DURABILITY OF WOODS 171 



years. The relation of special conditions of moisture is further 

 exhibited in these cases in the fact that ties which were perfectly 

 sound on the exposed sides are very often found to be in an 

 advanced state of decay throughout the buried parts. Mutilation 

 is an important factor in the introduction of decay, and Dudley 

 has shown (14) that where spikes have been driven into the ties, 

 and where the structure has thereby suffered mechanical altera- 

 tion, decay finds an opportunity for speedy entrance into the 

 interior tissues, which it rapidly permeates and destroys. This 

 relation of cause and effect is in perfect harmony with what 

 has long been known to occur in living trees where broken or 

 badly amputated limbs afford an opportunity for fungi to pene- 

 trate and destroy otherwise healthy tissues. 



The preceding considerations have directed attention to the 

 fact that coniferous woods may be preserved indefinitely, pro- 

 vided they are completely excluded from fresh supplies of free 

 oxygen and are maintained under conditions of low tempera- 

 ture, --in other words, hermetically sealed in an impervious 

 medium. While we are thus in a position to understand the 

 conditions under which a very large proportion of woods are 

 preserved as fossils in the more recent geological strata, no 

 explanation is offered which will adequately account for the 

 mode of preservation of the large number of plants met with 

 in the older rocks, even as far back as the Devonian and 

 Silurian, and it is desirable that examples of these should be 

 passed in review. In this connection four principal forms of 

 preservation may be noted, -- (i) carbonization, (2) silicification, 

 (3) calcification, and (4) pyritization. 



Carbonization. This form of preservation is essentially char- 

 acteristic of plants derived from the coal measures, and it is 

 represented by coal itself. It depends essentially upon a gradual 

 withdrawal of the elements of water from the original cellulose 

 substance, whereby a relative excess of carbon is developed. It 

 is a change which takes place under exclusion of air, and it is 

 no doubt facilitated by the action of heat and possibly also of 

 pressure. It is obvious, however, from the nature of the changes 



