396 NATURAL SCIENCE. Dec, 



while along the western coasts of Scotland and Ireland, which are 

 exposed to the waves of the wide and deep Atlantic, the true mud-line 

 may be situated at a depth of about 150 or 200 fathoms." {Trans. Roy. 

 Soc. Edin., vol. xxxvii., p. 482.) 



" In the shallower reaches of the ocean the materials on the 

 bottom are assorted and distributed by currents in a way that 

 produces a great variety of conditions. In some places there are 

 siliceous or calcareous sands, in other places dead shells and pebbles ; 

 on submarine banks, rocks and boulders prevail ; in depressions, fine 

 muds and clays. On each of these bottoms there is usually a very 

 different assemblage of animals. So that, although the trawl may not 

 in shallow water bring up such a variety of organisms in any single 

 locality as from deeper water, still the total number of genera and 

 species inhabiting the whole area of depths less than 50 fathoms is 

 recognised as much greater than in deeper water. With increasing 

 depth, not only the nature of the deposits, but the other physical 

 conditions, become more and more uniform, till a depth is reached 

 along the continental shores facing the great oceans immediately 

 below which the conditions become nearly uniform in all parts of the 

 world, and where the fauna likewise presents a great uniformity. This 

 depth is usually not far above nor far below the loo-fathom line, and 

 is marked out by what I have elsewhere designated as the Mud-line. 



" In all modern seas the depth at which minute particles of 

 organic and detrital matters in the form of mud begin to settle on the 

 bottom of the ocean is important both from the physical and biological 

 points of view. This depth is determined by the distribution of 

 land and water. It is dependent on the depth and extent of the 

 ocean or basin, and varies temporarily with seasons of strong winds 

 and calms. In small enclosed arms of the sea, like those of the west 

 of Scotland, the mud-line is situated at depths of from 5 to 20 

 fathoms; but where currents rush through narrow passages or over 

 submarine barriers, it is much deeper. In the North Sea, the oceanic 

 mud-line to the north occurs at a depth of about 80 fathoms, but on 

 the coasts of Scotland facing the great stormy Atlantic it is often 

 found at a depth of over 100 fathoms. In the Faroe Channel the 

 currents rush over the Wyville-Thomson Ridge with sufticient force 

 to prevent mud forming on the summit, and the mud-line is met with 

 on either side of this ridge at about 300 fathoms. In basin-like 

 depressions within the loo-fathom line — like the Silver Pits of the 

 North Sea — mud is formed, while it cannot rest on the rims of these 

 basins. All the minute organic particles washed down from the land, 

 carried away from the shallow waters by currents, or derived from 

 the decay and death of pelagic organisms, ultimately find a resting 

 place on the bottom in deep water, principally just about and beyond 

 the mud-line." (" Summary of Results," pp. 1433- 1434.) 



These quotations show that Murray fully recognises the fact that 

 fine m.ud may be found at a great range of depth, and that he considers 



