Ch. IV.] CONSOLIDATION OF STRATA. 33 



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 commu- 

 nicate 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 pe- 

 riods filled with salt water. 



The Lyra-Fiord in Jutland offers an excellent illustration of analogous 

 changes ; for, in the course of the last thousand years, the western ex- 

 tremity 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 having been ' as often formed and removed. 

 The last irruption of salt water happened in 1824, when the North Sea 

 entered, killing all the freshwater shells, fish, and plants ; and from that 

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

 ters and other marine mollusca, have succeeded the Cyclas, Lymnea, 

 Paludina, and Charm* 



But changes like these in the Lym-Fiord, and those before mentioned 

 as occurring at the mouths of great rivers, will only account for some 

 cases of marine deposits of partial extent resting on freshwater strata. 

 \Vhen we find, as in the southeast of England, a great series of fresh- 

 water beds, 1000 feet in thickness, resting upon marine formations and 

 again covered by other rocks, such as the cretaceous, more than 1000 

 feet thick, and of deep-sea origin, we shall find it necessary to seek for a 

 different explanation of the phenomena.f 



CHAPTER IV. 



CONSOLIDATION OF STRATA AXD PETRIFACTION OF FOSSILS. 



Chemical and mechanical deposits — Cementing together of particles — Hardening 

 by exposure to air — Concretionary nodules — Consolidating effects of pressure — 

 Mineralization of organic remains — Impressions and casts how formed — Fossil 

 wood — Goppert's experiments — Precipitation of stony matter most rapid where 

 putrefaction is going on — Source of linie in solution — Silex derived from de- 

 composition of felspar — Proofs of the lapidification of some fossils soon after 

 burial, of others when much decayed. 



Having spoken in the preceding chapters of the characters of sedi- 

 mentary formations, both as dependent on the deposition of inorganic 

 matter and the distribution of fossils, I may next treat of the consolidation 

 of stratified rocks, and the petrifaction of imbedded organic remains. 



Chemical and mechanical deposits. — A distinction has been made b^' 



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



f See below, Chap. XVIII., "on the Wealden. 



