192 STRATIFIED OR SEDIMENTARY ROCKS. 



structure of the stem — viz., pith, wood, and bark — not only the radiating 

 silver-grain and the concentric rings of growth, are discernible, but 

 even the microscopic cellular structure of the wood, and the exquisite 

 sculpturings of the cell- walls themselves, are perfectly preserved, so 

 that the kind of wood may often be determined by the microscope with 

 the utmost certainty. Yet not one particle of the organic matter of 

 the wood remains. It has been entirely replaced by mineral matter ; 

 usually by some form of silica. The same is true of shells and bones 

 of animals ; but as shells and bones consist naturally partly of organic 

 and partly of mineral matter, very often it is only the organic matter 

 which is replaced, although sometimes the original mineral matter is 

 also replaced by silica or other mineral substance. The radiating 

 structure of corals or the microscopic structure of teeth, bones, and 

 shells, is often beautifully preserved. This kind of preservation for 

 shells and corals is most common in limestones and clays ; for wood, in 

 gravels. 



Theory of Petrifaction. — If wood be soaked in a strong solution of 

 sulphate of iron (copperas) and dried, and the same process.be repeat- 

 ed until the wood is highly charged with this salt, and then burned, 

 the structure of the wood will be roughly preserved in the peroxide of 

 iron left. Also, it is well known that the smallest fissures and cavities 

 in rocks are speedily filled by infiltrating waters with mineral matters. 

 Now, wood buried in soil soaked with some petrifying material becomes 

 highly charged with the same, and the cells filled with infiltrated mat- 

 ter, and when the wood decays the petrifying material is left, retaining 

 the structure of the wood. But this is not all, for in Nature there is 

 an additional process, not illustrated either by the experiment or by 

 the example of infiltrated fillings. As each particle of organic matter 

 passes away by decay, a particle of mineral matter takes its place, until 

 finally the whole of the organic matter is replaced. Petrifaction, there- 

 fore, is a process of substitution, as well as interstitial filling. Now, 

 it so happens, probably from the different nature of the process in the 

 two cases, that the interstitial filling always differs, either in chemical 

 composition or in color, from the substituted mate- 

 ct ? rial. Thus the structure is still visible, though the 



mass is solid. If Fig. 174 represent a cross-section of 

 three petrified wood-cells, the matter filling the cells 

 (b) is always different from the matter forming the 

 cell-wall (a). 



"Fir 174 



The most common petrifying materials are silica, 

 carbonate of lime, and sulphide of iron (pyrites). In the case of petri- 

 faction by pyrites the process is quite intelligible, but the structure is 

 usually very imperfectly preserved. If water containing sulphate of 

 iron (FeSOJ come in contact with decaying organic matter, the salt is 



