38 THE MEDALS OF CREATION. Crap. III. 
formations are often completely petrified, that is, permeated 
by, or transmuted into, stone. 
The words fossil and petrifaction are so commonly used as 
synonymous terms, even by educated persons, that it is 
necessary to define the sense in which they are employed in 
these volumes. 
Fossils are the durable parts of animal and vegetable 
structures imbedded in rocks and strata by natural causes 
at a remote period ; thus wood in a 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 decay- 
ing state, or permeated by calc-spar, flint, or iron, and con- 
verted into a hard mineral substance, are alike fossil bones 
or shells. 
Petrifactions are the remains of animals and vegetables in 
which the original structure is converted into stone, or, in 
other words, is petrified ; such are the silicified stems of — 
trees from Antigua, and Germany, and the calcified bones 
and shells in the Oolitic and Wealden limestones. Such pe- 
trifactions may be correctly termed fossil plants, bones, or 
shells ; but similar organic remains, though of equal anti- 
quity, which have not undergone such changes, are not 
petrifactions in the proper meaning of that term. 
The process by which petrifaction is effected is still in- 
volved in obscurity ; mineral solutions have permeated the 
original tissues, and the organic molecules have been replaced 
by mineral molecules, but how this transmutation is pro- 
duced is not understood. Mr. Dana’s observations and Mr. 
Jeffery’s experiments have, however, thrown much light on 
the process of silicification.* 
Incrustations.—Another prevalent error is that of con- 
sidering Incrustations to be fossils or petrifactions ; a mistake 
which is sanctioned by the custom of calling waters that are 
* See Wond. p. 100. 
Sm 
