832 



MR G. W. TYRRELL ON 



types in time, if not in space. The development of each type is shown in the 



following table 





Total No. 



of 

 Specimens. 



Tuffs, etc. 



Argill. 

 and Aren. 

 Sediments. 



Limestones. 



Cumberland Bay Series, Upper Division . 

 „ „ Middle „ 

 „ ,, Lower ,, 



Cape George Series ..... 



8 

 35 

 64 



4 



7 

 7 

 6 

 



1 

 23 

 57 



4 





 5 

 1 

 



Igneous material is present throughout the Cumberland Bay Series. Its amount 

 appears to be relatively greatest in the upper division, and it decreases steadily 

 through the lower divisions. Calcareous rocks are most abundant in the middle 

 division, but are throughout very subordinate in quantity to the dominant arenaceo- 

 argillaceous series. Amongst the latter, radiolarian rocks occur sparingly in each of 

 the three divisions. Furthermore, the grade of metamorphism of the rocks increases 

 steadily downwards through the series, and reaches its greatest development in the 

 phyllites and " augen "-grits of the lower division of the Cumberland Bay Series. The 

 four specimens belonging to the Cape George Series are phyllitic, and do not sensibly 

 differ from many of those of the lower division of the Cumberland Bay Series. 



From all available sources of information South Georgia may therefore be re- 

 garded as consisting of an extensive sedimentary series, mainly arenaceous in character, 

 but with a considerable development of argillaceous types and of bedded trachytic 

 tuff's. A few calcareous grits or gritty limestones are found intercalated with the 

 dominant greywackes. Some of the shales, slates, and fine-grained tuffs are radio- 

 larian. Other fossils have been found, but their evidence as to the age of the series 

 is decidedly ambiguous. The rocks are highly folded, and in some cases sheared and 

 metamorphosed. Some of the coarser sheared quartzose rocks of the lower division 

 of the Cumberland Bay Series may be porphyroids, but it is desirable not to press 

 this identification until further and less equivocal evidence is obtained. 



Igneous rocks other than interbedded tuft's do not appear to be extensively de- 

 veloped. ITkim found an " altvulcanischer " area consisting of basalts and melaphyres 

 at the south-east end of the island. With this, perhaps, the diabasic intrusion found 

 by Mr Ferguson in the lower division of the Cumberland Bay Series may be cor- 

 related. Heim also collected rocks of granitic and dioritic character from the 

 moraines and beaches of Slosarczyk and Larsen Fiords. These are probably derived 

 from a granito-dioritic complex now largely buried beneath the central ice-cap. The 

 'jiMiiite-porphyry collected by Mr Ferguson from the moraine at Moraine Fiord, 

 Cumberland Bay, may be associated with this series. 



The principal problem on which the geology of South Georgia may be expected 

 to throw light is the nature of the tectonic connection between the Andes and the 



