clviii THE VOYAGE OF H.M.S. CHALLENGER. 



those which we may with probability expect from the tropical eastern Atlantic, wUl be sure to 

 increase very widely our knowledge of the class. 



238. Glohigerina Ooze. — Next to the Eadiolarian ooze proper the Globigerina ooze is 

 the deposit which is richest in the remains of Eadiolaria. Often these are so abundant 

 that it is doubtful to which category the specimen should be referred {e.g., Stations 270 

 and 271, see § 237). In fact, the two pass without any sharp boundary into each 

 other, and both present transitions to the Diatom ooze. Next to red clay (§ 239), 

 Globigerina ooze is the most widely distributed of all sediments, and forms a large part 

 of the bed of the ocean at depths of 250 to 2900 fathoms (especially between 1000 and 

 2000 fathoms). It covers extensive areas at depths below 1800 fathoms, and in still 

 deeper water is replaced by red clay. It is a fine-grained white, grey, or yellowish 

 powder, which sometimes becomes coloured rose, red, or brown owing to the admixture 

 of oxides of iron and manganese. True Globigerina ooze consists for the most part of 

 the accumulated calcareous shells of pelagic Foraminifera, pi'incipally Glohigerina and 

 Orbulina, but also Hastigerina, Pulvinulina, &c. It contains usually from 50 to 80 per 

 cent, of calcium carbonate, the extreme values being 40 and 95 per cent. After this has 

 been removed b}^ acids, there remains a residue, which consists partly of the siliceous 

 shells of Eadiolaria and Diatoms, and partly of mineral particles identical with the volcanic 

 elements of the red clay. 



Eegarding the composition and significance of the Globigerina ooze, see John Murray (L. N. 27, 

 pp. 523-525, and L. N. 53, vol. i. p. 919). Eecently this author has separated from the Globigerina 

 ooze {sensu stricto), the PtcroiMd ooze, distinguished from the former by the greater abundance of 

 Pteropod shells and calcareous shells of larger pelagic organisms which it contains. It is found in 

 moderate depths (at most 1500 fathoms), and contains fewer Eadiolaria. 



239. Red Clay. — This is quantitatively the most important of all deep-sea deposits, 

 covering by far the greatest extent of the three great ocean basins at depths greater than 

 2200 fathoms. It thus far surpasses in area the other deposits, both Eadiolaria and Globi- 

 gerina oozes, and commonly forms a still deeper layer beneath them. Probably these three 

 deep-sea deposits together cover about three-eighths of the whole surface of the earth, 

 that is, about as much as all the continents together, whilst only two-eighths are covered 

 by the terrigenous deposits. Eed clay is principally composed of silicate of alumina, 

 mixed in various proportions with other finely granular substances ; its usual red colour, 

 which sometimes passes over into grey or brown, is more especially due to admixture of 

 oxides of iron and manganese. Calcareous matter is usually entirely wanting, or present 

 only in traces, whilst free silica is found in very variable, often considerable quantities. 

 The chief mass of the red clay consists of volcanic ashes, pumice, fragments of lava, &c., 

 whilst a large part of it is generally composed of shells of Eadiolaria or fragments of 



