238 CORAL AND ATOLLS 



In other words, where, below the surface of the sea, will 

 dropping particles fail to come to rest, and cease to form banks ? 

 We do not know, and beyond saying that somewhere in the 

 ocean there is a level, below which particles may rest and 

 sedimentation take place, and above which particles are pre- 

 vented from settling by the movements of the surface waters, 

 — we cannot definitely go. This level is one of such great 

 importance that I propose to look upon it as a definite plane, 

 occurring over every ocean, and to give to it the name of the 

 Limiting Line of Sedimentation. 



We may attempt to find out where this line may be ex- 

 pected to exist by ascertaining how far beneath the surface of 

 the ocean the effects of wave action are felt ; but on this 

 point our knowledge seems to be rather scanty. Dana, in 

 his "Corals and Coral Islands," 1875, has collected a certain 

 amount of information concerning this question, and he quotes 

 M. Siau (Comptes Bendus, t. xii. p. 744) for the assertion that 

 ripple marks are present at a depth of 94 fathoms to the 

 north-west of St. Paul's Koads. Various other estimates 

 have been made, and 83 fathoms, 50 fathoms, and 15 fathoms 

 are some of the figures arrived at. De la Beche is respon- 

 sible for the statement that sand and mud are stirred up, so 

 that they discolour the water, at the depth of 1 5 fathoms ; — 

 it is evident therefore that this depth is above the Limiting 

 Line of Sedimentation. 



It would not be expected by any one with a knowledge of 

 the sea that the level of the Limiting Line of Sedimentation 

 would be the same for all waters ; for the normal rollers of 

 the Trade-swept ocean must affect a far greater depth than 

 the short seas of land-locked basins. It would not be ex- 

 pected even that it would be at the same level at all times 

 in the same sea, for in monsoon areas it would be probably 

 different in two monsoon seasons. Still, for all parts of all 

 oceans there is an average line separating the wave-stirred 

 upper layers from the still, quiet waters of the ocean depths. 



Sedimentation Banks. — It is, therefore, an easy matter 

 to picture the development of a sedimentation bank in the 



