It is quite possible that pH may undergo considerable change 
during the course of the process. During the initial stage of the 
process, pH may have been quite high (alkaline), promoted by 
weathering of the source material of the silica — volcanic ash, 
for example. Alkali and alkaline earth metal ions would be 
among the first components to be released to solution. This 
would cause arise in pH which, in turn, would promote silica 
dissolution. In this manner, a large quantity of soluble silica 
can be concentrated which later is available for emplacement in 
wood as the metal ions migrate from the site and pH is lowered. 
In addition, perhaps the alkaline conditions might expedite the 
silicification process by partially removing some of the lignin 
polymers enmeshing the architectural framework of the wood, 
the cellulose microfibrils, providing greater exposure of the 
latter to the petrifying agent and freeing active sites on the 
cellulose for bonding attachment with silica. Then, with de- 
crease in pH and lowering of silica solubility, perhaps abetted 
by the humic material formed earlier from lignin degradation, 
as well as base metal loss, silica deposition is enhanced. Or- 
ganic matter in solution may further aid the process by ac- 
celerating removal of aluminum and iron. Under somewhat 
acid conditions, there is a much greater tendency for aluminum 
and iron to be chelated into mobile complex ions than silicon. 
The ashen gray eluviated horizons observed in the profiles of 
podzol soils are developed in this fashion. 
Product 
The physical state of silica in all newly formed petrifactions 
is best characterized as amorphous or nearly amorphous. Dur- 
ing the early history of a petrified wood, the organic substance 
is still present and subject to deterioration, and this wood 
substance may ultimately be lost from the petrifaction without 
physical alteration of the siliceous lithomorph. This is a conse- 
quence of the highly hygroscopic and permeable nature of the 
initial silica deposit. The propensity of vascular tissue as a 
depository or sink for silica may be related to the potentiality 
for hydrogen bonding that exists between soluble silica and the 
dominant molecular constituents of wood — in particular, the 
polysaccharides. Any porous material can be infilled with silica 
by direct precipitation, but the replication of fine detail proba- 
bly requires chemical ‘fixing’. As noted before, the process 
28 
