however, can be observed in woods taken from such springs. 
H.E. Merwin (in Allen and Day, 1935) macerated several wood 
specimens removed from the Norris Geyser Basin and other 
areas in Yellowstone Park, Wyoming, and found that woody 
features had indeed been replicated in silica. The lumen cast 
appearing in Figure 5 on Plate 6 was prepared by the authors 
from tissue taken from the interior of a large, heavily encrusted 
limb of lodgepole pine (Pinus contorta) recovered from Tree 
Top Spring —a siliceous thermal spring in the Sylvan Springs 
area of Yellowstone Park. At the time of collection, pH was 5.5 
and temperature 85 Celsius. Since this spring did not exist prior 
to 1959, having formed as a result of an earthquake at that time, 
the total time of immersion of the wood in the spring could not 
have exceeded 13 years. 
The role of water in petrifaction is of paramount importance. 
Water is a necessary agent for ash alteration and mineral 
diagenesis. Saturation of the sediment serves to exclude oxy- 
gen, thereby inhibiting deterioration of tissue structure, 
through the maintenance of reducing conditions. Water- 
logging dispels entrapped air, and maintains the wood in a 
swollen and plastic state, thereby maintaining maximum per- 
meability. Also, it is the medium for transport and dispersal of 
soluble silica into and through the specimen. 
Conditions of temperature and pressure during petrifaction 
are probably very near those close to ambient shallow depth 
sedimentary environments. Excessive pressures would result 
in severe deformation of wood shape and tissue structure; 
excessive temperatures (greater than 100°C) would result in 
destruction of wood substance. 
The pH of the permeating fluid, within the wood, during 
silica emplacement is probably in the near neutral or, even 
more likely, somewhat acidic range of the pH scale. Either 
highly acidic or especially highly alkaline conditions rapidly 
destroys all wood substance, and hence, the initial template for 
deposition. Furthermore, high alkalinity would prevent silica 
deposition. Above the pH 9, soluble silica dissociates, causing 
an abrupt increase in silica solubility with rising pH — whereas 
below this approximate value, silica solubility is essentially 
independent of pH (Siever, 1972). Moreover, strongly basic 
media would interfere with hydrogen bond establishment be- 
tween silicic acid and the hydroxylated organic substances in 
wood. 
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