ACCUMULATIONS AT THE MOUTHS OF RIVERS. 165 



coarser is the sediment which it may be able to transport. While the 

 Po, slackening its velocity, deposits its gravel where it joins the Trebia, 

 west of Piacenza, 130 miles from the sea; and the Ganges 180 miles 

 above the commencement of its delta, and 400 miles above the present 

 line of coast ; the rough bed of the Yorkshire Tees is pebbly quite 

 down to the sea ; and the streams which descend by a short and 

 furious course from the Maritime Alps, bear down pebbles into the 

 Mediterranean. 



From these instructive examples of pebbly, sandy, argillaceous, 

 and calcareous strata, forming at the same era, in different basins of 

 the sea, and even in different parts of the same basin, under similar 

 conditions, enveloping deposits entirely marine, entirely freshwater, 

 or partly marine and partly freshwater, we may turn with advantage 

 to the contemplation of the older strata of conglomerate, sandstone, 

 clay, marl, and limestone ; and by carefully noting points of agree- 

 ment and circumstances of difference, may frame satisfactory notions 

 of the conditions under which they were respectively deposited. 

 Especially we may be guided in our decision concerning the extent 

 and connection or separation of the several basins of the ancient 

 oceans, and the relative influence of ancient and modern rivers. 



Bars at the Mouths of Elvers. Rivers which discharge them- 

 selves into the sea, where tides and currents contend with the freshes, 

 may, like the Rhine, be enabled for a certain time to deposit their sedi- 

 ment in a delta, and to increase this even to a vast degree, in conse- 

 quence of entering the sea at a deep emargination of the coast, or amidst 

 shallow sands which impede the action of the tide. But in such a 

 case the accretion of land will gradually diminish, and at length the 

 movements of the sea must balance the current of the river. In this 

 case a line of sand-banks will be formed varying in position according 

 to the alternate predominance of the contending forces, and the 

 river entrance will have a bar. The Rhine, the Thames, and all the 

 eastern rivers of England, are nearly in the same case. The sea, 

 indeed, has again reclaimed from the Rhine, by most destructive 

 floods, the large space of the Zuider Zee and the Bies-Bosch. 



Thus also the seaward growth of the Nilotic delta, once so rapid, 

 is greatly retarded or almost annihilated by a current of the Medi- 

 terranean ; and the rivers of Western Africa, as well as the mighty 

 Amazon, no longer extend themselves into the sea, but meet its cur- 

 rents in furious strife, drop the sand at their mouths, and resign 

 their finer sediment to its disposal. The distance to which 

 the ocean can waft this sediment on its surface along with 

 the lighter fresh water is very great. General Sabine supposed him- 

 self to have crossed the discoloured waters of the Amazon 300 miles 

 from its mouth, where it still retained its comparative lightness, and 

 kept its place on the surface of the sea. 



Thus may the sediments of distant countries be mixed or alter- 

 nate, in deposits far from shore, and even in the deep sea. The dis- 

 tinctness of currents of water which flow down the same river 

 channel, even with a rapid descent, has often been noticed. Thus 



