DURABILITY OF WOODS 173 



Carbonization necessarily involves a more or less profound 

 obliteration of structural details. This is especially true in those 

 cases in which an absence of infiltrated mineral matter has pre- 

 vented a retention of the original structural details, and where 

 pressure in conjunction with heat, as in hard coal, has pro- 

 duced a secondary effect. From this point of view it is true 

 that a highly carbonized cortex rarely presents any structural 

 details. Lignites and some of the softer coals not infrequently 

 present well-defined structure, but the same cannot be expected 

 of the hard coals, in which extreme alteration has been effected. 

 In many cases, such as may be found in the Devonian and later 

 formations, carbonization is joined to silicification or calcification 

 and gives rise to resultant forms of preservation, which will be 

 discussed more fully and with more propriety in the next chapter; 

 but attention may be directed to the general fact that where 

 carbonization operates by itself the fossil acquires an opacity 

 which renders it very difficult to determine details, while the 

 structure also becomes so friable as to make special methods of 

 section cutting imperative. 



Silicification. This is by far the most common form in which 

 the stems of plants are preserved in the older rocks. It depends 

 upon the slow infiltration of a solution of an alkaline silicate into 

 the tissues, whereby the entire structure eventually becomes con- 

 verted into a mass of silica, as in the trees of the petrified for- 

 ests of Arizona, or as may be seen in some of the larger algae, 

 such as Nematophycus from the Devonian. According to the 

 rate of infiltration, relatively to the operation of decay, all struc- 

 tural details may be observed. Under ordinary circumstances, 

 however, such a method of preservation is one of the most 

 advantageous for the purposes of scientific study, because of 

 the transparency of the mass and the permanent form of the 

 material. 



Calcification. This form of mineralization is much less com- 

 mon than silicification, with which it may be combined. In 

 some cases, however, calcite constitutes the entire mass of the 

 infiltrated material, as in the case of Osmundites skidegatensis 



