PART I. CHAPTER III. 



49 



Alternations of Marine and Freshwater Formation?. 



The alternation of marine and freshwater formations both on 

 a small and large scale, are facts well ascertained in geology. 

 When it occurs on a small scale, it may have arisen from the 

 alternate occupation of certain spaces by river-water and the 

 sea ; for in the flood season the river forces back the ocean and 

 freshens it over a large area, depositing at the same time its sedi- 

 ment ; after which the salt water again returns, and, on resum- 

 ing its former place, brings with it sand, mud, and marine shells. 



There are also lagoons at the mouths of many rivers, as the 

 Nile and Mississippi, which are divided off by bars of sand from 

 the sea, and which are filled with salt and fresh water by turns. 

 They often communicate exclusively with the river for months, 

 years, or even centuries ; and then a breach being made in the 

 bar of sand, they are for long periods filled with salt water. 



The Lym-Fiord in Jutland offers an excellent illustration of 

 analogous changes ; for, in the course of the last thousand 

 years, the western extremity of this long frith, which is 120 

 miles in length, including its windings, has been four times fresh 

 and four times salt, a bar of sand between it and the ocean hav- 

 ing been as often formed and removed. The last irruption of 

 salt water happened in 1824, when the North Sea entered, kill- 

 ing all the freshwater shells, fish, and plants ; and from that 

 time to the present, the sea-weed Fucus vesiculosus, together 

 with oysters and other marine mollusca, have succeeded the 

 Cyclas, Limnea, Paludina, and Charae.* 



But changes like these in the Lym-Fiord, and thos^ before- 

 mentioned as occurring at the mouths of great rivers, will only 

 account for some cases of marine deposits resting on freshwater 

 strata. When we find, as in the south-east of England, a great 

 series of freshwater beds, resting upon one marine formation of 

 great thickness, and again covered by another more than 1000 

 feet thick, we shall find it necessary to seek for a different expla- 

 nation of the phenomena, t 



* See Principles of Geology, Index, " Lym-Fiord." 

 t See account of Wealden, Part II. 



