244 ANNUAL OF SCIENTIFIC DISCOVERY. 



and eddies alternating with each other, the final result of which is to 

 transport, in the direction of the flood, the movable materials which the 

 waves and atmospheric agents have detached from the coast-beaches. 

 This is particularly striking on the coast of the United States. The al- 

 luvial deposits form, at first, only a narrow line on the coast of Florida ; 

 this line enlarges insensibly on the coasts of the Carolinas, Virginia, and 

 New Jersey ; it becomes wider on the coast of Massachusetts, and final- 

 ly attains the maximum of development in the Grand Banks of New- 

 foundland. 



This process is of the highest importance in the economy of nature, 

 if we consider that the banks thus formed 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 important fisheries are carried on, 

 because these are the abodes of myriads of invertebrate animals (worms, 

 mollusks, and zoophytes), which serve for the food of fishes, while 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 in- 

 fluence river-deposits. Hitherto, the formation of deltas, such as those 

 of the Mississippi, the Nile, the Orinoco, and other rivers, has been at- 

 tributed 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. 



Mr. Davis, on the contrary, shows 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 consider- 

 able. 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, accumulates 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 off to sea the principal 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 gen- 

 erally composed of sea-sand brought by the tide, and not of fluviatile 

 deposits. 



INFLUENCE OF TIDES ON ANCIENT GEOLOGICAL FORMATIONS. 



IN applying the principles of the tide-theory of Mr. Davis to the study 

 of the deposits of former geological epochs, Mr. Desor states, "that it is 

 easy to show, by a geological map of the United States, that the same 

 laws which now regulate the deposition of sand-banks have been in op- 

 eration during the diluvial, tertiary, and cretaceous epochs ; the deposits 

 of those epochs forming so many parallel zones successively following 

 the great backbone of the Alleghanies. 



"The diluvial deposits, in Europe as well as in America, merit a 

 special attention in this respect. No doubt, during the diluvial 



