50 



LYELL'S ELEMENTS OF GEOLOGY. 



Consolidation of Strata. 



CHAPTER IV. 



CONSOLIDATION OP STRATA AND PETRIFACTION OF FOSSILS. 



Chemical and mechanical deposits — Cementing together of particles — Hard- 

 ening by exposure to air^ — Concretionary nodules — Consolidating effects of pres- 

 sure— Mineralisation 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 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 forms of 

 stratification, 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 scorice thrown out by a 

 volcano, which have fallen into their present place by the force 

 of graf itation. 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 car- 

 bonate of lime is often thrown to 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.* 



In coral reefs, large masses of limestone are formed by the 

 stony skeletons of zoophytes ; and these, together with shells, 

 become cemented together by carbonate of lime, part of which 

 is probably furnished to the sea-water by the decomposition of 

 dead corals. Even shells of which the animals are still living, 



♦See Principles of Geology, Index, " Calcareous Springs," &c 



