May 1, 1898.] 



KNOWLEDGE 



95 



eye. It is, perhaps, hardly necessary to remark that the 

 carbonate oflime in this deposit is not all due to foraminifera, 

 for many other organisms contribute calcareous material, 

 some living on the surface waters, and others on the sea 

 bed. Thus we find that remains of molluscs, echinoderms, 

 annelids, corals, and polyzoa were nearly always met with in 

 dredging up samples of this ooze. Sometimes, in tropical 

 or sub-tropical regions, the shells of pteropods' and 

 heteropoil.i were found in great abundance, and then the 

 globigerina ooze passes into a pteropod ooze (see Chart I). 

 The well-known coccoliths and rhabdoliths are often 

 abundant, and sometimes make up as much as 15 per cent, 

 of the deposit. These curious minute organisms, which a 

 few years ago were a puzzle to naturalists, are now regarded 

 as calcareous pelagic algae. Remains of the radiolaria, 

 diatoms, and siliceous sponges are almost always present. 



Life is much more abundant on the globigerina ooze 

 areas than on those of the red clay or radiolarian coze. 

 Where the former ooze occurs, numerous animals (fishes 

 and invertebrates) were dredged up. Of the 118 samples 

 reported on by the Challenger naturalists, pelagic foramini- 

 fera made up on the average 53 per cent., and other 

 organisms 9 per cent. ; the bottom-living foraminifera were 

 only over 2 per cent. 



Pteropod Oose. — Although the remains of pteropods are 

 abundant everywhere in the surface waters of tropical and 

 sub-tropical regions, yet there dead shells are wholly 

 absent from the deposits in all the deeper waters. The 

 reason of this is that the delicate shells expose a large 

 surface to the sea water as they fall through it, and are 

 therefore dissolved before they reach the bottom where it 

 is deep. Pteropod ooze abounds on submarine ridges that 

 rise to within 1000 fathoms of the surface. A few traces 

 were sometimes met with at 2000 fathoms, but only in 

 lesser depths do they make up any appreciable part of the 

 globigerina ooze. In all deposits near continents and 

 islands, where tropical oceanic waters occupy the surface, 

 heteropod and pteropod shells are more or less abundant. 

 Thirteen samples are described in the ('luiUemjer report, 

 with an average depth of 1014 fathoms. Foraminifera 

 made up about 50 per cent. It differs from globigerina 

 ooze in the larger amount of carbonate of lime, due to the 

 delicate pteropod shells. It is most typically developed on 

 the central ridges of the Atlantic, where the depth is not 

 more than 1000 fathoms or so (see Chart I). It is to be 

 met with off the West Indies, the Azores, and some of the 

 Fiji Islands. Many inter-tropical islands are apparently 

 surrounded by this deposit. 



We now pass on to terriyenous deposits. Omitting the 

 coarser deposits of sand and shingle beds nearer shore, we 

 have now to deal with fine detrital matter beyond the 100 

 fathom line. All these deposits are laid down on what 

 may be called the continental slope, or that area extending 

 irom the 100 fathom line down to the ocean basins, where 

 they merge into pelagic deposits. 



Blue Mud. — This deposit is particularly interesting to the 

 geologist, because it throws light upon the history of the 

 numerous formations of slate, shale, and clay fomid iu the 

 series of stratified rocks. It is met with in the deeper 

 waters round continental land, and in all enclosed seas, 

 partly cut off from the open ocean. It is, of course, a product 

 of the land denudation. Its blue colour is due to chemical 

 changes produced by decomposing vegetable and animal 

 substances, in presence of the sulphates of seawater, which 



* The class Pteropoda are small free-swiraniing niollusca, found on 

 the surface of the ocean, often in enormous numljers. They are lower 

 down in the scale of life than the Gastropoda, and represent the 

 transient larval stage of sea-snails. The Heteropoda are also pelagic 

 and free-swimming, but may or may not have a shell. 



are reduced to sulphides and decompose the ferric oxide 

 abounding in all deposits into sulphide of iron and ferrous 

 oxide. Blue muds smell of sulphuretted hydrogen. Fifty- 

 eight samples are described, with an average depth of 1411 

 fathoms. In some cases pelagic foraminifera make up 2.5 per 

 cent, of the deposit, while in others there is no trace of them. 

 Roimded grains of quartz are abundant. The mineral 

 particles are mostly derived from adjacent land. Blue 

 muds surround nearly all the coasts and fill most enclosed 

 seas like the Mediterranean. 



Bed Mud. — Along the Brazilian coasts of South America 

 the terrigenous deposits differ from other such in being 

 reddish instead of blue or green. The cause of this is to 

 be found in the large amoimt of ochreous matter biought 

 down by the Amazons, Orinoco, and other South American 

 rivers, and distributed by ocean currents. Probably there 

 is not enough organic matter to reduce the whole of the 

 iron peroxide to protoxide, nor does the sulphide of iron 

 accumulate here. Similar red muds are met with oft' the 

 Chinese coast, near the mouth of the Yang-tse-Kiang. 



Gree7i Muds and Sands. — These deposits are chiefly 

 characterized by the presence of glauconitic grains and casts 

 of calcareous organisms. They are generally met with 

 along bold exposed coasts where no very large rivers pour 

 their detrital matters into the fea. They do not occur in 

 very deep water (usual depth, 100— 900 fathoms). In these 

 regions pelagic deposits are found to approach much nearer 

 to the shores than where the blue muds prevail. So much 

 is this the case that were it not for the grains of glauconite 

 (a greenish mineral) they might be taken for globigerina 

 ooze. (Jreen Sands were found in rather shallower waters 

 (average depth, 449 fathoms). Forammifera make up 

 3C per cent, of the deposit. This glauconite appears to be 

 precipitated inside the foraminiferal shells, thus forming 

 casts of their interiors which remain after the shells have 

 been dissolved. The chemistry of the ocean is at present 

 little understood, but it is suggested that the glauconite may 

 be the result of chemical changes undergone (in the presence 

 of organic matter) by finely divided particles derived from 

 land. 



Volcanic Muds and Satids. — Oceanic islands of volcanic 

 origin are surrounded by volcanic muds or sands formed 

 by the wearing down of volcanic rock and its subsequent 

 partial decomposition by the chemical action of sea water, 

 the fragments of shells which are present being often 

 coated with peroxide of manganese. Near shore, within 

 the region of wave action, the deposits are mostly sands, 

 mixed with fragments of calcareous organisms. They 

 vary in colour, being grey, brown or black, and pass in 

 some places into muds, or coral sands, or, with greater 

 depth, into oozes ; thirty-eight samples of volcanic muds 

 are described, with an average depth of 1033 fathoms. 

 Volcanic sands chiefly differ in the absence of the fine 

 clayey and calcareous matter so abundant in the muds. 



Coral MikIs and Sands are formed from the debris of 

 structures such as are shown in our plate, copied by the 

 kind permission of Mr. Sa%'ille-Kent from one of the illus- 

 trations in his work on " The Great Barrier Reef of Aus- 

 tralia." Several forms of coral are shown iu this plate, with 

 masses of madrepore and the shrub-like growth known as 

 stag's-horn coral. Around such coral reefs the deposits are 

 chiefly made up of fragments of organisms living in shallow 

 water — such as algfe, corals, molluscs, polyzoa, annelids, 

 echinoderms, and foraminifera. As might be expected in 

 shallower waters, these form a coarse sand or gravel ; but 

 beyond the limits of wave-action there is a fine mud con- 

 sisting principally of ground-up calcareous matter. As we 

 get further away from the reefs and in deeper water, the 

 deposits gradually pass into some kind of ooze. Sixteen 



