426 



DEPTHS OF THE OCEAN 



every reason to believe that the physical conditions in these 

 depths have been essentially the same at least for thousands of 

 years. 



We possess, of course, no information as to the time required 

 for the distribution of a species into oceanic depths. In shallow 

 waters we know quite well that new physical conditions may 

 permit a species to migrate into new areas and to multiply 

 enormously in a short space of time (as an instance may be 

 mentioned the immigration of cod into the Liimfjord after 

 the North Sea broke through at Thyboroen). At all events it 

 seems reasonable, first of all, to look for factors in operation at 

 the present day, the influence of which may be investigated, 

 before we fall back on the hypothetical conditions prevailing in 

 a previous geological period. 



In his " Challenger " Summary, Sir John Murray has 

 attempted an explanation of the quantitative distribution of 

 organisms in different depths, which not only throws much 

 light on these important geographical questions, but also possesses 

 the great advantage of containing in itself a whole programme 

 of future research. He found that many deep-sea animals — the 

 hydroids, for example — had developed special apparatus for 

 catching the minute shells and particles of food that fall from the 

 surface waters, and the holothurians and other echinoderms — 

 the most abundant of deep-sea animals — had their intestines 

 always crammed with the surface layers of the deposit on which 

 they were captured, either Blue mud, Diatom ooze, Globigerina 

 ooze, Pteropod ooze, or Red clay. 



We have seen in Chapter IV. that marine deposits may be 

 separated into two main groups : terrigenous deposits and 

 pelagic deposits, the former occurring in deep and shallow 

 water around all continents and islands within an average 

 distance of one hundred or two hundred miles from the coast, 

 and the latter occurring in the deeper water towards the central 

 parts of the great ocean basins. 



It is a well-known fact that the detrital matter which is 

 carried into the sea by rivers is rapidly deposited on meeting 

 salt water, but in shallow water, where currents and wave-action 

 produce their maximum effect, these fine detrital matters are not 

 allowed to settle on the bottom, but are moved along till they 

 reach the lower limit of wave-action. In enclosed seas this may 

 be at a depth of only a few fathoms, but along coasts facing the 

 great oceans the waves are so long and so high that to a depth 

 of several hundred fathoms minute particles of sand may be dis- 



