96 GEOLOGY. 



of new islands. There is no delta formed at the mouth 

 of the Amazon, because there is an ocean current that 

 sweeps by, and this carries the sediment discharged by 

 that river up by the coast of Guiana, where it is deposit- 

 ed, forming immense muddy shoals and swampy tracts. 



178. Amounts of Deposits from Rivers. The amount 

 of sediment deposited by large rivers is enormous. The 

 Ganges pours such a quantity of mud and sand into the 

 Bay of Bengal that the water is seen to be colored by 

 it sixty miles from the shore. It is calculated that in 

 the rainy season each year this river discharges into the 

 sea an amount of solid matter equal in weight to fifty-six 

 pyramids, estimating a pyramid to contain 6,000,000 of 

 tons of granite. Mr. Lyell states that " if a fleet of more 

 than eighty Indiamen, each freighted with one thousand 

 four hundred tons weight of mud, were to sail down the 

 river every hour of every day and night for four months 

 continually, they would only transmit from the higher 

 country to the sea a mass of solid matter equal to that 

 borne down by the Ganges in the flood season, as the 

 exertions of a fleet of about two thousand such vessels 

 going down daily with the same burden, and discharg- 

 ing it into the gulf, would be no more than equivalent to 

 the operations of the great river. Yet, in addition to 

 this, it is probable that the Burrampooter conveys an- 

 nually as much solid matter to the sea as the Ganges." 

 The amount of land made by such enormous quantities 

 of sediment is very great. Most of the lower part of 

 Louisiana was formed by sediment brought down by the 

 Mississippi, and the land has encroached upon the water 

 several leagues since New Orleans was built. The solid 

 matter annually discharged by the Mississippi is two 

 thousand million (2,000,000,000) tons. This is sufficient 

 to cover a township six miles square with earth thirty 

 feet deep. 



179. Extent of Deltas. The deltas of very large riv- 

 ers are of great extent. That of the Nile is about as 



