89 



PART IV. 



PETROGRAPHY OF STONES DREDGED FROM THE 

 VICINITY OF THE SHAG ROCKS 



INTRODUCTION 



One of the most remarkable geological features of the West Antarctic region is the existence of an 

 eastwardly-directed loop of submarine ridges and islands which connects Staten Island in Tierra 

 del Fuego, through the Burdwood Bank, Shag Rocks, South Georgia, Gierke Rocks, South Sandwich 

 Islands, the South Orkneys, and the Elephant and Glarence Group, with the Graham Land peninsula 

 and its adjacent archipelagos. It represents an extension of Circum-Pacific orogenic structures for more 

 than I GOG miles into the heart of the alien geological region of the South Atlantic. This loop or arc 

 has been called the Southern Antilles on the basis of a supposed analogy with the Antilles con- 

 necting North and South America; but a better 

 term is the Scotia Arc, coined by J. M. Wordie, 

 since the loop surrounds the Scotia Sea. The 

 geological constitution of the Scotia Arc is con- 

 sistent with the view, put forward by E. Suess 

 and others, that it represents an orogenic tectonic 

 connexion between South America and Graham 

 Land.i 



Something is known of the geology and petro- 

 graphy of all the connecting links of the Scotia 

 Arc with the exception of the Shag Rocks. It is 

 fortunate therefore, that two Discovery dredgings 

 have been made in the vicinity of the Shag Rocks 



*'=. .Shag Rocks 



Fig- 13- 



(see map. Fig. 13), which have provided sufficient material to enable us to assess the geological character 

 of the Scotia Arc in this hitherto unknown region. These dredgings were made on 12 November 1930 

 by the 'Discovery 11' at Sts. 474 and 475. The exact positions and depths are as follows: 



St. 474. One mile west of the Shag Rocks. Depth 199 m. 



St. 475. Long. 53° 30^ S., lat. 42° 44* W. (about 25 miles west of the Shag Rocks). Depth 748 m. 



PETROGRAPHY 



Fourteen stones came from St. 474 and five from St. 475. Of these nineteen stones, fifteen are 

 practically identical and consist of tremolite-epidote-greenstone or greenstone-schist, one is a 

 feldspathic quartzite, and three are quartz-vein rocks. The four last-named stones all came from 

 St. 474, nearest to the Shag Rocks. The overwhelming preponderance of the greenstones in this 

 collection makes it tolerably certain that this rock constitutes the Shag Rocks themselves and the 

 submarine ridge on which they stand to at least 25 miles to the west. 



The stones range in size from 4 in. to i in. in greatest diameter. Fifteen of them, as above stated, 

 are ' greenstones '—dense, compact rocks of grey-green colour, showing an ill-developed cleavage 

 along which they tend to split. Only two are definitely slaty or phyllitic in aspect. The quartzite is 

 a fine-grained rock of a pale buff tint, and obviously contains much feldspar. The quartz-vein rocks 

 are white and coarse-grained. 



1 Recent summaries of the evidence have been given by O. Holtedahl, 'On the Geology and Physiography of Some 

 Antarctic and Sub-Antarctic Islands', Scientific Results of the Norwegian Antarctic Expeditions 1927-8 and 1928-9, instituted 

 and financed by Consul Lars Christensen, No. 3, Norske Vidensk.-Akad., Oslo, 1929, pp. 104-18. G. W. Tyrrell, 'Petrography 

 and Geology of South Georgia', 'Quest' Exped. Report {Brit. Mus. Nat. Hist.), 1930, PP- 5i-4- H. F. P. Herdman, Report 

 on Soundings taken during the Discovery Investigations, 1926-32', Discovery Reports, vi, 1932, pp. 214-19. 



