54 DISCOVERY REPORTS 



being conspicuously porphyritic, and in being almost or quite devoid of olivine. All but one of the 

 six specimens available come from Dr Mackintosh's collection, and were obtained from both sides 

 of the entrance channel to Deception Harbour. The ground-mass is of the same type as that of the 

 basalts, that is, composed of andesine laths, and granules of augite and iron ores. A plagioclase of 

 somewhat more basic character forms numerous laths which run in wavy flow-lines through the 

 ground-mass. A few large phenocrysts of augite may occur, but olivine, if present at all, is always 

 in very small quantity, and is altered to brownish serpentine. The ground-mass varies in texture 

 from coarsely intergranular to fine-grained intersertal, with a brownish glassy base blackened with 

 iron-ore dust. 



These rocks are adjudged to be the same as those described by Barth and Holmsen ((4), p. 9) as 

 andesine-basalt and pillow-lava of bandaitic type, of which they have provided chemical analyses 

 (p. 58). Dr Thomas, too, described what is apparently the same type, in the more basic varieties 

 of his 'hyaloandesites' ((2), p. 82). Both Barth and Holmsen, and Dr Thomas, mention hypersthene 

 as a constituent of this rock type, but the writer was unable to identify orthorhombic pyroxene with 

 certainty in the material at his disposal. 



Andesite {hyalo-aiidesite). This is the most abundant rock type in both Mr Ferguson's and 

 Dr Mackintosh's collections. As the analyses show (p. 58), there is a continuous series of com- 

 positional types from the basic bandaites to the relatively acid oligoclase-andesites (santorinites), 

 varying chiefly in silica percentage and proportion of ferromagnesian to feldspathic minerals and 

 quartz. As many of the rocks are of glassy facies, these variations are masked, at least mineralogically, 

 by the glassy matrix; in thin section the rocks present a relatively unvarying appearance and, 

 except for one or two more crystalline types, may be grouped as hyalo-andesites. Dr Thomas ((2), p. 82) 

 described several rocks from Deception Island under this heading. 



In hand specimens these rocks are black or dark grey in colour, usually slaggy, vesicular or even 

 pumiceous, and are obviously of glassy nature. Even the more crystalline varieties are black and of 

 dense texture. From these black slaggy types there are all transitions to dark, non-vesicular, glassy 

 rocks, resembling pitchstones, which are, however, more acid than the majority of the types grouped 

 under the name hyalo-andesite, and properly belong to the oligoclase-andesites or santorinites. 



In thin section many of these slaggy rocks are found to be composed of a brownish glass, dusted 

 thickly with black specks of iron ores, and often highly vesicular. They always show swarms of 

 plagioclase microlites (oligoclase to andesine), usually in parallel fluxional streams, but occasionally 

 felted together with the production of pilotaxitic texture. Microlites of pyroxene can often be detected 

 in varying numbers by their bright polarization tints and oblique extinction. Some microlites, 

 however, which are indistinguishable from the pyroxenes in their appearance under ordinary light, 

 have a very high double refraction and straight extinction. ^ It is probable, therefore, that these are 

 olivines. Olivine does actually occur in very small amount in a few of the rocks as micro-phenocrysts, 

 and is almost invariably altered with the production of a reddish serpentine. There are also occasional 

 micro-phenocrysts of andesine and augite. 



From these highly vitreous types there are all gradations to almost holocrystalline (micro-crystalline) 

 types consisting of a very dense intergranular admixture of plagioclase microlites with granules ot 

 augite and iron ore, which carries fluxional streams of plagioclase laths. 



Dr Thomas detected well-formed ciystals of tridymite lining steam cavities and planes of flow in 

 these rocks ((2), p. 84). Barth and Holmsen ((4), p. 1 1 e^ seq.) found no less than 17 per cent of 

 tridymite lining steam cavities in one of the more acid types. The writer found abundant tridymite 

 in only one of the vesicular hyalo-andesites. It lines and fills steam cavities and fracture cracks m 

 Barth and Holmsen ((4), p. 9) have also noted small elongated crystals of olivine in the ground-mass of these rocks. 



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