342 On Tidal Action as an Agent of Geological Change. 



with is infinitely greater. It is by tidal influence that the 

 material brought down by continental rivers and that frayed 

 by waves and currents from the coast is gradually worked out 

 to the ocean depths. The North Atlantic is distinguished by 

 a more widespread distribution of sediment over its floor than 

 most of the oceans, doubtless due, as I have elsewhere pointed 

 out, to the more numerous and greater sedimentary contribu- 

 tions of the rivers draining the surrounding land areas. 



Without tidal action this sediment would be left to shoal 

 the inshore waters instead of being gradually worked out to 

 great depths, which is what actually takes place. Unfortu- 

 nately at present our knowledge of the nature of the ocean- 

 bottom is extremely limited, and quite insufficient to justify 

 the wide generalizations of some too-ardent naturalists who 

 would paralyze inquiry by their rigid systems. 



Tidal Movements in the Ocean. — Although it is on the lit- 

 toral margins and the shallow seas opening into the oceans 

 that the resistless force of the tides is the most obvious, the 

 tidal influence is felt in the profoundest depths of the ocean. 

 Sir John Herschel says that " a tide-wave of 4 feet of total 

 height, advancing over a sea 30,000 feet deep, implies an 

 advance and recess of 2800 feet, which being spread over six 

 hours is nowhere very rapid."* 



The mean depth of the ocean is probably not more than 

 12,500 feettj and there is every reason to believe that the slow 

 movements of these enormous bodies of water, by concentration 

 in deep channels favourably circumstanced, get translated into 

 rapid currents that scour the bottom. Formerly it was sup- 

 posed that the waters of these great depths were profoundly 

 and universally still, but the information gained in laying 

 deep-sea cables proves that swift currents sometimes prevail. 

 Sir James Anderson, the eminent telegraph-engineer, says, in 

 a letter to me : — " Perhaps the most marked experience we 

 have had of currents at great depths was in the case of the 

 Falmouth cable near Gibraltar. At 500 fathoms the wire 

 was ground like the edge of a razor, and we had to abandon 

 it and lay a cable well inshore. Captain Nares, of the sur- 

 veying ship 'Nemesis,' I think with tangles could get no 

 specimen of the bottom whatever, and he thinks he got suffi- 

 cient evidence to prove the existence of a perfect swirl at that 

 depth." % 



* 30,000 feet is deeper than the deepest soundings yet recorded. 



t Dr. John Murray makes it 2070 fathoms (Sco'ttish Geographical 

 Magazine; paper read before the Royal Society of Edinburgh, Decem- 

 ber 19, 1887). 



X Quoted by me in " The North Atlantic as a Geological Basin." Pre- 

 sidential Address, Liverpool Geol. Soc. Session 1885-0. 



