14 THE CHAIN OF LIFE. 



by infiltration or have more or less completely replaced their 

 substance. Of course, as a rule, the softer and more putrescible 

 organic matters have perished by decay, and it is only the 

 harder and more resisting parts that remain. Even these have 

 often yielded to the enormous pressure to which they have 

 been subjected, and if at all porous, have been changed by the 

 slow action of percolating water charged with various kinds oi 

 mineral matter in solution. 



It thus happens that many fossils are infiltrated with mineral 

 matter. Wood, for example, may have the cavities of its cells 

 and vessels filled with silica or silicates, with sulphide or car- 

 bonate of iron, or with limestone, while the woody walls of the 

 cells may remain either as coaly matter or charcoal. I have 

 often seen the microscopic cells of fossil wood not only filled 

 in this way, but presenting under a high power successive coats 

 of deposit, like the banded structure of an agate. 



In some cases not only are the pores filled with mineral 

 matter, but the solid parts themselves have been replaced, and 

 the whole mass has actually become stone, while still retaining 

 its original structure. Thus silicified wood is often as hard and 

 solid as agate, and under the microscope we see that the wood 

 has entirely perished, and is represented by silica or flint, dif- 

 fering merely in colour from that which fills the cavities. In 

 this case we may imagine the wood to have been acted on by 

 water holding in solution silica, combined with soda or potash, 

 in the manner of what is termed soluble glass. The wood, in 

 decay, would be converted into carbon dioxide, and this as 

 formed would seize on the potash or soda, leaving the silica in 

 an insoluble state, to be deposited instead of the carbon. Thus 

 each particle of the carbon of the wood, as removed by decay, 

 would be replaced by a particle of silica, till the whole be- 

 came stone. By similar chemical changes corals and shells 

 are often represented by silica, or by pyrite, which has taken 

 the place of the original calcareous matter; and still more 

 remarkable changes sometimes occur, as when the siliceous 



