200 DISCOVERY REPORTS 



The Burdwood Bank shows a ground exceeding iooo square miles in area of very 

 coarse gravel of group E on its summit, and one of about 850 square miles of medium 

 gravel of group A on its northern slope. 



The main features of the texture of the bottom deposits shown by charting the 

 deposits according to the grade are the two chief grounds, silt to the north and east, fine 

 sand to the south and west. Extensive but smaller grounds of various coarser grades of 

 sand and gravel occupy large areas of the continental shelf west of long. 6o° W and south 

 of lat. 47 S. Though the areas covered by the various types of deposit do not coincide 

 with the grounds grouped according to texture, it is noticeable that the contours 

 delimiting deposit types frequently coincide in part of their course with those de- 

 limiting the grounds. 



DISCUSSION 



The sparseness of sampling in the region examined is such as to preclude any detailed 

 discussion of the causes of the distribution of the bottom deposits. The entire region is 

 characterized by the presence of enormous masses of fine sand and silt. A line running 

 first in a northerly and then in a north-westerly direction from the Falkland Islands to 

 the South American coast separates the silt grounds on the north and east from the 

 fine-sand grounds to the south and west. The currents and prevailing winds of the part 

 of the South Atlantic comprised in the region examined travel approximately from 

 south-west to north-east ; to these agencies and to the heavy swells which prevail the 

 segregation of the silt grounds to the north and east of the region is no doubt due. A 

 chain of coarser grounds also extends in unbroken line for nearly 300 miles from the 

 eastern extremity of Tierra del Fuego in a north-north-easterly direction as far as 

 lat. 50 S. Thereafter the line is interrupted, but patches of coarse material lie on the 

 course of the line for another 300 miles to lat. 45° S. Grounds of coarser texture also 

 extend in a north-easterly direction from the north-east corner of the Falkland Islands. 

 It is possible that the position of these grounds, extending in a south-westerly to north- 

 easterly direction, is correlated with the currents and prevailing winds of the region 

 which travel in the same direction. In support of this it is noticeable that the contours 

 enclosing the areas covered by the different types of deposit (see Plate XIII) in many 

 cases run in a south-westerly to north-easterly direction. This is particularly marked 

 in the area covered by deposits of group F which extends across the region from south- 

 west to north-east west of the Falkland Islands. On the other hand, the coarse grounds 

 stretching across the area in a north-easterly direction may also be correlated with rock 

 outcrops occurring there. 



It appears probable that some at least of the deposits are being formed in situ on the 

 sea bottom and are not derived from the land or other regions. On the Burdwood Bank, 

 Macfadyen (1933) has deduced the occurrence of rock outcrops from the presence of 

 fossil Foraminifera in the deposits. The same deposits also contained portions of the 

 rock from which the fossils had been washed out. The occurrence of glauconite in the 



