118 FOSSILS THEIR NATURE AND ARRANGEMENT. 



Generally speaking, in recent accumulations, such as sand- 

 silt, peat-moss, and the like, the remains of plants and 

 animals are found little altered. The more volatile matters 

 are expelled from the plant, and the more perishable in- 

 teguments and softer tissues of the animal have perished. 

 Boots, trunks, "branches, and the harder fruits, if excluded 

 from the action of the atmosphere, become darker, denser, 

 and assume a peaty aspect. In like manner, bones, horns, 

 teeth, shells, and crusts, lose a portion of their animal 

 matter, and become denser and heavier through some slight 

 absorption of mineral ingredients. But in all the older 

 formations the vegetable, if not converted into coal, is 

 thoroughly lapidified that is, changed by a slow chemical 

 process into flint, ironstone, limestone, or sandstone, as the 

 case may be and merely retains its organic form ; while 

 animal remains undergo a similar conversion, and are 

 recognisable only through their individual forms and 

 textures, which continue unchanged and persistent. There 

 is nothing more marvellous than this process of petri- 

 faction : particle after particle as the organic matter dis- 

 appears, so particle after particle the mineral matter takes 

 its place, and this so delicately that scarcely a cell or 

 fibre is ever ruptured or displaced ! Of course where the 

 mineral solutions percolating the earth are so numerous 

 there will be great variety of petrifactions, some being 

 calcareous or limy, some siliceous or flinty, some ferruginous 

 or irony, and others bituminous or coaly. But in whatever 

 state they may occur the process seems the same namely, 

 a gradual dissipation, through decay, of the organic atoms, 

 and a gradual substitution, through permeation, of the 

 mineral or inorganic. Great differences will also arise from 

 the chemical nature of the organisms themselves wood, 

 bone, horn, teeth, shells, crusts, and corals, each having its 



