64 Prof. J. W. Gregory — Geological Relations of S. Georgia. 



Duriug a study of the microscopic structure of tliese rocks 

 Mr. Tyrrell found a series of Eadiolaria in various specimens, and 

 they are especially well preserved in one band. Some sections have 

 been kindly examined by Dr. Hinde. He reports : " The following 

 genera appear to be represented : Cenosphcera, Cetiellipsis, Amphi- 

 brachium, Archicapsa, I)icolocapsa, Tricolocapsa, Dictyomitra, and 

 Stichocapsa. The three first-mentioned genera would be included in 

 Haeckel's Spumellaria, the other six in his Legion Nasselaria. At 

 present I should consider from the general character of the forms in 

 these two sections that they are post-Palseozoic and pre-Tertiary in 

 age, and they might come in between the Triassic and the Cretaceous. 

 But this view might be modified by further knowledge from the 

 contents of fresh sections." 



Hence the Middle Cumberland Bay Series includes rocks which, 

 according to the Cephalopod, are probably Cretaceous; according to 

 the Eadiolaria are probably Jurassic ; and according to Mr. Ferguson's 

 fossils the lower division is probably Silurian. 



The available evidence, therefore, regarding the geology of South 

 Georgia unfortunately leaves the relations of the island inconclusive. 

 Mr. Ferguson is strongly of the opinion that his three divisions of 

 the Cumberland Bay Series form one continuous succession, and 

 his excellent photographs support that conclusion. Moreover, the 

 similarity between the materials of the three divisions show that 

 they were laid down under similar conditions. Each division, as 

 shown by Mr. Tyrrell's description of the rocks, includes bands of 

 tuff and layers containing Radiolaria, and all these divisions are 

 cleaved, though the rocks of the lower division are much more 

 sheared and altered than those in the two upper divisions. The whole 

 of the Cumberland Bay Series appears to have been formed off the 

 shores of a volcanic land, which included some sedimentary rocks, the 

 debris of which formed the grits and grey wackes of the lower division. 



The palaeontological evidence, however, suggests that the lower 

 division is Ordovician or Silurian, and that the middle and upper 

 divisions are Mesozoic. That at least a part of the series is Palaeo- 

 zoic is supported by the fact that, according to Mr, Ferguson, both 

 he and Dr. Pirie regard the rocks as strikingly similar, lithologically, 

 to the graptolite-bearing rocks of the South Orkneys. We are 

 therefore left with two alternative explanations. South Georgia 

 may consist mainly of Mesozoic marine sediments, interstratified, 

 as in the Andes, with contemporary volcanic material, and the rocks 

 may have been folded and faulted by movements belonging to the 

 Andean System. On the other hand, if the presence of marine 

 Ordovician or Silurian sediments and the absence of Kainozoic 

 volcanic activity be confirmed, these facts would be less favourable 

 to the view that the island is part of the Andean Mountain System. 

 The island might be a folded fragment of the Flabellites Land of 

 Professor Schwarz, and the loop of the Andes which connected 

 Grahamland with Southern Patagonia would have passed to the west 

 of South Georgia. We must await better fossils and further evidence 

 as to the igneous rocks before choosing definitely between these 

 hypotheses. 



