process which occurs in a continuous sequence of stages. It 
should be further noted here that silica, when in the amorphous 
or near-amorphous condition of a gel or low ordered opaline 
state, is hygroscopic and highly permeable to fluid flow. And 
the rate of transformation to the relatively impervious crystal- 
line state is extremely slow, even in terms of geologic time. 
Thus, even long after silicification has begun, wood substance 
can continue to degrade and migrate from the specimen, 
thereby providing additional space for further silica deposition. 
Moreover, the degradation products from the former process 
may have arole in promoting or abetting the latter process, as 
both proceed simultaneously (see Section IV). In nature, this 
mechanism of wood removal and directed mineral deposition 1s 
never totally perfect. Nor does it usually go to completion. 
Most silicified woods, even many which have converted to 
microcrystalline quartz, contain some organic matter. A 
specimen of Callixylon wood from the Olentangey shale of 
Ohio, for example, is 4.1 per cent by weight in organic carbon, 
suggesting much of the original wood substance is still present, 
albeit chemically altered. This specimen is Upper Devonian in 
age, approximately 400 million years old! 
Many mineralized woods, when subjected to stress, tend to 
fracture radially — in the same preferred direction of breakage 
displayed by non-mineralized woods when they check and 
rupture through differential shrinkage upon drying. This ten- 
dency toward radial longitudinal fracture in many petrified 
woods can probably be attributed to a combination of two 
factors — one, the presence of substantial amounts of wood 
substance in the petrifaction and, two, an uneven distribution 
of silica through the specimen, witha pattern of discontinuities 
predetermined by the original wood structure at the time of 
petrifaction. In those cases in which the silica underwent crys- 
tallization to quartz, with substantial crystal growth not dic- 
tated by the structure of the original organic template, no 
preferred direction of fracture can be expected during break- 
age. 
If a specimen of wood should lose all organic substance 
during silicification and, in so doing, is entirely infilled with 
only silica, a question which arises is how can one recognize 
the object as a result of petrifaction. If the silica was deposited 
continuously and homogeneously, completely filling the entire 
volume of the former sample of wood, it is quite likely that one 
20 
