246 M. Siau on the Action of Waves at great Depths, 



in that case tlie tallow will be of a concave shape. Finally, at great 

 depths^ two zones of substances may be brought up at once, of different 

 specific gravity, and in that case it will be observed that the heaviest cover 

 a protuberance, and the lighter a depression in the tallow. 



Such are the considerations which have led us to believe that, in these 

 latitudes, the agitation of the sea is felt at a greater depth than it has 

 been proved to extend to by other observers. 



We regret that we had neither the means nor the time to carry our re- 

 searches to the length we desired, the nature of the bottoms on which we 

 operated often affording facilities for observations of this nature by the 

 intermixture of the substances, of very different specific gravity and co- 

 lours, of which they were formed. 



The deepest sounding rigorously determined, is that of 188 metres 

 (578 feet) obtained on the north-west of St Paul's Roads, on a bottom of 

 sand and basaltic gravel, and there the existence of zones has been recog- 

 nised in the most obvious manner. 



We have made soundings to a much greater depth ; and, although they 

 seemed to us to hold out the highest probability of affording analogous 

 results, we do not, in the mean time, refer to them more particularly be- 

 cause they have not been repeated. 



M. E. De Beaumont made the following remarks on M. Siau's obser- 

 vations — 



The result to which M. Siau has come, in attempting to trace to their 

 last limit in regard to depth, the indication of the agitation of waves be- 

 comes of greater interest when we compare it with the ascertained facts, 

 relatively to the greatest depths in the sea at which we find animals 

 fixed, and consequently obliged to wait for their nourishment approach- 

 ing them. It would appear that the two limits come very near each 

 other, and do not, in general, much exceed the depth of 200 metres. 



M. de la Beche has printed at the end of his Researches in Theoretical 

 Geology J a table drawn up by M. Broderip, in which the upper and lower 

 limits between which a great number of shells are found in the sea are 

 noted. It would appear from this table, that the shells which can endure 

 the greatest depths are the terebratulse, which have been found adhering 

 to rocks at a depth of 90 fathoms. 



From the researches of MM. Quoy and Gaymard, Ehrenberg, Darwin, 

 and many other modern voyagers, it would seem that the greater part of 

 the fixed polypiers live only at small depths. The coral on the coast of 

 Algiers is perhaps the species of adherent polypiers whose existence at 

 the most considerable depths has been ascertained. M. Milne Edwards 

 has sometimes dredged coral near Bonne, from the depth of 531.4 feet, 

 (1G2 metres) ; but the coral-divers do not think that it exists below 800 feet, 

 or 244 metres. 



A madrepore is mentioned, obtained by Ellis in the Greenland seas, 

 from a depth of about 1377 feet, or 420 metres, but it was an unattached 

 madrepore with a corneous support.* 



* From the Comptes Rendus dcs Sceaaccs dc TAcademie dcs Sciences, t. xii. p. 774, 



