could not recognize it, as no cellular structure; not even in thin 
section, would be apparent. Moreover, without any clear or- 
ganismic identity, it is questionable whether such an object can 
even be referred to as a petrifaction. It is quite possible that 
some of the opaline and cherty objects occurring as scattered, 
isolated bodies in sedimentary rock units may have originated 
in exactly this fashion. It is not uncommon to find in a locality 
of silicified woods, specimens which are recognized as such 
only because of their associated occurrence with petrifactions 
displaying traces of botanical identity. Specimens in a single 
petrified forest usually display considerable variety in degree 
of structure retention. And even in certain individual petrifac- 
tions, it is quite common to find that some portions are totally 
devoid of any structure. It might be noted here that sometimes 
gross morphology such as growth rings is retained in silica, 
providing the only evidence that the object once was a part of a 
living system. 
The visual appearance of cellular detail in a petrifaction can 
be retained or assumed in more than one manner. These are: 
(a) by the actual presence of wood substance itself, in essen- 
tially intact form; 
(b) by degradation products formed from deteriorated wall 
substance, immobilized in silica at or very near their site of 
origin; 
(c) by variations in the mineralogy (color and texture) of 
different generations of silica, deposited at intervals during the 
silicification sequence (Schopf, 1971), and the presence of 
impurities in trace amounts; 
(d) by infiltration of foreign tar-like organic matter into the 
specimen (Went, 1972); 
(e) by patterns of entrapped air which darken silica surfaces 
(Went, 1972). 
Before continuing further, it should be emphasized that the 
emplacement of silica in wood does not proceed as a 
‘molecule-for-molecule’ replacement of organic polymers by 
silica, a popular misconception which persists to this day, in 
spite of a number of reports offering evidence to the contrary 
(St. John, 1927; Arnold, 1941; Barghoorn, 1960; Schopf, 1971). 
Petrifaction is fundamentally a process of infiltration and im- 
pregnation, wherein wood substance serves as an active 
template for silica deposition. Examination of the lithomorphs 
appearing in Plates | through 6 strongly support this postulate. 
21 
