302 DISCOVERY REPORTS 



south of the Falkland Islands, and the Shag Rocks, west of South Georgia. As Herdman 

 has shown, there appears to be a gap of about 60 miles (long. 47-49 W) where the sound- 

 ings show greater depths than usual. This depression is occupied by Globigerina ooze 

 (Sts. WS 521, WS 522), while the diatom ooze under discussion is accumulating on the 

 eastern slope at a depth of 2549 m. (St. 429). This is about an average depth for the 

 occurrence of diatom ooze, though the deposit is recorded from 5000 m. at St. 11, 

 north-east of South Georgia. The deposit is remarkably uniform in character over the 

 whole region. The genera which occur most constantly are Coscinodisciis, Cocconeis, 

 Fragilaria, Thalassiothrix and Rhizosolenia, while other forms, such as Tholassiosira, 

 Biddulphia and Achnanthes are occasionally present. There is a minimum of terrigenous 

 material and an entire absence of the perishable organic debris which is so characteristic 

 of diatomaceous muds. Nearer to South Georgia the diatomaceous deposits contain a 

 considerable proportion of terrigenous material evidently transported from the land 

 area; these deposits are assigned to the category next to be described. 



Diatomaceous mud. The great majority of the twenty-six stations which yield 

 diatomaceous mud lie in the Scotia Sea, north of the South Shetland and South Orkney 

 Islands. The samples from these stations conform very closely to those from the regions 

 of South Georgia and the South Shetlands, but there is some variation in general con- 

 stitution. In this connection, the sample from St. WS 204 may be quoted as illustrating 

 gradation from diatomaceous mud to diatom ooze, for it is almost of sufficient purity to 

 be included in the latter category; moreover the perishable part of the organic debris 

 has almost disappeared. The diatomaceous muds occur at greatly varying depth; St. 

 WS 768 yields diatomaceous mud at a depth of 108 m., and a similar deposit occurs at 

 St. WS 201 at a depth of 4134 m. In both deposits some of the mineral grains reach a 

 diameter of 0-5 mm., a size which is surprising at the latter depth. It must be inferred 

 that depth in itself is unimportant compared with distance from land and the influence 

 of detritus-bearing currents. Among the inorganic constituents, quartz and green horn- 

 blende are the most abundant and widespread minerals. The presence of a few other 

 minerals, noted incidentally during the general examination of the deposits, may be 

 recorded here, though the significance of their occurrence can only be determined when 

 the mineralogical analysis of all the samples is completed. Well-formed crystals of 

 hypersthene are noted in the samples from Sts. WS 400 and WS 471 ; this mineral also 

 occurs in samples from the west of the Falkland Islands (p. 305). The presence of 

 glaucophane at St. WS 403 doubtless has some connection with its widespread occur- 

 rence at stations in the western part of the Bransfield Strait further south (p. 309). The 

 interesting zeolite-mineral phillipsite recorded from St. WS 470 is of sporadic occur- 

 rence and is probably connected with the chemical and physical conditions which de- 

 termine the decomposition of volcanic rocks on the sea floor. The organic constituents 

 may include Radiolaria and sponge spicules, but diatoms are preponderant. Coscino- 

 disciis, Tholassiosira, Fragilaria, Cocconeis, Rhizosolenia and Corethron are the genera 

 most commonly seen, though from time to time other forms such as Triceratium, 

 Biddulphia and Asteromphalus are noted. Only occasionally are calcareous shells present 



