I 4 PRINCIPLES OF PALEONTOLOGY. 



(fig. 2.) The whole, however, instead of being composed 

 of the original carbonaceous matter of wood, is now converted 

 into flint. The only explanation that can be given of this by no 

 means rare phenomenon, is that the wood must have undergone 

 a slow process of decay in water charged with silica or flint in 

 solution. As each successive particle of wood was removed 



Fig, 2. Microscopic section of the 

 silicifled wood of a Conifer (Sequoia] cut 

 in the long direction of the fibres. Post- 

 tertiary? Colorado. (Original.) 



Fig. 3. Microscopic section of the wood 

 of the common Larch (Abies larix), cut 

 in the long direction of the fibres. In 

 both the fresh and the fossil wood (fig. 

 2 ) are seen the discs characteristic of con- 

 iferous wood. (Original.) 



by decay, its place was taken by a particle of flint deposited 

 from the surrounding water, till ultimately the entire wood was 

 silicified. The process, therefore, resembles what would take 

 place if we were to pull down a house built of brick by succes- 

 sive bricks, replacing each brick as removed by a piece of stone 

 of precisely the same size and form. The result of this would 

 be that the house would retain its primitive size, shape, and out- 

 line, but it would finally have been converted from a house of 

 brick into a house of stone. Many other fossils besides wood 

 such as shells, corals, sponges, &c. are often found silicified ; 

 and this may be regarded as the commonest form of fossiliza- 

 tion by replacement. In other cases, however, though the prin- 

 ciple of the process is the same, the replacing substance may be 

 iron pyrites, oxide of iron, sulphur, malachite, magnesite, talc, 

 &c. ; but it is rarely that the replacement with these minerals is 

 so perfect as to preserve the more delicate details of internal 

 structure. 



