ig4 DISCOVERY REPORTS 



The Thule Island andesites are dark grey or purplish grey compact rocks without 

 conspicuous crystals, although many, very small, microporphyritic feldspar crystals may 

 usually be detected on close inspection. Some of the specimens are highly vesicular or 

 slaggy; one is pumiceous. 



In thin section the analysed rock, collected from a shore boulder, shows numerous 

 microphenocrysts of euhedral plagioclase, with much less abundant hypersthene and 

 augite, and fresh but rare olivine, in a ground-mass consisting of minute laths of 

 plagioclase feldspar, intermingled with grains of pyroxenes and iron ores. Only a very 

 small amount of cryptocrystalline or glassy base is present. As a whole the rock is 

 highly feldspathic, and is of andesitic, not basaltic, character, notwithstanding the 

 presence of a Uttle olivine. The phenocrystic feldspar is highly zonal, ranging from 

 labradorite (AbaAug) at the core to oligoclase on the margin. These feldspars also carry 

 numerous inclusions of ground-mass constituents which are always arranged in regular 

 zones with reference to the margins of the crystals. The ground-mass feldspar is of 

 somewhat more sodic composition and seems to be mainly andesine of composition 

 about AbgAng . The hypersthene is faintly pleochroic ; the augite, however, is practically 

 colourless. 



Other rocks of much the same character, but with a finer ground-mass, and consider- 

 ably richer in an obscure glassy base, come from the 50 ft. escarpment, and from the 

 slaggy lava surface (or base) at the 100 ft. level. One of the shore specimens may also be 

 associated with this group, but shows in addition a well-marked flow structure. Another 

 specimen from the 50 ft. escarpment shows the phenocrystic constituents embedded in 

 red and black glassy matter which carries only a few scattered microlites of feldspar and 

 pyroxene. 



The olivine-andesites are represented by three beach pebbles. In their general 

 characters these rocks conform very closely with the second type of andesite lava described 

 above, but are somewhat richer in olivine which also occurs in larger crystals. A pebble 

 of andesitic pumice in thin section shows a solidified froth of colourless isotropic glass in 

 which scattered crystals of plagioclase, hypersthene and augite are entangled. 



The tuff which occurs at the 150 ft. level at Beach Point is composed of little angular 

 fragments of andesite and dacite, mostly of the more vitreous, slaggy, vesicular, and 

 altered types, embedded in a paste consisting of comminuted particles of the same 

 materials, mingled with a few angular grains of quartz, and detached feldspars. 



In the only published account of rocks from the South Sandwich Islands O. 



Backstrom^ has described rock types very similar to the above from Candlemas Island. 



He states that the lavas of Candlemas are mostly reddish porphyritic rocks in which the 



feldspar phenocrysts are occasionally so numerous as to produce a whitish appearance. 



The feldspar is a basic labradorite (AbggAngs), and the pyroxene is hypersthene with 



mantles of augite. The latter mineral also occurs independently. Corroded crystals of 



olivine are present in a few of the rocks, which Backstrom regarded as basalts transitional 



^ " Petrographische Beschreibung einiger Basalte von Patagonien, Westantarktika, und den Siid- 

 Sandwich-Inseln", Bull. Geol. hist. Upsala, xiii, pp. 115-82 (1915). South Sandwich Islands, pp. 163-76. 



