IX.] THE WORK OF RAIN AND RIVERS, 143 



sediment by means of river-floods is, however, best illus- 

 trated by reference to the overflowing of the Nile. After 

 the rainy season in the southern part of the course of the 

 river, there is a sudden rush of water, charged with detritus, 

 which spreads far over the banks of the river in its lower 

 course, and deposits the rich alluvial mud of Egypt. 



When a river approaches the sea, the inclination of its 

 basin usually diminishes, its speed is slackened ; and, con- 

 sequently, it deposits more or less of the matter which it 

 liolds in suspension. If the sea, near the mouth of the river, 

 is not much disturbed by currents, as in a protected bay, the 

 sediment will accumulate, and form a tract of alluvial land 

 whic:h is generally fan-shaped. In Lower Egypt, the Nile 

 has, in this way, produced an enormous alluvial tract, which 

 was called by the Greeks the Delta, in allusion to its shape 

 resembling that of their letter A. About 120 miles above 

 its mouth, the Nile divides into two main streams, of which 

 the western is known as the Rosetta branch, and the eastern 

 as the Damietta branch, these names referring to two towns 

 situated at their respective mouths. The two streams in- 

 close, with the Mediterranean sea on the north, a triangular 

 tract of aUuvial land, intersected by a network of channels. 

 The apex of this triangle, forming what is called the head 

 of the delta, is situated about 25 miles below Cairo. Fig. 

 35 sboA's the form of the Nilotic delta. 



From being originally applied to the triangular land 

 about the mouths of the Nile, the term " delta " has come 

 into general use and is now extended to all similar alluvial 

 deposits. Even the land, which has been described as 

 formed in the Lake of Geneva by deposits from the Rhone, 

 may be called a lacustrine delta. If the silt is thrown down 

 under tranquil conditions, on a tolerably level bed, it will 

 fall in nearly horizontal layers, regularly spread one upon 



