570 



IMEEDDING OF AQUATIC SPECIES 



[Ch. XLVIII. 



If we had no liistorical information respecting the former 

 existence of an inlet of the sea, in this valley and of its 

 gradual obliteration, the inspection of the section above 

 described would show, as clearly as a written chronicle 

 the following sequence of events. First, there was a salt- 

 water estuary peopled for many years by species of marine 

 Testacea identical with those now living, and into which 

 some of the larger Cetacea occasionally entered. Secondly, 

 the inlet grew shallower, and the water became brackish, or 

 alternately salt and fresh, so that the remains of freshwater 

 and marine shells were 

 sediment of its bottom. 



mm 



fled in the blue argillaceous 

 Thirdly, the shoaling continued 

 until the river-water prevailed, so that it was no longer hab- 



marine 



fluviatile species and aquatic insects. Fourthly, a peaty swamp 



where some trees grew, or perhaps 



morass was formed 



were drifted during floods, and where terrestrial quadrupeds 



Finally, the soil being flooded by the river only 



mired 



became 



/ 



It was before stated, that 



on the sea-coast, in the delta of the Ganges, there are eight 



some 



must 



period, served in its turn as the principal channel of dis- 

 charge.^ As the base of the delta is 200 miles in length, it 



happen that, as often as the great volume of river- 

 water is thrown into the sea by a new mouth, the sea will at 

 one point be converted from salt to fresh, and at another 

 from fresh to salt; for, with the exception of those parts 

 where the principal discharge takes place, the salt water not 

 only washes the base of the delta, but enters far into every 

 creek and lagoon. It is evident, then, that repeated alter- 

 nations of beds containing freshwater shells, with others 

 filled with marine exuviae, mav here be formed. It has also 



been shown by artesian 

 p. 478), that the delta ( 



borings 



at Calcutta (see Vol. I. 

 ided much farther than 



now into the gulf, and that the river is only recovering from 

 the sea the ground which had been lost by subsidence at some 

 former period. Analogous phenomena must sometimes be 



* Vol. I. p. 472. 



( 



\ 



