488 G. W. Tyrrell — Petrography of South Georgia. 



series of igneous rocks in and about Larsen Harbour at the south- 

 eastern end of the island. The tectonic inferences to be drawn 

 therefrom are so important that it was thought advisable to send the 

 slides to Dr. J. S. Flett, F.R.S., for his opinion as to their spilitic 

 nature. He has very kindly confirmed this identification. The 

 diabases and melaphyres, described (macroscopically) by Heim, no 

 doubt belong to this series. Furthermore, the epidotized lavaform 

 and tuffaceous rocks of doubtful afiinities, described by me in the 

 last paper, almost certainly belong to the series.^ 



Associated with the dark-grey or grey-green spilites are light- 

 coloured rocks which were previously identified as quartz-felsites. 

 Di'. Flett prefers to call them soda-felsites ; and this is undoubtedly 

 the better term since the rocks are very rich in albite, and must be 

 regarded as consanguineous with the albite-rich spilites. The 

 alaskite from Cooper Island, described in the last paper, is perhaps 

 also to be correlated with this series. 



Furthermore, the badly decomposed rocks here described as green- 

 stones were probably once dolerites with a sodic plagioclase, oligoclase 

 or albite, petrographic types which elsewhere are closely associated 

 with spilites and soda-felsites. The ophitic dolerite from Cumber- 

 land Bay and the epidiorite (meta-dolerite) of Gold Harbour, 

 described in previous papers, are probably also to be associated with 

 the spilitic series. These rocks, however, differ entirely from the 

 ophitic dolerites and basalts from Larsen and Slosarczyk Harbours in 

 the last collection.* The latter rocks are very fresh, with a much 

 more calcic plagioclase than is general in the spilitic series, and 

 furthermore have escaped the epidotization which is so characteristic 

 of that suite. Hence it is believed that they belong to an intrusive 

 series of much younger date than the spilitic series. 



From consideration of the evidence now accumulated South 

 Georgia consists principally of a folded sedimentary complex, 

 including greywackes, slates, mudstones, and tuffs, of uncertain 

 age, probably Palaeozoic in the main, but perhaps ascending into the 

 Mesozoic. It contains a spilitic series of igneous rocks at the 

 south-eastern end of the island, and intrusive rocks of the same group 

 occur within the sediments at least as far away as Cumberland Bay. 

 Rising behind the dark, " altvulcanischer " region around Larsen and 

 Drygalski Harbours, Heim saw a great, light-coloured massif, the 

 composition of which was believed to be granito-dioritic from the 

 evidence of the moraine material within the area.^ The specimen of 

 granite-porphyry collected by Mr. Ferguson from the glacial material 

 at Moraine Fiord probably belongs to this or a similar complex.* 



Spilites are characteristically associated with rocks of Palaeozoic or 

 Pre-Palseozoie age; and if. this relation holds in South Georgia, 

 the presence of spilites may be held to reinforce the view that the 

 sedimentary series of the island is mainly of Palaeozoic age. The 



1 Ibid., pp. 439-40. 

 ^ Ibid., p. 437. 



* Op. cit., p. 454. 



* Trans. Koy. Soc. Edin., vol. 1, pt. iv, p. 830, 1915. 



