THE BUILDING OF THE EARTH 



The sea is not the only carrier which is thus laying 

 down beds of material. The rivers are doing the same 

 thing. Every shower of rain washes some dirt by which 

 we mean sand or gravel or loam or chalk from the land 

 into the nearest rivulet. The rivulet hurries with it 

 down to the neighbouring river, and the river carries it 

 down to the sea. If the river is going very fast it carries 

 most of its dirt along with it, and we generally find the 

 river muddy after rain. But when the river slackens its 

 pace, as it usually does when it nears the sea and meets 

 the sea's tides, then it lets the dirt fall ; and thus at the 

 river's mouth we find mud-banks or sand-banks. If a 

 river is left long enough to its own devices, these sand- 

 banks will so increase in bulk that the mouth of the river 

 will become shallower and shallower and will spread. It 

 silts up, and when a river is needed for the navigation of 

 ships large sums of money have to be spent, as in the 

 Scheldt or at the mouth of the Thames, in dredging this 

 mud so as to keep the channels clear. 



There are many striking examples of this land-building 

 by rivers ; and the deltas of rivers, so called from their 

 resemblance to the Greek letter A, form in some instances 

 great areas. The Mississippi, the Nile, and the Ganges, 

 for example, are surrounded by great tracts of land at 

 their mouths, which are formed entirely from matter 

 brought down by the rivers and deposited at lower levels 

 than those at which the rivers originated. The Missis- 

 sippi, which drains a river basin of 1,147,000 square 

 miles, has an annual discharge of sediment of no less 

 than 7,459,267,200 cubic feet. The Italian River Po, 



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