162 ELEMENTS OF SCIENCE 



action is largely aided by ice, for, as we have seen,* water 

 expands when it freezes, and thus it must enlarge any 

 cracks and fissures into which it may have made its way 

 and frozen. By these means the land is being 

 continually torn down and carried off to be deposited 

 either in estuaries, or at the mouths of rivers, or in the 

 bed of the ocean. The mass of matter thus carried to 

 the sea by some of the largest rivers is enormous. It has 

 been calculated that the Ganges carries down every year 

 as much land as could be carried down by 730,000 ships, 

 each of 1400 tons burthen. The substances carried 

 down by the Mississippi have formed at the mouth of 

 that river, in the Gulf of Mexico, a deposit extending 

 over an area of 30,000 square miles, and is known to be, 

 at least in some parts, several hundred feet in thickness. 

 A deposit thus formed at a river's mouth is generally 

 more or less triangular in shape, on which account it is 

 called a delta, from its resemblance in shape to the 

 Greek letter so named. It is only when seas are more 

 or less enclosed (as, e.g., the Gulf of Mexico and the 

 Mediterranean), or where the ocean currents are weak, 

 that the transported materials are deposited so as to 

 form deltas. Egypt largely consists of the delta formed 

 by the Nile, and it has been calculated that not less 

 than 17,000 years have been required for its formation. 

 Deposits of this kind carried down by rivers into fresh- 

 water lakes, also form " deltas " therein. 



The eroding action of water is notorious. When the 

 gradient of a river is considerable (as is commonly the 

 case in the upper courses of rivers), its excavating action 

 is also considerable, and if such a gradient be maintained 

 to the coast, the river will excavate a deep channel 



* See ante, p. 91. 



