3I0 DISCOVERY REPORTS 



traverse from Adelaide Island shows that the continental shelf extends some 80 miles 

 from the coast, and that the edge of the shelf is very steep, the depth increasing from 

 500 to 3000 m. in 15 miles. A profile section (adapted from Herdman 1 ) is shown on 

 Plate XXI. On the shelf the deposits from Sts. WS 509-5 1 5 have all the characteristics of 

 diatomaceous mud, and the assemblage of diatoms is similar to that seen in samples 

 from the Scotia Sea. Near the edge of the shelf (St. WS 516), at a depth of 261 1 m., 

 the bulk of the deposit is a coarse sand containing angular grains of quartz and opaque 

 fragments of rock. Farther to the north-west (St. WS 517) the detrital grains are of 

 smaller size, and the organic remains become more important so that the deposit must 

 be classified as diatomaceous mud. The presence of coarse sand at so great a depth on 

 the continental slope, and at so great a distance from land, raises interesting questions 

 as to the cause of its occurrence. But speculation must remain in abeyance for the 

 present, as no other sample from a similar situation is available and generalization is 

 hardly possible on a single example. 



The remaining seven samples are from stations farther south and west, as far as lat. 

 70 S and long. ioo° W. St. WS 495, north-west of Alexander I Island, yields diatom- 

 aceous mud of the usual type at a depth of 2582 m. At St. WS 508 the deposit (taken 

 from a depth of 309 m.) is a coarse sand or fine gravel which may indicate a bottom of 

 bare rock. The samples from Sts. WS 506 and WS 507 at a depth of about 580 m. can only 

 be classified as terrigenous mud ; for though flocculent material appears as conspicuous 

 as the small mineral grains enclosed therein, there are few recognizable organisms. The 

 same brown unctuous mud occurs farther west at greater depths, namely at Sts. WS 505 

 (1500 m.) and WS 503 (4073 m.); it is probably typical of the sea-floor in these high 

 latitudes, for Herdman states that " the average depth of the ocean bed in the Bellings- 

 hausen Sea varies very little", the average depth being approximately between 3900 

 and 4400 m. A brown mud from a depth of 4334 m. at St. WS 502, however, is classi- 

 fied as diatomaceous mud, because recognizable diatoms are present in some quantity, 

 and their occurrence seems to account for the (presumably organic) flocculent material. 

 But the abundance of terrigenous material at these great depths and at so great a 

 distance from land is surprising, and seems to demand some special means of transport. 

 It may be that much detrital material has been contributed by the melting of icebergs, 

 as Pirie 2 has suggested for the muddy deposits of the Weddell Sea. 



THE WESTERN COAST OF SOUTH AMERICA (Plate XXII) 

 Samples of sea-floor deposits are available from fifty-one stations off the western 

 coast of South America, between the equator and lat. 40 S, along the course of the 

 Humboldt current. The majority of the samples are remarkably uniform in character, 

 thirty-six being classified as diatomaceous mud, eight as terrigenous mud and eight as 

 sand. The first-named deposit has much the same general constitution as its equivalent 



1 Herdman, 1932. Report on Soundings, etc. Discovery Reports, VI, p. 228. 



2 Pirie, J. H. H., 1914. Deep-Sea Deposits of the Scottish National Antarctic Expedition. Trans. Roy. 

 Soc. Edin., xlix, p. 677. 



