Page:The Journal of geology (IA journalofgeology11893univers).pdf/66

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 Paraña 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 ther 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