8o DISCOVERY REPORTS 



IGNEOUS ROCKS 



Porphyritic micro-diorite {quartz-diorite-porphyry). This is a fine-grained rock consisting of diversely 

 arranged laths of plagioclase (oligoclase-andesine), with subordinate chlorite representing an original 

 ferromagnesian mineral, probably hornblende, irregular grains of titano-magnetite, a little interstitial 

 quartz, and an abundance of thin needles of apatite. The porphyritic constituents are few and consist 

 solely of badly altered plagioclase (probably andesine). This rock resembles the quartz-diorite 

 porphyries which are abundant in the South Shetlands, the Palmer Archipelago and Graham Land. 



Porphyritic honibleiide-micro-granite [Iioniblende-quartz-porpJiyry). This is an interesting and 

 unusual rock with very numerous euhedral phenocrysts of feldspar, quartz, hornblende, biotite, and 

 ilmenite, with apatite in well-formed crystals as an abundant accessory, embedded in a pale brown, 

 glassy to crypto-cr}stalline ground-mass. The feldspars are much sericitized and consist of orthoclase 

 and oligoclase (AbjAnJ in roughly equal proportions. Quartz occurs as large embayed cr^'stals up 

 to 0-5 cm. in greatest diameter, often with edges and corners rounded by corrosion. The hornblende 

 forms prisms and plates of green to pale yellowish brown pleochroism, and is often partially or 

 completely altered to chlorite of high d.r. The biotite is completely altered to a pale green chlorite 

 of anomalous 'ultra-blue' polarization colour, with the disengagement of magnetite. Ilmenite 

 altering to leucoxene occurs in large scattered crystals. The phenocrysts form more than half the rock. 



Spherulitic quarts-porphyry. This rock contains a few small embayed phenocrysts of quartz, rather 

 more abundant euhedral phenocrysts of very turbid orthoclase and a few of albite, in a micro- 

 crystalline and spherulitic ground-mass. The spherulites are often perfect; they may be isolated in 

 the ground-mass, but more often they are grouped around the phenocrysts. The only ferromagnesian 

 minerals are a few small areas of chlorite with separated magnetite, and one or two large crystals of 

 titano-magnetite. 



Rhyolite. This rock consists mainly of a crypto-cr^'stalline but obviously quartzose ground-mass, 

 with numerous parallel streaks of micro-granitic material. The latter consists of quartz and turbid 

 orthoclase intergrown with the production of a rough micrographic structure. A few small pheno- 

 crysts of oligoclase, orthoclase and quartz occur, but the only ferromagnesian constituents are 

 represented by ragged patches of titano-magnetite, and a few flakes of chloritized biotite, which are 

 associated with the streaks of micro-granite. This rock may be regarded as a rhyolite with flow structure. 

 It may represent a lava, or perhaps more probably, a small dike. 



These acid volcanic or dike rocks may have come from the extreme northern tip of Graham Land, 

 where O. Nordenskjold has described a similar series, mostly tuflFs, at Flora Bay.^ Also, at Hoffnungs 

 Bay,- he found acid porphyries and porphyry tuffs, apparently concordant with folded and meta- 

 morphosed Jurassic sediments. 



SEDIMENTARY ROCKS 



Only three of the stones can be regarded as unmetamorphosed sediments. These are all greywackes, 

 one of sand grade, and the other two of silt grade. 



The coarser greywacke is grey-green in colour and quartzite-like in aspect. In thin section it is 



seen to consist mainly of ver^^ angular fragments of quartz and feldspars, with a little biotite (altered 



to chlorite and magnetite), pale pink garnet, and some epidote, sericite, and chlorite developed as 



secondary minerals. In addition to the mineral fragments there are numerous rock chips, including 



carbonaceous shale, chert, fine-grained quartzite, sericite-schist, and fragments of the ground-mass 



of trachytic and felsitic igneous rocks. Most of the quartz shows a marked undulose extinction 



1 ' Untersuchungen aus dem westantarktischen Gebiete', Bull. Geol. Inst. Upsala, vi, 1900, p. 239. 

 ^ 'Antarctis', Handbiicli dcr Rcgioiialen Geologic, Bd. viii, Abt. 6, 1913, p. 9. 



