ESTUARIES. 31 



at 13,600 square miles, and the annual mud-discharge at 7,400,000,000 

 cubic feet.. Upon these data he makes the probable age of the delta 

 33,500 years. To this he adds half as much for the age of the river- 

 swamp, making in all 50,000 years. 



It is evident, however, that this estimate can not be relied on as 

 even approximately accurate. For there is no reason why the time of 

 river-swamp deposit should be added to that of the delta, for they were 

 both probably formed at the same time — one by deposits higher up the 

 river, the other by deposits at the mouth. Again, on the other hand, 

 the estimate takes no account of the submarine extension of the delta, 

 in area certainly, and in cubic contents probably, much greater than 

 the subaerial delta. Figs. 22 and 23 are an ideal section and a map of 

 a delta in which a is the aerial and b the submarine portion. This 

 would greatly increase the time. 



It is evident, therefore, that although the problem is one of great 

 interest, we are not yet in possession of data to make a reliable estimate. 

 Every estimate, however, indicates a very great lapse of time. 



But it must not be imagined, as all estimaters seem to do, that this 

 time, be it greater or less than Mr. Lyell's estimate, belongs all to the 

 present geological epoch. Prof. Hilgard has shown that the true allu- 

 vial deposit of the Mississippi is only fifty to one hundred feet thick. 

 Beneath this the deposit belongs to the Quaternary or preceding geo- 

 logical epoch. 



7. — Estuaries. 



We have already seen that rivers which empty into tideless seas 

 communicate with the sea by numerous branches traversing an alluvial 

 flat, formed by the deposits of the river ; while rivers emptying into 

 tidal seas communicate by wide mouths or bays, formed by the erosive 

 action of the flowing and ebbing tide. Such bays are called estuaries. 

 We have fine examples of estuaries in the Amazon and La Plata Rivers, 

 in the Delaware and Chesapeake Bays, in the friths of Scotland and the 

 fiords of Norway ; in fact, at the mouths of all the rivers emptying into 

 the Atlantic on our own coast as well as on the EurojDean coast. The 

 mouth of the Columbia River is a good example on the Pacific coast. 

 The phenomena of a delta and an estuary are sometimes combined in 

 the same river. This is the case to some extent in the Ganges. 



Mode of Formation. — Estuaries are evidently formed by the erosive 

 action of the inflowing and outflowing tide. Their shape, narrow above 

 and widening toward the sea, gives great force to the tidal current, 

 which, entering below and concentrated in the ever-narrowing channel, 

 rushes along with prodigious velocity and rises to an immense height. 

 In the Bay of Fundy the tide rises seventy feet, and at Bristol, England, 

 it rises forty feet, in Puget Sound twenty-five feet. Sometimes, from 

 obstructions at the mouth of the river, the tide enters as one or more 



