THE VICTORIAN NATURALIST. 51 



and the final products of the slow oxidation of organic material 

 are the same as those produced by its combustion by fire. How- 

 ever, in slow decay a vast number of chemical bodies are 

 formed, one after the other, as more and more oxygen is seized 

 on by the decaying substances, and this decay is — it is hardly 

 necessary to remind you — brought about almost entirely by the 

 action of living bacteria, at any rate in its earlier stages. 

 Amongst these chemical substances is a series of acids called 

 collectively " humic " acids, from their constant occurrence in 

 humus or decaying vegetable matter. Some of these acids have 

 as active a solvent power as the so-called mineral acids, such as 

 hydrochloric or nitric. Amongst them two, crenic and apocrenic 

 acids, stand out in efficiency. Most of the salts that they 

 form are soluble in water, and are in their turn readily oxidized 

 to carbonates and oxides, most of which are insoluble. Thus we 

 have surrounding the roots, both dead and alive, but more 

 especially when dead, a series of substances set free which are 

 capable of completely dissolving the shelly fragments, and on 

 further oxidation of redepositing the lime as carbonate in a 

 compact form. With the complete disappearance of the root the 

 increase of the tube ceases, and a period of decay sets in, brought 

 about by a similar series of changes, or, when the tubes are 

 exposed on the surface, partly by unequal contraction and 

 expansion due to variations in temperature and partly by the 

 carbonic acid in rain water, and once more we get loose sand. 



We see, then, that the name " petrified wood," which is 

 popularly applied to these objects, is a misnomer, for in a true 

 petrification the organic material is replaced particle by particle 

 by the petrifying substance. Nor are they casts, as Darwin held, 

 for the root is still there while they are forming, and a cast is 

 made by filling a space left after the removal of an object. That 

 from which a cast is taken is a mould, but as one of their 

 essential characters is their structural difference from the loose 

 sandy matrix in which they lie the term mould or impression is 

 inapplicable. The term incrustation, which I have used, and 

 which is implied in Moseley's explanation, is in itself open to 

 objection, since we generally understand that an incrustation is 

 closely applied to the incrusted object, whereas in the present 

 case a zone of non-indurated material often surrounds the root. 

 Again, it does not 'seem right to apply the term concretion, for the 

 material composing a concretion is collected from the neighbour- 

 hood and concentrated at a given spot by what the wise term 

 concretionary action, and we have no evidence of concentration 

 in the present case. However, the least objectionable term seems 

 to be the one adopted, namely, " incrustation," and moreover it is 

 one which most nearly suggests the method of formation of these 

 objects, which are so frequent a source of wonder to the seaside 

 holiday-maker, and which are generally spoken of as petrified trees. 



