SEA-FLOOR DEPOSITS. PART I 311 



in the Scotia Sea, containing a considerable proportion both of mineral grains and of 

 flocculent diatom debris ; the latter is usually sufficient in quantity to impart a green 

 colour to the mud. In the South American samples, however, the flocculent aggregates 

 constantly enclose numerous brown resting spores which presumably belong to various 

 algae; moreover similar spores are often seen in position within the frustules of 

 Coscinodiscus and other diatoms. While this peculiarity is undoubtedly of a seasonal 

 character, it tends to support the previously formed conclusion that the flocculent 

 material is largely formed of the disorganized cell contents of algae. It is worthy of 

 note, too, that the assemblage of diatoms in the deposits under discussion differs con- 

 siderably from that found in the samples from the Scotia Sea. Coscinodiscus is the pre- 

 dominant genus in both regions, and it is particularly large and abundant in most 

 samples from South American waters. Here it is constantly accompanied by Actino- 

 ptychus and Navicula, while Aster ompholns, Bacteriastrum, Entopyla, Grammatophora, 

 Pleurosigma, Licmophora, Choetoceros and Triceratium are more sporadic in occurrence. 

 Many of these genera have not been observed in the deposits from the Scotia Sea during 

 the present investigation. There (it may be recalled) Thalassiosira, Fragilaria, Rhizoso- 

 lenia and Corethron are (after Coscinodiscus) most consistent in occurrence, but these 

 genera are virtually absent from the South American assemblages. Another feature of 

 minor interest is the presence of the widespread silicoflagellate Dictyocha fibida, Ehr., 

 in several of the South American samples. 



As in other regions, the accumulation of diatomaceous mud shows little relation to 

 depth, for such deposits are found at the slight depth of 29 m. (St. WS 676), while they 

 are also present at depths of over 4000 m. (Sts. WS 686, WS 703-705). This bathy- 

 metrical range is similar to that shown by the terrigenous mud which occurs in a depth 

 of 369 m. at St. WS 396, and at St. WS 617 in 4864 m. It must be noted, however, 

 that some of these deposits differ from diatomaceous mud only in the proportion of 

 flocculent matter and in the paucity of recognizable organic remains. In any case, the 

 great vertical range of terrigenous material seems to be due to the circumstance that 

 the continental shelf is very narrow along the western coast of South America, with the 

 consequence that areas of considerable depth are within the range of currents which 

 transport detritus from the land surface. The irregular distribution of diatoms and 

 other organisms is probably due to local differences in chemical and physical conditions. 



The mineral content of the diatomaceous muds is, in general, similar to the detrital 

 material which makes up the bulk of the terrigenous deposits. Quartz is the most 

 abundant mineral and accounts for the bulk of the detrital grains in all the deposits. 

 Green hornblende also is of widespread distribution, having been noted in thirty-five 

 of the fifty-two samples, but brown hornblende is apparently rarer and of sporadic 

 occurrence. Hypersthene is noted only occasionally in these South American deposits 

 at three stations (WS 591, WS 596, WS 604) between 32 and 36 S lat. though volcanic 

 glass occurs in about half the samples. The presence of plagioclase felspar at three 

 stations probably indicates that the mineral has not always been distinguished by the 

 writer from other colourless grains ; complete mineralogical analysis of the samples will 



