SOUNDINGS TAKEN DURING THE DISCOVERY INVESTIGATIONS, 1932-1939 61 



The terms 'ridge' and 'rise', however, would appear to be sufficiently expressive to provide an 

 adequate description of the bottom relief between the deep basins of the various oceans, but if it is 

 considered really necessary to have this additional term we would then suggest that the word 'sill' 

 is much preferable as the English version of the term ' Schwelle '. 



THE SCOTIA ARC (Plate XXIII)i 



In our previous report on soundings (1932, pp. 214-9) rfiuch evidence was presented in support of 

 an arcuate connexion between Tierra del Fuego and Graham Land, by way of Staten Island, the 

 Burdwood Bank, the Shag Rocks, South Georgia, the Gierke Rocks, the South Sandwich Islands, the 

 South Orkney Islands, Elephant and Clarence Islands and the South Shetlands. Such a connexion 

 had long been forecast by some geologists, but evidence from rock specimens was scanty or even 

 contradictory and, prior to the work of the Discovery Committee's ships, very little information was 

 available from soundings. 



The term ' Scotia Arc' was adopted on the suggestion of Mr J. M. Wordie (see Herdman, 1932, 

 p. 214) but does not appear to have met with universal approval; Mosby (1940, p. 96), for instance, 

 prefers the term ' South Atlantic Arc' and German authorities, such as Stocks (1937, 1939) and Wiist 

 (1933), have persisted in the view that their term 'South Antilles' is more correct. Reasons have 

 already been given (1932) for preferring the term Scotia Arc to that of the South Antilles and, as the 

 name ' Scotia Sea' is generally accepted for the area around which the major portions of the Arc are 

 grouped, it does not appear that the term South Atlantic Arc is a better alternative. In our opinion 

 this latter name is also slightly misleading, and in this report the name Scotia Arc will be retained. 

 Between 1932 and 1939 many thousands of additional soundings in this area were taken by the 

 ' Discovery II ', and, by landings and dredgings, we have also been able to obtain relevant geological 

 material. Samples of rock were obtained from the South Shetlands, parts of Graham Land and the 

 off-lying islands to the north-west, Gibbs Island (near Clarence Island), and Saunders Island, in the 

 South Sandwich group. Dredged material from near the Shag Rocks, from Clarence Island, and from 

 four stations in the South Sandwich group completed the collection. Ahhough the evidence from the 

 South Sandwich group is inconclusive, Tyrrell's report on these specimens (1945) supports the theory, 

 favoured by Suess and others, of a tectonic connexion between South America and West Antarctica 

 along the Scotia Arc. 



Tyrrell considers that the predominant basic lavas of the South Sandwich Islands show a closer 

 affinity with the comparable rocks of the Antilles of North America than with those of the Andes, 

 and he suggests that the South Sandwich Islands may not lie on the main line of the Scotia Arc, but 

 may form an easternmost ridge parallel to and in echelon with it. In support of this he notes the south- 

 eastward trend of the axis of South Georgia, and the northward extension of the 3000 m. contour 

 (shown in pi. xlv of our previous report of 1932) leading down to the South Orkney Islands. Since 

 the publication of that chart, however, further soundings have provided valuable evidence of a far 

 more prominent ridge which appears to connect the South Orkney with the South Sandwich Islands 

 (see pp. 73-4), and there seems little doubt that this represents the main line of the Arc. Unfortunately, 

 we have had no new soundings to the south-east of South Georgia but, in our opinion, those already 

 obtained are of sufficient density to preclude the possibility of a direct connexion between the South 

 Orkneys and South Georgia. 



1 As in our earlier bathymetric chart soundings from sources other than the Discover}- Committee's vessels are shown in 

 Pis. XXIII to XXV as open circles. Our soundings up to May 1932 are shown as black dots; subsequent to that date crosses 

 have been used to indicate the positions of ' Discovery ' soundings. Since the scale is small it has not been possible to show 

 all the soundings taken when they were closely spaced, or when continuous soundings were being taken with the recorder. 



