352 THE BOTTOM OF THE SEA. 



the coast. A dam constructed at a given distance, 

 and turning the current towards the offing, dimi- 

 nishes the accumulation of the sand. 



In consequence of the same action, sandy or muddy- 

 deposits are produced at the mouths of rivers, in part 

 embarrassing ilia issue of the waters, and sometimes 

 becoming sufficiently important to constitute islands 

 extending to a greater or less distance into the sea, 

 and called " deltas." 



We have already seen that rivers carry a large 

 amount of rubbish of all kinds into the sea. Kocks 

 of any size do not travel far beyond the mountains 

 from which they are torn ; coarse gravel goes a little 

 farther, but does not always reach the sea. In the 

 case of the Ganges, it is found 400 miles from the 

 mouth; whilst in that of the Po, it is not dis- 

 coverable beyond Piacenza. Bodies carried in sus- 

 pension go farther in proportion to their lightness. 



Mud and sand, therefore, form essentially the base 

 of the delta. Fresh-water or land shells, the remains 

 of salt-water animals, and more rarely marine shells, 

 help to increase these deposits. 



Eemains of animals of large size are also found in 

 such situations, whether they have been carried there 

 by the river-current, or whether the delta has served 

 them for a habitation. The delta of the Ganges is 

 inhabited by tigers and alligators; all the human 



