in it was found tridymite lining steam cavities exactly as 

 reported by Barth and Holmsen. 



These rocks resemble some of the more basic pitchstones of the Tertiary igneous episode in the 

 west of Scotland, notably the types called leidleite and inninmorite,^ especially the latter, which is 

 reported to contain anorthite phenocrysts. Indeed, the text-figures of the microscopic appearance of 

 leidleite and inninmorite (e.g. figs. 47, 48) given in the Mull Memoir cited above might pass for some 

 of the hyalo-andesites and oligoclase-andesites of Deception Island. 



Tiijf and agglomerate. Every account of Deception Island emphasizes the abundance of fragmental 

 volcanic rocks — tufi^ and agglomerate — in the constitution of the volcano. Five specimens from 



1 The Royal Society Expedition to Montserrat, B.W.L: 'The Volcanic History and Petrology of Montserrat, with Obser- 

 vations on Mont Pele, in Martinique', Pliilus. Trans., B, ccxxix, 1938, pp. 58-61. 



- G. W. Tyrrell, ' Some Tertiary Dykes of the Clyde Area', Geol. Mag. 1917, p. 31 1- 



3 'Tertiary and Post-Tertiary Geology of Mull, Loch Aline and Oban', Mem. Geol. Surv. Scotland, 1924, pp. 281-4. 



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