60 G. W. Tyrrell — The Rocks of South Georgia. 



latitic fragments are dominant, and occasional fragments of yellowish 

 glass enclosing euhedral orthoclase are seen. Many of the rocks have 

 suffered some degree of scapolitization. The tuffs pass into siliceous 

 greywackes still with some recognizable volcanic material, and are 

 interbedded with fine banded raudstones which appear to represent 

 volcanic muds. Radiolaria have been recognized in these rocks as 

 well as in the tuffs. The mndstones occasionally become some- 

 what laminated and pass into black shales. An imperfect cleavage 

 traverses many of these rocks at a high angle to the bedding. These 

 tuffs doubtless represent the interbedded ' diabas - schalstein ' of 

 Thiirach.-' 



The Middle Division of the Cumberland Bay Series consists mainly 

 of cleaved mudstones and siliceous shales or fine-grained grits, with 

 a few bands of compact dark limestone, and one or two crystal tuffs, 

 similar to those of the Upper Division. In thin section many of the 

 grits are highly sheared, and a lamination has been set up parallel to 

 the plane of shearing. The laminae are frequently streaked witli 

 carbonaceous material and wind around uncrushed ' augen ' of quartz. 

 The more shaly bands are finely puckered and folded, and are traversed 

 by quartz veins. Some of them contain a development of secondary 

 mica. Rocks intermediate between shale and grit — siliceous shite or 

 argillaceous grit — are common, and are frequently banded with 

 alternations of more sandy and more shaly material. Most of these 

 rocks are dark in colour and are distinctly cleaved. Some of the 

 mudstones, however, are very hard and cherty, and almost devoid of 

 cleavage-planes. The limestones are generally well cleaved. Only 

 one shows traces of organic structures. Others contain a quantity of 

 volcanic ash, or a considerable development of angular quartz chips, 

 and are to be regarded as calcareous tuffs and grits. 



The Lower Division of the Cumberland Bay Series consists of very 

 similar types of rocks to the above, but exhibiting a higher grade 

 of metamorphism. The mudstones are here transformed to black 

 graphitic slates, which pass into phyllites by an increasing develop- 

 ment of secondary mica on the cleavage-planes. Associated with 

 these are highly sheared argillaceous grits, showing good augen 

 texture in thin section. This association is probably the phyllit and 

 phyllit-gneiss of Thiirach. Less cleaved and altered mudstones are 

 to be correlated with his thonschiefer. The slates and grits frequently 

 occur in thin alternating bands, which show fine folding in the 

 quartzose and extraordinary strain-slip and puckering in the argil- 

 laceous layers. Crystal tuffs, similar to those in the Upper and 

 Middle Divisions, are also to be found amongst the rocks assigned to 

 this division, especially at Port Gladstone, Cumberland Bay. One 

 of these contains Radiolaria. A sheared limestone from Moraine 

 Fiord is included in the collection. 



Cape George Seeies. — Only four specimens of this series occur in 

 the collection. They consist of grey, phyllitic, siliceous slate and 

 black slate, much crushed and veined by quartz, otherwise not very 

 different from the lower part of the Cumberland Bay Series. 



' H. Thiirach, " Geognostisehe Beschreibung der Insel Siid-Georgien " : 

 Internat. Polarforschung, 1882-3. Die deut. Esped. , vol. ii, No. 7, p. 116, 1890. 



