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. When we find, as in the south-east of England, a great series of freshwater 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.[33-B] FOOTNOTES: [28-A] See Synoptic Table in Blainville's Malacologie. [29-A] Gray, Phil. Trans., 1835, p. 302. [31-A] For figures of recent species, see below, p. 183., and figs. of fossils, see p. 228. [32-A] See Index of Principles, "Fossilization." [33-A] See Principles, Index, "Lym-Fiord." [33-B] See below, Chap. XVIII., on the Wealden. CHAPTER IV. CONSOLIDATION OF STRATA AND 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--Goeppert's experiments--Precipitation of stony matter most rapid where putrefaction is going on--Source of lime in solution--Silex derived from decomposition 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 sedimentary 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 by geologists between deposits of a chemical, and those of a mechanical, origin. By the latter name are designated beds of mud, sand, or pebbles produced by the action of running water, also accumulations of stones and scoriae thrown out by a volcano, which have fallen into their present place by the force of gravitation. But the matter which forms a chemical deposit has not been mechanically suspended in water, but in a state of solution until separated by chemical action. In this manner carbonate of lime is often precipitated upon the bottom of lakes and seas in a solid form, as may be well seen in many parts of Italy, where mineral springs abound, and where the calcareous stone, called travertin, is deposited. In these springs the lime is usually held in solution by an excess of carbonic acid, or by heat if it be a hot spring, until the water, on issuing from the earth, cools or loses part of its acid. The calcareous matter then falls down in a solid state, encrusting shells, fragments of wood and leaves, and binding them together.[34-A]