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APPENDIX 



REPORT ON ROCK SPECIMENS FROM THULE ISLAND, 

 SOUTH SANDWICH ISLANDS 



By G.W.TYRRELL, .A..R.C.Sc., D.Sc, F.G.S., F.R.S.E. 



Lecturer in Geology, University of Glasgow. 



The specimens, fifteen in number, were collected from the scarp at Beach Point (see the 

 foregoing Report, pp. 188-9). Eight of them were obtained from rock hi situ on the face 

 of the cliff and at its summit. The remaining seven were beach pebbles. It is stated in 

 the Report (p. 188) that the ridge is about 150 ft. in height, and is composed of hard grey 

 rock with outcrops of red tuff, and a soft crumbling black rock, perhaps volcanic ash, 

 at the summit. The specimens collected in situ bear out this description. From the labels 

 attached to the material, and from the petrographical examination, the following ad- 

 ditional facts have been elicited. Six specimens were obtained from a steep escarpment 

 at about 50 ft. above sea level. Of these, four are dacite lavas with good flow structures, 

 and two are pyroxene-andesites containing both monoclinic and orthorhombic pyroxenes. 

 From the fact that a black slaggy andesitic lava with red crusts, diagnosed in the field as 

 "tuff", was obtained at a height of 100 ft. above sea level, it is inferred that the upper 

 part of the cliff, from the 100 ft. level at any rate, consists of a flow, or flows, of andesite 

 lava, whilst the dacite specimens probably came from underlying flows. At the top of 

 the cliff, 150 ft. above sea level, a true andesite tuff was obtained, which probably repre- 

 sents the final explosive discharge of the volcano after the andesite lava had been ejected. 

 The seven beach cobbles and pebbles consist mainly of dacites and andesites entirely 

 similar to those found in situ. In addition there is a specimen of a basic type of andesite 

 containing a notable amount of olivine, and one of andesitic pumice. 



PETROGRAPHY. 



The dacites (or dacitoids) are greyish green, reddish, or purplish, compact, non-por- 

 phyritic rocks, the colours of which obviously vary according to the state of oxidation of 

 their iron. Colour streaks and bands, and a somewhat platy fracture, indicate a flow 

 structure which is prominent in all specimens when seen in thin section. 



Under the microscope these rocks are found to consist mainly of feldspar microlites 

 which are arranged in more or less parallel streams indicating flow, embedded in a 

 dense, irresolvable, cryptocrystalline base. The microlites are of plagioclase, and are 

 referred to oligoclase as they give straight or nearly straight extinctions. The only other 

 identifiable constituents of the ground-mass are minute spots and streaks of iron oxides 

 which, when in the form of haematite or limonite, produce the prevailing purplish 

 and reddish tints of the rocks. There are also tiny crystals of strong birefringence 

 which may be identified as augite in coarser streaks and bands which are to be found 

 in one of the specimens. Rare microphenocrysts of andesine (AbjAna), which are some- 

 times euhedral, and sometimes curiously corroded, are the only other crystalline 



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