38 DYNAMICAL GEOLOGY. 



into a breaker, which advances at a rate sometimes twenty 

 miles an hour. It is evident that the erosive or land- 

 destroying power of such currents is enormous. What- 

 ever is thus gathered in its upward course, together with 

 whatever is brought down as sediment by the river, is 

 all carried by the ebb-tide out to sea, and therefore lost 

 to the land. In fact, of all places along the water-front, 

 the mouths of rivers are the most vulnerable to the attacks 

 of the sea.* 



Deposits at the Mouths of Rivers. The retreating 

 tide carries away to the sea both what is gathered by the 

 advancing tide and what is brought down by the river. 

 Therefore the estuary is scoured out, rather than receives 

 deposit. Yet, in certain sheltered coves such as repre- 

 sented in Fig. 17, at a and b stratified deposits will often 

 be found. These are peculiar. They consist of an alterna- 

 tion of fresh-water, brackish-water, and salt-water deposits, 

 known each by the shells which they contain. The reason 

 is this : In dry seasons these coves are occupied by salt 

 water only, and in flood seasons by fresh water only. 



Also along the water-front of deltas some parts will 

 be receiving sediments from the river, while other parts, 

 receiving no such sediments, will be inhabited by marine 

 animals. With the shifting of the mouth of the river, 

 these latter may be again covered with river sediments. 

 Therefore, in all deposits made at the mouths of rivers 

 there will be an alternation of fresh- and salt-water de- 

 posits. And, conversely, stratified deposits, consisting of 

 such alternations, wherever found, are judged by geologists 

 to have been formed at the mouths of ancient rivers. 



_i 6. Bars. 



\ The formation of bars is an admirable illustration of 

 the laws of sediment-laden currents. Bars are formed at 



* Estuaries are also often formed by subsidence of the land. They 

 are then the drowned lower courses of rivers. 



