ELEPHANT AND CLARENCE GROUP 8i 



indicative of strain. The feldspars include orthoclase and albite (always turbid), and clear fresh 

 andesine (Ab^Anao). The rock is traversed by thin veins of secondary quartz, epidote and calcite. 



The remaining tw^o rocks have the same composition as that above-described, but the grain-size 

 is coarse silty. They contain a greater abundance of biotite, chlorite and garnet, but rock chips are not 

 so much in evidence, probably because of the finer grain. A few crystals of apatite occur in these rocks, 

 and in one of them carbonaceous streaks delineate the bedding planes. The same slide shows a plane 

 of shearing along which coarse sericite and chlorite have been developed. 



These rocks are probably due to the rapid waste of a terrain of miscellaneous rocks, including acid 

 and intermediate volcanic types, shales, cherts, quartzites, and schists. The abundance of quartz 

 with undulose extinction points to the presence of gneisses, or, more likely, of a quartz-porphyry 

 formation which has undergone extreme mechanical deformation, within the area of erosion. 

 A mylonized porphyry formation of this character covers great areas in Patagonia and Tierra del 

 Fuego, and has also been found in West Antarctica as far to the south as Adelaide Island (see this 

 Memoir, p. 74). 



Greywackes and greywacke-siltstones of ancient aspect are common in Tierra del Fuego in forma- 

 tions of Late Palaeozoic and Early Mesozoic ages;^ and O. Nordenskjold {op. cit. supra, p. 238) has 

 described non-schistose slates and greywackes underlying fossiliferous sediments of Jurassic age in 

 Hope Bay at the northern end of Graham Land. He also asserts the abundance of porphyries and 

 porphyry tuffs in the same area. It is therefore possible that the above-described stones came from 

 this region ; but, from the identity in composition of the stones, and the fact that they were associated 

 together in the same dredging, it is considered to be at least as likely that they were derived from the 

 nearest land, i.e. Clarence Island. 



METAMORPHIC ROCKS 



Twenty-eight, or four-fifths, of the dredged stones belong to metamorphic types. The great majority 

 of these are due to the dynamic metamorphism of sedimentary rocks resembling the Scottish ' faikes ', 

 alternate laminae of carbonaceous shales and quartzose siltstone or sandstone. These rocks have been 

 intricately folded, sheared, crushed, and converted into carbonaceous sericite-phyllites alternating 

 with quartzose phyllite and quartz-sericite-schist. Some of the rocks contained a significant amount 

 of calcareous cement which has been recrystallized as calcite. This mineral is occasionally so abundant 

 that the rocks have to be recognized as calc-sericite-schists. 



Thin flakes of sericite are profusely developed in both the siliceous and argillaceous laminae. 

 Calcite and chlorite are formed mostly in the coarser quartzose bands. The chlorite, developed from 

 ferromagnesian impurities in the original sediments, is usually a pale green variety with ' ultra-blue ' 

 polarization colours. It is often vermicular and then almost isotropic. Epidote is sparingly developed 

 in the earlier stages of metamorphism, and generally in the slaty laminae. 



Some of the rocks are minutely folded and puckered, even within the limits of a thin section (Fig. 11), 

 and the thicker laminae of phyllite acquire a strain-slip cleavage parallel to the axes of folds in the 

 coarser quartzose layers. Others are sheared and smashed into small fragments with the production 

 of crush-breccias. These crush-breccias are often rolled out and a kind of flaser structure is developed, 

 consisting of lenticular fragments of the brittle quartzose layers around which the phyllite laminae 

 have been forced to wind. The quartz grains grow during this process and uhimately form a coarse 

 mosaic. Similarly the size and amount of the sericite flakes increase with the degree of internal 

 movement. These rocks develop into well-crystallized quartz-sericite-schists at the climax of the 

 metamorphic reconstitution. 



1 E. H. Kranck, 'Geological Investigations in the Cordillera of Tierra del Fuego', Ada Geographica, iv, no. 2, Helsinki, 

 1932, PP- 231- 



