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THE VOYAGE OF H.M.S. CHALLENGER. 
the autl other islands of the Pacific. Indeed many inter-tropical islands are apparently 
surrounded, between depths of 400 and 1400 fathoms, by deposits which might in most 
c-ases be called Pteropod Oozes. In northern temperate and polar regions this deposit 
could not occur, as the shells do not live in the surface waters of these regions in sufficient 
abundance. 
II. Terrigenous Deposits. 
At the outset of this chapter it was pointed out that all marine deposits might be 
divided into two great groups, viz.. Pelagic and Terrigenous (see pp. 185 and 186). It was 
likewise stated that the terrigenous deposits were for the most part made up of materials 
immediately derived from the great land masses, which had been subject, in a greater or 
less degree, to the mechanical effects of erosion. A very large part of the terrigenous 
deposits does not, however, fall to be considered in detail in this work, which is limited to 
a description of deep-sea deposits, or, according to our definition of the term, to those 
deposits forming in the ocean beyond a depth of 100 fathoms. The terrigenous deposits 
of the littoral and shallow-water zones surrounding the land are primarily of the same 
nature as those forming in the deep-sea zone. In consequence, however, of the different 
physical conditions prevailing in these three zones, the deposits are more diverse, 
heterogeneous, local, and coarser in the shallower zones than in the deeper one, for the 
deposits become more and more uniform, homogeneous, fine grained, and widely distri- 
buted as the deep water of the ocean basins is approached. 
It is well known that fresh water carries a much larger amount of sediment in 
suspension than salt water, and that wherever a mixture of these waters takes place along 
the borders of the continents almost the whole of the sediment falls rapidly to the bottom, 
thus contributing a great mass of material to the terrigenous deposits in process of forma- 
tion.* ^lurray and Irvine^ have shown that a considerable quantity of clayey matter can 
be held in suspension in sea-water, the amount being greater in waters of a low, than in 
waters of a high, temperature, and they point out that Radiolarians and Diatoms probably 
obtain their silica from this source. This does not, however, in any way lessen the import- 
ance of the fact that the great bulk of detrital matters borne from the land to the ocean 
is deposited in somewhat close proximity to the coasts. The combined effect of rivers, 
winds, waves, currents, and tides on the materials of the land and shallow-water areas, is 
to transport all the fine particles out to depths in which they may fall to the bottom in 
comparatively still water, and where they may accumulate in the form of various kinds 
of muds. AVc have seen that while the depth at which these muds form in enclosed seas 
* Th. SchetTCT, Pwjij. Ann, Bd. Ixxxii. p. 419, 1851 ; Fr. Schulze, IhuL, Ikl. cxxix. p. 368, 1866 ; Sidell in Abbot 
and IlaniphivyB’ Report on the MiHsiKaippi, Aj)p. A. No. 2, 1876 ; llilgnrd, Amer. Joum. Sci., ser. iii. vol. xvii. p. 205, 
l'*7y ; Brewer, “ On the Sulwidence of Particlea in Lifjuiile,” Mem. Nat. Actul. Sci., Washington, vol. ii. p. 165, 1883. 
* “Silica and Siliceous Organisms in Modern Seas,” Proc. Roy. Soc. Edin., 1891 ; see also Chapter VI. 
