MARINE DEPOSITS OF PATAGONIA 197 



" representative number " of the sample. Thus a sample containing only large fragments 

 would have a representative number of 15, while one containing only silt would have a 

 representative number of o. All other proportions of the grades are represented by the 

 numbers between o and 15, the representative number increasing in value as the 

 coarseness of the texture increases. The samples are then referred to the grade con- 

 taining particles whose diameter in millimetres corresponds with the representative 

 number. Thus a sample with representative number 0-31 is referred to grade fine sand, 

 which contains particles 0-1-0-5 mm. in diameter, whilst a sample with representative 

 number 7-61 is referred to grade coarse gravel, which contains particles 5-10 mm. in 

 diameter. Samples with representative numbers below o-i are classed as silt. The 

 deposits are then plotted on the chart according to the grades to which they are 

 assigned. 



The finest two grades, fine sand and silt, cover the greater part of the region con- 

 sidered and form extensive grounds, while the coarser grades occur in smaller grounds 

 and patches. A line running east from Cape Tres Puntas to the edge of the continental 

 shelf, and then turning southerly and following the edge of the shelf to the north-east of 

 the Falkland Islands, where it turns eastward again, divides the two main grounds of 

 the region, the silt grounds lying to the north and east, and the fine sand grounds to 

 the south and west of the line. 



THE SILT GROUNDS 



The silt grounds, about 60,250 square miles in area, are composed of deposits of the 

 groups C, D, E and F. Group E deposits occur in the western part, those of group F 

 on the continental slope and in the middle of the continental shelf. Deposits of group C 

 cover the northern part and a wedge-shaped area at the centre of the continental shelf, 

 while those of group D lie to the east of this latter area. North-east of the Falkland 

 Islands between lat. 50 and 51 S the silt ground extends over the edge of the con- 

 tinental slope on to the continental shelf and reaches in towards the north coast of the 

 Falkland Islands at the northern end of Falkland Sound. This part, both on the con- 

 tinental slope and shelf consists of deposits of group B. 



Smaller coastal silt grounds about 800 square miles in area lie off Point San Julian 

 and Point Gallegos, the former with deposits of group F, the latter of group E. Another 

 small area of about 850 square miles lies on the continental slope west of the deep water 

 lying north-east of the Burdwood Bank. Its deposits belong to group F. Grounds of 

 coarser texture lie in smaller areas within the main silt ground as detailed below. 



COARSER GROUNDS WITHIN THE SILT GROUNDS 



There is a coastal belt of fine sand 2800 square miles in area, with deposits of group E, 

 between Cape dos Bahias and the Valdes Peninsula, while at the edge of the con- 

 tinental slope between lat. 45 and 47 S lies a ground of fine sand of about 1200 square 

 miles area. Its deposits are of group E to the north, and of group D at the south, with 



