consistent time (geological age) correlation with either the 
amount of wood or its state of preservation. 
Silicified woods average around one per cent in total organic 
carbon, ranging from trace amounts to roughly five per cent. 
Considering that elemental carbon constitutes about fifty per 
cent of the dry weight of most woods and that the average 
specific gravities of wood and silica are roughly in the ratio of 
one to five, the average volume occupied by organic tissue in 
petrifactions is estimated to be on the order of ten per cent, 
assuming no compression. 
The physical state of silica in silicified woods of Tertiary age 
is commonly opaline.? Ancient specimens, including all those 
of Paleozoic age, are composed of microcrystalline quartz, 
either as submicroscopic equant grains or radiating fibers of 
chalcedony. Originally, probably all siliceous petrifactions 
were amorphous, forming from the build-up in concentration 
and polymerization of monosilicic acid molecules within the 
woody matrix. With time, the resulting precipitate loses water 
(syneresis) and becomes opaline. Silica can persist in the 
Opaline state for millenia. Eventually, it will become more 
compact and take on order —1.e., transform ultimately to the 
more thermodynamically stable crystalline state. 
Il. LABORATORY SILICIFICATION OF PLANT TISSUE 
AT LOW TEMPERATURE 
The critical factors for the natural process of wood petrifac- 
tion are not completely known. Nor has a laboratory process 
been developed which can readily produce specimens compa- 
rable in quality to natural petrifactions like those from the 
Clarno formation (Eocene) in the John Day Basin of Oregon, 
for example. Attainment of this objective is desirable for the 
insight it might provide into the actual mechanisms of petrifac- 
’The terms of opaline silica and opal are not distinguished here and are used ina very 
general sense, referring to all hydrous silicas — including both those which are 
compact and vitreous and those which are friable or dispersed. The various structural 
forms of the hydrous silicas have been subdivided on the basis of their x-ray diffraction 
patterns into three separate categories by Jones and Segnit (1971). These are: opal-A 
(highly disordered, near amorphous), opal-CT (disordered a-cristobalite), and opal-C 
(well-ordered a-cristobalite). This classification might be amended to include the 
occurrence of tridymite-like structures which have been observed in a number of 
natural hydrous silicas, including the wood opal described by Mitchell and Tufts 
(1973). 
° 
