674 DR J. H. HARVEY PIRIE ON DEEP-SEA DEPOSITS 



cent, in the most northerly. The siliceous organisms constitute respectively 55, 80, 

 and 30 per cent, of the whole, going from south and north. Diatoms in all three make 

 up the greater part of this, but radiolaria are also always present. The predominant 

 species are given below in Dr Gran's report. Murray and Philippi, in their Valdivia 

 report, refer to the absence in the deposits of certain species common in the surface 

 waters, presumably through solution during the long process of sinking ; whether this 

 holds good also in the Weddell Sea and S. Atlantic area it is not yet possible to say, 

 until the results of the examination of the tow-netting specimens are available. The 

 absence of clayey matter and rock flour in these Diatom oozes, particularly in the most 

 southerly, is very striking. Evidently the transition from the glacial clay to the south, 

 in which the "fine washings" of the most northerly saujple obtained formed 97 per 

 cent, of the whole deposit, is a fairly sharp one. The transition to Globigerina ooze 

 on the northern side of the Diatom ooze band in this region appears, on the other hand, 

 to be more gradual. This is in keeping with the results obtained by the Valdivia and 

 the Gauss ; but no isolated patches of Globigerina ooze within the Diatom ooze area, 

 or between it and the glacial clay, as found by the Valdivia, were come across 

 by the Scotia. 



The relative amounts of diatoms in the surface waters and in the bottom material 

 from the same place form a very marked contrast. Diatoms live entirely in the upper 

 layers of the ocean, within the depth to which light permeates, and within the pack-ice 

 area are exceedingly abundant, colouring the ice as it is seen broken up in summer, of 

 a brownish colour. Over the whole of the Weddell Sea area, where the bottom material 

 is a glacial clay, diatoms flourish in the surface waters, but in the deposits they are 

 either entirely absent or in insignificant amounts (under 2 per cent.). Where they are 

 most predominant in the deposits — in about 51° or 52° S. — on the other hand, they are 

 comparatively infrequent in the surface waters, flow is this striking difference to be 

 accounted for? It is not a question of depth and solution before reaching the bottom, 

 for the difference in the two zones is inconsiderable — about 2400 to 2700 fathoms for 

 the glacial clays, and 2100 to 2500 fathoms for the Diatom oozes. Nor can the rapid 

 accumulation of glacial detritus with hiding of the diatoms account for this remarkable 

 distribution, for in deposit No. 44 from 61° 21' S., 13° 02' W. — the most northerly glacial 

 clay obtained in this area, and where the accumulation of detritus must be much slower 

 than nearer the Antarctic continent — not a single diatom could be found. Carrying 

 away of the diatoms by surface currents set in motion by the prevailing winds also fails 

 as an explanation, for in the southern part of the Weddell Sea the drift is to the west, 

 in the northern portion to the east, whereas a northerly carriage is required to fit in 

 with the facts of distribution. One is thrown back on the explanation tentatively put 

 forward by Dr Philippi (6) in his preliminary report on the deposits obtained by the 

 German Antarctic Expedition, and subsequently elaborated by himself. This is in the 

 existence of a northerly undercurrent produced by the melting of ice within the pack-ice 

 zone. Some indication of strong undercurrents was got by the Scotia while trawl- 



