^o DISCOVERY REPORTS 



There still remains the gap in the Arc between the meridians of 48° W and 49^ W. It is now seen 

 that the width of this is considerably less than hitherto shown, though from the evidence of the 

 soundings alone its existence must be considered as certain. Hydrological observations do not either 

 confirm or deny its presence, although there is some slight evidence that a northward movement of 

 cold bottom water may occur at this point. Soundings, however, show the gap to be connected with 

 a narrow trough of moderately deep water which lies east and west, between the Arc proper and 

 a parallel ridge to the north. This parallel ridge, which extends east from the Falkland Islands for 

 some 600 miles, at depths less than 3000 m. (1641 fm.), and which has an area of 5000 sq. miles less 

 than 2000 m. (1094 fm.) in depth at its eastern end, is an interesting feature. It extends east to the 

 longitude of AV 30' W and there can be no doubt that from this position to some distance west of the 

 Falkland Islands it is completely separated over its whole extent from the main line of the Arc. The 

 trough which constitutes the separation is narrow, but for the greater part of its length is more than 

 3000 m. (1641 fm.) in depth. It can finally be traced, by the 500 m. (273 fm.) depth contour, to 

 approximately 53" 30' S, 63° 30' W; about 150 miles west of the mean longitude of the Falkland 

 Islands. 



The soundings shown on our original bathymetric chart gave a strong indication that such a trough 

 existed, although the extent of the northern ridge, as determined by the 3000 m. (1641 fm.) contour, 

 was shown only to the longitude of 47° W. Our more recent soundings in this area, however, comprise 

 two lines between the Falkland Islands and South Georgia, five lines in a north and south direction, 

 and four on north-westerly and south-easterly courses. All these series of soundings cross this trough, 

 and four of the north and south lines and three of the others crossed completely the eastern end of 

 the Falkland Ridge. The area less than 2000 m. (1904 fm.) in depth, of which only the southern part 

 is shown on our present bathymetric chart (PI. XXIII), was thus fairly extensively sounded out and 

 its limits determined with reasonable accuracy. 



Geologically, this ridge parallel to the Scotia Arc raises some interesting questions. For example, 

 the structure of the Falkland Islands bears not the least resemblance to any structure so far determined 

 in the various sectors of the Arc, and yet there can be no doubt of the connexion between the Falkland 

 Islands and the comparatively shallow area which lies some 500 miles farther east, thus forming a 

 parallel fold, such as is symptomatic of the Arc in general. It is not, perhaps, within the scope of this 

 report to discuss these implications, but it should be noted that soundings provide evidence of rather 

 similar conditions north-east from South Georgia, fuller details of which will be given when we 

 consider the soundings in that area. 



SOUTH GEORGIA AND THE SHAG ROCKS 



Soundings taken since 1932 immediately to the west of the Shag Rocks have already been mentioned, 

 their effect on the contours here being quite marked ; but new soundings north and south close to the 

 Rocks, and eastwards to South Georgia, cause little alteration in our original conception of the contours 

 in this area. Two lines of soundings, however, which crossed this part of the ridge in a north-easterly 

 direction, enable us to complete the 2000 m. contour south of the ridge, which had been in doubt; 

 but the new contour follows closely the original tentative line. South of the Shag Rocks the 3000 m. 

 contour is found to lie some miles north of the line previously shown, thus reducing the width of the 

 ridge to a considerable extent. 



Our soundings clearly show that the Shag Rocks constitute an important link in the Scotia Arc; 

 and it is of considerable interest to find geological evidence in support of this in Tyrrell's recent report 

 (1945). Stones dredged from a depth of 199 m. (109 fm.) at St. 474 (i mile west of the Shag Rocks) 

 showed a preponderance of greenstones which, from a close examination, Tyrrell considers to be 



