114 THE FAIRY-LAND OF SCIENCE. 
mud is so thin that it takes a thousand years foi 
it to become 2 or 3 feet thick ; but besides that which 
falls in the valley a great deal is taken to the mouth 
of the river and there forms new land, making what is 
called the " Delta " of the Nile. Alexandria, Rosetta, 
and Damietta, are towns which are all built on land 
made of Nile mud which was carried down ages and 
ages ago, and which has now become firm and hard 
like the rest of the country. You will easily remember 
other deltas mentioned in books, and all these are 
made of the mud carried down from the land to the 
sea. The delta of the Ganges and Brahmapootra in 
India, is actually as large as the whole of England and 
Wales,* and the River Mississippi in America drains 
such a large tract of country that its delta grows, 
Mr. Gelkie tells us, at the rate of 86 yards in a year. 
All this new land laid down in Egypt, in India, in 
America, and in other places, is the work of water. 
Even on the Thames you may see mud-banks, as at 
Gravesend, which are made of earth brought from 
the interior of England. But at the mouth of the 
Thames the sea washes up very strongly every tide, 
and so it carries most of the mud away and prevents 
a delta growing up there. If you will look about when 
you are at the seaside, and notice wherever a stream 
flows down into the sea, you may even see little 
miniature deltas being formed there, though the sea 
generally washes them away again in a few hours, 
unless the place is well sheltered. 
This, then, is what becomes of the earth carried 
down by rivers. Either on plains, or in lakes, or in 
58,311 square miles. 
