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On the Action of JFaves at great Depths. By M. Siau, 

 Civil Engineer.* 



The observations, of which we are about to give an account, were made 

 on a bottom of madreporic white sand, and basaltic black sand: they took 

 place while studying a plan for establishing a harbour at St Gilles, where 

 there is a natural passage pierced in the coral bank which prevails along 

 the coast. 



When the sea is sufficiently calm to enable us to see the gravelly sand 

 in the bottom of the passage, we notice that it forms in it parallel undula- 

 tions, the transverse section of which increases according to the state of 

 the sea which produces them. We have estimated the distance between 

 two hollows or two consecutive summits of the undulations, when a 

 proper view could be obtained of them, at from 30 to 50 centimetres ; 

 and the depth of the hollow below the summit was found to be about 

 from 10 to 16 centimetres. 



In the hollow of tlie undulation the heaviest substances are accumu- 

 lated, such as coarse sand, gravel, and small pebbles ; on the summit we 

 see only the finest sand. 



When the undulation is.forraed of substances of the same size and of dif- 

 ferent specific gravities, such as basaltic and calcareous sands, it is observed 

 that the heaviest matters are in the hollow, and the lightest at the summit. 



The undulations arc the effect of the action of waves, and admit of an 

 easy explanation. Wlien the water is limpid, so that we can see the bot- 

 tom, the waters exercise little action upon it, but when they were much 

 agitated, all substances were put in motion by them. In proportion as 

 the wave diminishes, its action is lessened, until the moment arrives when 

 it cannot set the heavier substances in motion. Then a selection or kind 

 of parting takes place; the lighter substances, being separated, have conti- 

 nued to advance by undulations, as is always the case, the wave acting on 

 the bottom of the hollows in order to carry them to the summit, and leav- 

 ing the weightiest bodies uncovered. 



Advancing into the passage towards the entrance, it is remarked that 

 the undulations always preserve the same parallelism, and that their section 

 more and more diminishes. The same thing appears in the open sea ; 

 there the undulations are parallel to each other, and very nearly parallel 

 to those' of the passage. We always distinguish these alternate zones of 

 heavier and lighter substances ; they can easily be distinguished when 

 the sea is calm and clear at a depth of at least 20 metres. 



If we advance into the open sea and take soundings, having the base of 

 the lead well covered with tallow, we will perceive, on hauling in the line, 

 that the zones of which we have spoken are impressed upon the tallow. 

 Sometimes a uniform zone of heavy substances will be brought up, and 

 then the adhesive matter at the end of the lead will have assumed a con- 

 vex form ; sometimes a zone of lighter substances will be obtained, and 



* Vide page 63 of this voluiuo of Journal for Trofcssor G, Forchammer's Obser- 

 vations on Lund-Furrows and Water-Furrows, or ripple-marksi— Edit. 



