LOADS CARRIED BY RIVERS. 55 



force of the stream itself or the action of frost, &c., they are, 

 of course, all angles, and as these prevent their rolling along 

 easily, they are carried but a little way at first, and then left 

 to be ground down into more manageable shapes and sizes. 

 Smaller, rounder pebbles are carried farther, and in the case 

 of short rapid rivers are not dropped until they reach the 

 sea ; and rounded pieces of porous pumice-stone are found 

 floating down the Amazons,one and two thousand miles away 

 from the volcanoes Cotopaxi, &c., whence they must certainly 

 have come, although the Brazilians, who use them to remove 

 rust from their guns, firmly believe them to be solidified river 

 foam. 



Having travelled so far, they would be pretty sure of 

 being floated out to sea, but their case is exceptional, and, 

 generally speaking, the nearer we come to the mouth of 

 a river the finer is the mineral matter which it carries, and, 

 before it has finished its course, its load has been so 

 thoroughly sorted by the dropping of all the heavier portions, 

 one after the other, that at last it carries nothing but the 

 finest mud, which, on reaching the sea, is then carried 

 farther by waves and currents, until finally deposited in the 

 largest department of the world's great lumber room. 



If, however, the river should fall into an almost tideless 

 sea, such as the Mediterranean, Carribean, or Gulf of 

 Mexico, much of the mud, instead of being swept away, is 

 deposited at its mouth ; and this is the origin of the tongue 

 of land at the mouth of the Mississippi, and also of the 

 great triangular plain of Lower Egypt called the Delta, 

 which, like all deltas, has been won from the sea. The 

 Nile has cut for itself in the rocky surface of the desert a 



