26 NATURAL SCIENCE. ^arch. 



of the iron oxide has taken place in consequence of the presence of 

 organic matter, and sulphide of iron is present in a finely divided 

 state. When fresh, these deposits usually have the smell of sul- 

 phuretted hydrogen. The proportion of carbonate of lime may vary 

 from a mere trace to 35 per cent. The mineral particles range in 

 size from -06 to '3 mm. in diameter, the average being '115 mm. 

 Quartz particles which are rare or absent in the pelagic deposits 

 are the most abundant of the definitely recognisable constituents ; 

 orthoclase, plagioclase, green hornblende, augite, white and black 

 micas, epidote, chlorite, zircon, and tourmaline have also been 

 determined. Glauconite is not characteristic, but may be found in 

 almost all the specimens collected during the cruise. The blue muds 

 differ from the red clays in containing a much larger proportion of 

 minerals (22-48 per cent, on the average), and a smaller proportion 

 of fine washings (61 '77 per cent.). They surround nearly all coasts, 

 and are found also in inland seas, such as the Mediterranean. 

 Off the coast of Brazil, a red mud is found in place of the more 

 widely-distributed blue mud. This is said to owe its colour to the 

 ochreous matter carried out to sea by the great South American 

 rivers. It contains, however, less iron than the blue mud, so that 

 the peculiar colour must be due to the absence of the reducing con- 

 ditions which prevail over the areas covered by blue mud. Similar 

 deposits occur off the Chinese coast near the mouth of the Yang-tse- 

 kiang. 



The green muds and sands owe their distinctive features to the 

 presence of glauconite. They grade off on the one side into blue 

 muds and on the other into globigerina ooze. Volcanic muds and 

 sands are found in the immediate neighbourhood of volcanic islands. 

 The volcanic material is mixed with a variable amount of calcareous 

 and siliceous matter, chiefly due to the remains of the usual pelagic 

 organisms. Coral muds and sands surround coral islands in the 

 same way. 



More than one hundred and fifty analyses of the deposits and of 

 special substances obtained from them are given in an appendix to 

 the Report. 



It will be seen from the brief account we have given of this most 

 valuable Report that the detailed study of specimens in the labora- 

 tory has confirmed and extended the conclusions arrived at during 

 the cruise. The deep-sea deposits are essentially made up of the 

 remains of pelagic organisms and volcanic products. Terrigenous 

 material is almost entirely absent. The proportion of carbonate of 

 lime decreases as the depth increases below 2,000 fathoms ; this is 

 owing to the solvent action of sea-water on the calcareous organic 

 remains. The deposits of the greatest depths are, therefore, almost 

 entirely derived from siliceous organisms and volcanic material. 



The broad features of the distribution of the more important 

 deposits may now be briefly summarised. Proceeding outwards 



