SOUTH SHETLAND ISLANDS 57 



the main part of the South Shetlands, and perhaps some part of the Palmer x'Vrchipelago and the 

 Graham Land coast. There is also one hyalo-andesite with good tridymite which certainly comes 

 from Deception Island and two others which probably come from the same source. Rocks of plutonic 

 aspect are also well represented in this collection. They include the quartz-pyroxene-diorites and 

 their porphyries which are common in the South Shetlands and adjacent regions. Diorite, tonalite, 

 granodiorite, biotite-granite, and their porphyries, together with granophyric granites and true 

 granophyres, which more probably come from the Palmer Archipelago and adjacent parts of Graham 

 Land, are also fairly abundant. Rarer types are represented by a basic diorite with abundant brown 

 hornblende, biotite, and apatite ; and a serpentine derived from augite-peridotite. 



The most interesting material, however, is provided bv specimens of sedimentary and metamorphic 

 character, which are unrepresented among the rocks in the Discovery collections obtained from actual 

 exposures. Little is known of these rock types in the South Shetlands and adjacent regions as they 

 have attracted little attention, perhaps owing to the relatively great abundance and conspicuous 

 characters of the igneous rocks. 



Many of the sediments represented among the dredged stones have suffered a low-grade cataclastic 

 metamorphism by crushing and shearing. Among the unaltered sediments are mudstone, siltstone, 

 greywacke, arkose and sandstone. There are two mudstones, and both appear to represent exceedingly 

 fine-grained washes from the weathered surfaces of basic lavas. Microlites of plagioclase can be 

 recognized in a chloritic and ferruginous clay matrix, and in one of them there is a sparse sprinkling 

 of angular quartz grains of silt grade. Another mudstone of similar type has undergone a little 

 crumpling and shearing with the development of thin quartz-chlorite veins. 



Seven pebbles appear to represent laminated sediments consisting of alternate beds of grey\vacke 

 and siltstone or slate in various stages of shearing and crushing. The least altered specimen shows 

 angular grains of quartz and subordinate feldspar in a siliceous ground-mass of silt grade in which 

 quartz is mingled with finely divided sericite, chlorite, epidote and iron ores. This material is pene- 

 trated by thin veins of secondary silica, now recrystallized to lines of granular quartz. The other 

 members of this series have undergone severe cataclasis, whereby ultimately quartz-chlorite-schist 

 has been developed from the greywacke bands and phyllite from the slaty bands. Three of the 

 specimens show signs of having first been broken up by crushing into an angular breccia in which, 

 by further shearing, the fragments have been drawn out with the production of a kind of mortar 

 structure, and with the development of much coarse chlorite and white mica. In one specimen, which 

 is relatively poor in quartz and rich in chlorite and epidote, it is probable that basic igneous rock 

 fragments made up the greater part of the original greywacke. The extreme term of alteration is 

 represented by a true schist consisting largely of quartz, biotite and sericite, in which mortar structure 

 is finely developed. 



One specimen is an interesting arkose consisting of extremely angular grains of quartz, alkali- 

 feldspar and plagioclase, small chips of andesite and keratophyre (?), a few bits of garnet and epidote, 

 and many flakes of unaltered biotite, in a ferruginous clay matrix. This composition suggests the 

 rapid waste of a mixed terrain consisting of granitic rocks, andesitic lavas, and perhaps some meta- 

 morphic rocks. 



Finally, there is a true sandstone consisting mainly of angular to subrounded grains of quartz, with 

 less abundant grains of alkali-feldspar and plagioclase, a few chips of slate and siltstone and, above 

 all, many large angular fragments of pale garnet. 



Mudstones, greywackes, quartzites and igneous breccias have been described from the South 

 Shetlands, but especially from the Palmer Archipelago ((i), p. 74; (4), p. 28). Ferguson {op. cit., p. 37) 

 described siliceous and argillaceous sediments interbedded with the lavas and tuffs of the older 



