situations involve protective isolation of the wood from atmos- 
pheric oxygen. Lignin is rapidly degraded only under aerobic 
conditions, or when oxygen can be transferred or dehydroge- 
nation can take place (Flaig, 1968). The nature of the sedimen- 
tary setting is a critical factor in fossilization in that it deter- 
mines, along with the wood itself, the associated microbiota 
and the biogeochemical factors affecting the specimen; for 
example, accessibility to air and water, Eh and pH of the 
microenvironment, and composition and flow of any permeat- 
ing fluids. 
The most enduring forms of structural preservation are those 
of fossilization through petrifaction. Although non-mineralized 
woods have been unearthed from Paleozoic sediments, Callix- 
ylon wood from the Berea Sandstone, for example, such an- 
cient occurrences are rare. Most commonly, the geologically 
older specimens are found as petrifactions. Fossil woods are 
usually discovered in situations where they have become ex- 
posed to view through erosion of their protective sedimentary 
cover, as along a stream bank or ona talus slope. Once exposed 
to the adverse conditions normally existent at the ground sur- 
face, the non-mineralized woods will be relatively short-lived, 
whereas those impregnated with resistant mineral matter may 
survive for millenia, although slowly oxidized and weathered 
in the regolith. 
Some forty minerals of widely diverse chemical affinities 
have already been noted in the literature as petrifying agents 
(St. John, 1927). The most common petrifactions are siliceous 
(either hydrous silica or microcrystalline quartz). Of less fre- 
quent occurrence are those composed of calcium carbonate 
and iron sulfide. Most of the remaining mineral forms are rare. 
The preeminence of silica as a petrifactant and wood as an 
object of petrifaction suggests some affinity to be operative 
between the two materials. The propensity of vascular tissue as 
a depository or ‘‘sink”’ for silica might be related to the poten- 
tiality for hydrogen bonding that exists between soluble silica 
and the ligno-holocellulosic complexes which constitute wood 
(see Section IV). The preponderance of silicified plant tissue 
over that of animal tissue is quite likely a further consequence 
of the nature of the plant cell wall, its semi-rigidity serving 
against collapse of internal structure and its permeability per- 
mitting infiltration of siliceous fluids (Rolfe and Brett, 1969). 
The most perfect preservation of original structure is found 
6 
