102 



DISCOVERY REPORTS 



During the third commission of the R.R.S. ' Discovery II ' some 120 stations were made 

 in the Antarctic zone at positions where the depth was greater than 3000 m. At seventy 

 of these stations there was most silicate at the depth of the lowest observation, usually 

 less than 500 m. from the bottom. At fifty stations the silicate content was greater 

 above the lowest observation, and at forty-four of these fifty stations the maximum 

 content occurred at depths between 500 and 1500 m. above the lowest observation. 

 Thus in the Antarctic zone during 1933-5 the silicate content of the bottom water was 

 always large and at seven out of twelve stations in this zone the actual maximum 

 occurred at depths less than 500 m. from the bottom, whilst at the other five out of 

 twelve maximum silicate occurred at a higher level in the bottom water. 



In the light of our present knowledge the geographic areas where the silicate content 

 decreases near the bottom lie chiefly in the Antarctic zone south of the Atlantic Ocean, 

 and south of the Pacific Ocean between 130 and 140 W. The temperature range at the 

 seat of the maximum in these areas is -0-31° to 0-31° C. south of the Atlantic and 

 o-i2-o-32°C. in the area 130-140 W. The reason why silicate is not always most 

 abundant near the bottom has not been ascertained. 



In the Scotia Sea a few extremely high values of 8000-8600 mg. were recorded 

 in December 1933, but a greater number of observations, in 1934-5, showed that the 

 usual content was not so high. The following figures give the silicate content from south 

 to north at the depth of the lowest observation which was at least 3000 m. 



The silicate content of the Antarctic bottom water along section 3 in the eastern part 

 of the South Atlantic Ocean in March 1933 is shown by the vertical section in Plate 

 VI. At the ice-edge the content at the deepest observation was 5300 mg., and there was 

 little change as far north as the Atlantic-Indian cross-ridge which rises in our section 

 to a saddle depth of 2633 m. in about 51$° S. This ridge impedes the northward flow 

 of bottom water which, however, passes into the Agulhas basin by a gap found to the 

 east of the section. North of the cross-ridge the silicate content of the bottom water 

 which is now a mixed water, falls steadily towards the north. In March 1935 the silicate 

 content of the bottom water in the Agulhas basin was 6000 mg. in the south and 

 4000 mg. on the northern side. This compares well with previous observations in April 

 1932 of 6000-6500 mg. in the south and 4300 mg. to the north. 



In the Atlantic-Antarctic basin a large number of observations were made in April- 

 May 1934 and February-March 1935, and at the great majority of stations the content at 

 the lowest observation, usually less than 500 m. from the bottom, was lower than that 

 of a higher level in the Antarctic bottom water. The difference varied from station to 



