49 2 THE JOURNAL OF GEOLOGY. 



the current will now set from shore into the estuary ; the reverse 

 of what happened with the rise of the La Plata. 



" By degrees the level of the estuary will again adjust itself to 

 mean sea-level. All the water which the tidal wave brought 

 from the sea will now have to be returned, and in addition the 

 whole volume which the great rivers have discharged into the 

 estuary ; and the currents will not only be stronger, but they will 

 also last longer, of which circumstance the outline of the tidal 

 wave bears evidence, the duration of the rise of the La Plata 

 being about six hours, its fall continuing for about seven hours." 



Revy further calls attention (page 23), to the fact that the 

 current with a given fall of the river is swifter in deeper, slower 

 in shallower water therefore deposit during flood-tide is 

 more copious over shallows, and is there less liable to scouring 

 during the ebb. It follows that the shallows become tide-flats, 

 tide-flats are raised to rush-grown islands, and the islands unite 

 to extend the river's banks. Thus the Parana has filled two-thirds 

 of the La Plata, which was 325 miles long, and the river will 

 ultimately replace the estuary, so that the future delta will be 

 built into the Atlantic, as that of the Mississippi extends into the 

 Gulf. 



If the sediment thus deposited consists of mingled sand and 

 clay it will be sorted to some extent by the alternate checking 

 and starting of currents. As with rising tide the current slows, 

 sand will first be dropped ; during the period of quiet water both 

 sand and clay will sink together, though at unequal rates ; and 

 when the ebb restores the outward current, the surface of the 

 latest deposit may be scoured, removing clay and leaving sand. 

 Furthermore the swifter currents of the channel may carry clay, 

 even though dropping sand, while the slower currents of the 

 shallows drop both. Hence there must be a tendency toward 

 alternation of more sandy layers with more clayey ones, and of 

 horizontal passage of sands into clays. 



Where rivers enter bays of such depth or expanse that the 

 fresh water does not displace the salt water, other conditions 

 than those governing estuarine deposition prevail. It is there 



