SOUTH SHETLAND ISLANDS 



49 



The 'conspicuous lava flow' north of the Headland is a doleritic type of andesite characterized 

 by an exceedingly coarse intergranular texture. The rock is mainly composed of laths of plagioclase 

 (AbjAnj), pale brown augite, and serpentinous patches which may represent vanished olivine or 

 hypersthene or both. The ferromagnesian minerals form clots as is common in this lava series. A 

 sparingly developed earlier generation of feldspars, slightly larger than the laths, is highly zonal, 

 both chemically and mechanically, and gives rhomboidal cross-sections. 



Four specimens of the boulders in the great conglomerate erratics consist of typical augite-andesites 

 differing among themselves only in the texture of their ground-masses. The numerous large pheno- 

 crysts consist of plagioclase (mainly about AbaAug), and pale brown augite. Many of the feldspars 

 show strong chemical and mechanical zoning. They are often full of inclusions except for a narrow 

 zone of oligoclase on the margins. Nevertheless, many of the feldspars are quite free from inclusions. 

 In fact the cloudy feldspars look rather like xenocrysts, especially when thev^occur in juxtaposition to 

 perfectly clear crystals. The facts that these rocks carry the bluish apatites, and occur in a hard 



HARMONY cove., NELSON STI^-, SOUTH SHETLPiNDS V;.n» ^oU«.i^ +.1 iS, 



67MI0N 1465 



ISi-ftMO STl?>Mr 





N A M. 



F'g- 5- 

 coarse conglomerate of well-rounded boulders indicating a long period of erosion, suggest that, 

 notwithstanding their freshness, they belong to the older series of lavas. 



Examination of a series of pebbles from the agglomerate in the vicinity of Martin's Head and 

 Lion's Rump shows that the majority consist of hornblende-augite-andesite lavas and their tuffs. 

 In addition, there is an altered doleritic andesite somewhat similar to that described above, a highly 

 epidotized andesite obviously belonging to the older lava series, and an altered tonalite in which the 

 feldspars have been thoroughly sericitized and epidotized, and the ferromagnesian minerals chloritized. 



The hornblende-andesite is an unusual type which has not hitherto been described from West 

 Antarctica. In the best-preserved specimen brownish green pleochroic hornblende in well-shaped 

 crystals comes next to plagioclase in abundance as phenocrysts, and is greatly preponderant over 

 augite. The ground-mass is dense and cryptocrystalline. 



"nelson island 



Harmonv Cove. Very little is known about the geology of Nelson Island. Mr Ferguson {op. cit. 

 p. 43) visited Harmony Cove, a harbour at the western corner of the island where Nelson Strait 

 joins Bransfield Strait, and collected a quartz-diorite-porphyry which appeared to be intrusive into 

 an igneous breccia. Dr Mackintosh collected four specimens from Harmony Cove. His account is 

 almost entirely topographical, but he has provided an excellent sketch of the rock exposures (Fig. 5). 



Study of these specimens confirms Mr Ferguson's results. One of them is a fine-grained norite 



