2 PETRIFACTIONS AND THEIR TEACHINGS. 



altered in appearance and composition by mineralization,, the 

 epithets figured stones, petrifactions, fossils, organic remains, 

 &c., are commonly employed to denote the various conditions 

 in which such relics occur. To avoid confusion it is, there- 

 fore, necessary to define the sense in which these terms are 

 used in the following pages ; especially as the words " petri- 

 factions/' and " fossils," are very generally regarded as syno- 

 nymous, even by well-educated persons. 



And here we must premise that the state of preservation of 

 an organic body, and the chemical changes which it may have 

 undergone in the mineral kingdom, have no necessary relation 

 to its antiquity ; for in comparatively modern deposits fossil 

 remains of animals and plants often have acquired a stony 

 hardness, while in rocks of the most ancient epochs they are 

 sometimes as little changed as if they had been entombed in 

 the strata but a few centuries. 



1. Fossils, may be denned as the durable parts of animal 

 and vegetable structures imbedded in rocks and strata by 

 natural causes at a remote period ; thus wood in the state of 

 lignite, bog-wood, and coal, or of siliceous or calcareous stone, 

 is fossil wood ; and bones or shells, whether in an earthy and 

 decaying state, or permeated by calc-spar, flint, or iron, and 

 converted into a hard mineral substance, are alike fossil bones 

 or shells. 



2. Petrifactions, are the remains of animals and vegetables 

 in which the original structure is converted into stone, or, in 

 other words, is petrified; l such are the silicified stems of trees 

 from Antigua and Germany, and the bones and shells in the 

 Oolitic and Wealden limestones. Such petrifactions may be 

 correctly termed fossil plants, bones, or shells ; but similar 

 organic remains, though of equal antiquity, which have not 

 undergone such changes, are not petrifactions in the proper 

 meaning of that term, 



3. Incrustations, are neither fossils nor petrifactions, but 

 simply durable parts of animals or vegetables invested with 



1 The process by which petrifaction is effected is still involved in 

 obscurity ; mineral solutions have permeated the original tissues, and 

 the organic molecules have been replaced by mineral molecules, but how 

 this transmutation is produced is not understood. Mr. Dana's observa- 

 tions and Mr. Jeffery's experiments have, however, elucidated the process 

 of silicification. 



