of extensive Conglomerate and Gravel Deposits. 169 



IV. Discharge of Rivers into Tidal Seas. 



Tidal rivers, when large, most frequently keep their mouths 

 open, though there is always a tendency to form bars and sand- 

 banks; as tor example, the Thames, the Severn, the Seine, the 

 Loire, the Tagus, the St. Lawrence. In such situations, the 

 detritus, if any, is small enough to be held in mechanical sus- 

 pension. 



Some tidal rivers, or rivers which discharge themselves 

 into tidal seas, form deltas when the force of the current is con- 

 siderable, the tides small, or the seas not much subject to storms 

 setting in shore, but merely to land and sea breezes. Of the 

 mouths of such rivers, the Missisippi, the Oronoco, the 

 Ganges, and the Yellow River of China, are examples. The de- 

 tritus brought down by such rivers to their mouths is either 

 sand or mud ; therefore they do not contribute towards the 

 formation of gravels at the bottom of the ocean. 



Tidal rivers, when small, have a tendency to be blocked up 

 by the sea, which often increases the bars into long banks of 

 pebbles or sand, and it is with difficulty that the rivers deliver 

 their waters into the sea: if the line of coast keeps the direc- 

 tion of the prevalent winds, the difficulty is increased, and the 

 river generally gains a cliff or some hard ground, for one of 

 its banks, before it can effect its escape into the sea. Good 

 examples of these embouchures are seen on our southern coast. 

 The Teign has a tendency to be blocked up by the bank upon 

 which part of Teignmouth is built, named the Den, which the 

 easterly winds, producing the greatest breakers on this beach, 

 drive across the mouth of the river from E. to W., and the 

 Teign escapes by the side of the Ness Point,which aflfords it sup- 

 port. The Axe is also deflected from its course by the pebble 

 bank thrown up from W. to E. by the prevalent W. and S.W. 

 winds, which here afford the heaviest breakers, and it escapes 

 into the sea by supporting itself against Axmouth cliff"; the 

 sea, however, is constantly endeavouring to bar up its pass- 

 age (fig. 2.). 



Ti)e harbour at Shoreham is a good example of a river 

 deflected from its straight course by banks thiv)wii np by the 

 sea (fig. 3). The river in this case escapes through a gnp 

 wliicli it has fornud in the bank itself. 



It is obvious that in these cases the sea rejects the detritus 

 it receives from the rivers, and forces it back, with the cliff" de- 

 tritus, upon the land. 



The great Hats on the western coast of South America are 

 excellent examples of mud and sandy detritus forced back 

 u))on the liind. 



.V. N. \()l. 7. No. 39. Mari/i 1830. Z V. Dis. 



