2i6 DISCOVERY REPORTS 



confirms Heim and Wordie's views on the structure of South Georgia. In discussing the 

 bearing of his observations on the problem of the South Antillean Arc, HoUedahl sees 

 no reason to beheve that South Georgia hes outside the main Andean fine of folding .^ He 

 has also endeavoured to show that there is a distinct geological likeness between South 

 Georgia and the Staten Island — Tierra del Fuego district. Concerning the remainder 

 of the arc he remarks: " the fact that various types of rocks, or even rocks of no doubt 

 different age, occur in various places, does not in any way contradict the assumption of 

 these land-masses being parts of the same old range", and with regard to the South 

 Orkneys he suggests that the divergence of the strike from the trend of the submarine 

 ridge may be purely local as "we know from many ranges that the strike may locally 

 diverge very considerably from the main trend of the folding belt". 



Wilckens," in his criticism of Staub's theory'* of the non-existence of an arcuate con- 

 nexion between the Andes and Graham Land, supports Holtedahl's contention that 

 Tierra del Fuego and South Georgia are of a similar geological structure and, in general, 

 considers that the existence of a submarine arc has been proved. 



The bathymetric chart of the Scotia Sea shown by Pratje,"* which is based mainly on 

 the echo-sounding profiles of the ' Meteor' during 1925 and 1926, supports the theory of 

 the island arc, though he shows a greater average depth between the Burdwood Bank and 

 the Shag Rocks and between South Georgia and the Shag Rocks than actually exists. 

 Comparison with our bathymetric chart of the same area (Plate XLV) will show that the 

 depth here is considerably less than that given by Pratje. 



The principal evidence that we are able to bring forward in regard to the Scotia Arc 

 is derived from soundings, and, as will be seen from the charts and plates accompany- 

 ing this paper, the soundings indicate clearly that well-defined submarine ridges exist 

 between Graham Land and South America along the generally accepted line. The only 

 geological evidence obtained during the course of the Discovery investigations relates 

 to the South Sandwich Islands. During our running survey of the group in March 

 1930 some rock specimens were collected from Beach Point, Thule Island, by Dr E. H. 

 Marshall and Mr T. J. Hart. These have been carefully examined and analysed by 

 Tyrrell,^ who pronounces them to be of a definite Andean type. 



Our soundings show very definitely that in many places there are concentric sub- 

 marine ridges which may in themselves be considered as favourable geological evidence 

 of an Andean loop. All the sections drawn across the various sectors of the ridge show 

 this peculiarity, though it is much more marked in the South Sandwich — South Orkneys 

 and the South Orkneys — Clarence Island sectors, as well as across the South Sandwich 

 Deep. Between the Burdwood Bank and the Shag Rocks the section (Fig. 3 b, p. 226), 

 drawn approximately down the 49th meridian, shows numerous rises; but between the 



^ Loc. n't., pp. 104-17. 



2 Wilckens, Natunv. Abt. Niederrheiii. Ges. Nat. Heilkunde, 1930-31, pp. 1-14, Bonn (1932). 

 ^ Staub, Die Bewegungsmechanismus der Erde, Berlin. 



* Pratje, Beitrage zur Bodengestaltung des Siidatlantischen Ozeans. Cent.f. Mill., Abt. B, p. 132 (1928). 

 "^ Kemp and Nelson, The South Sandwich Islands, with a report on rock specimens by G. W. Tyrrell, 

 Discovery Reports, in, pp. 191-7 (1931). 



