Tides Illmtrative of Geological Phenomena. 225 



maximum of its development in the Grand Bank of New- 

 foundland. 



This process is of the highest importance in the economy 

 of nature, if we consider that the banks thus formed by the 

 tidal currents are the principal seats of animal life in the 

 ocean. It is upon the banks which border the coast of the 

 United States that the most extensive fisheries are carried 

 on (particularly the St George's and Newfoundland Banks), 

 because these are the abodes of those myriads of invertebral 

 animals (worms, molluscs, and zoophytes), which serve for 

 the food of fishes, whilst the great depths of the ocean, at a 

 short distance from the banks, are almost deserts. 



The tides are not less important, from the manner in which 

 they influence river-deposits. Hitherto the formation of 

 deltas, such as those of the Mississippi, the Nile, the Orinoco, 

 and other rivers, has been attributed too exclusively to the 

 great quantities of mud which these rivers transport. It 

 seems to be forgotten that other rivers, such as the Amazon, 

 the Rio de la Plata, the Delaware, and others, are not less 

 muddy, and yet, instead of forming deltas at their mouths, 

 they empty into wide bays. 



Captain Davis, on the contrary, shews that deltas are in an 

 inverse ratio to the tides, so that they exist only where the 

 tides are feeble or null ; whilst we find estuaries wherever 

 the tides are considerable. Take, for example, the rivers 

 of the eastern coast of the United States, and most of the 

 rivers of Europe which empty into the Atlantic Ocean. And 

 this is perfectly natural. The tide, on entering a river, ac- 

 cumulates during the flood, and keeps back the water of the 

 stream, so that when the ebb begins, the water, in escaping, 

 forms a current strong enough to carry ofi^ to sea the princi- 

 pal part of the materials held suspended in the river-water. 

 Mr Davis remarks on this point, that, where bars exist in 

 such estuaries, they are generally composed of sea-sand 

 brought by the tide, and not of fluviatile deposits. 



In connection with Captain Davis, we have endeavoured to 

 apply the above results to the study of the deposits of former 

 geological epochs ; and we think it is easy to shew, on a geo- 

 logical chart of the United States, that the same laws which 



