itself is one of impregnation and emplacement — not chemical 
replacement. As the wood is serving as a template for deposi- 
tion, faithfulness in replication of histological detail is contin- 
gent on the condition of the wood at the time of silica emplace- 
ment. 
Time 
In terms of geologic time, the emplacement of silica in wood, 
as amolecular film, probably occurs rapidly. Conversion of the 
initial deposit to the opaline state — a change characterized by 
decrease in permeability, water content, and hydroxyl content, 
and an increase in refractive index, hardness, and specific 
gravity, all consequences of the assumption toward order — 
probably requires a much longer time. The mineralogy of the 
specimen of Cordyline mentioned earlier, which can be only 
eighty years old at the most, lies somewhere between these two 
arbitrary states. The ultimate fate of all silica is transformation 
to low quartz. Under the conditions normally present at or near 
the surface of the earth, this conversion may require millions of 
years (Siever, 1972, estimates a time scale of from 107 to 10° 
years for the transformation). In the presence of organic mat- 
ter, however, crystallization to quartz may be appreciably 
accelerated (C. Wohlberg, personal communication). 
