Fossils. 
45 
the change of say a piece of wood to stone. “ If an 
organic substance is exposed in the open air to the 
action of the sun and rain, it will in time putrefy, or 
be dissolved into its component elements, consisting 
usually of oxygen, hydrogen, nitrogen, and carbon. 
Those will readily be absorbed by the atmosphere to 
be washed away by rain, so that all vestiges of the 
dead animal or plant disappear. But if the same 
substances be submerged in water, they decompose 
more gradually, and if buried in the earth, still more 
slowly, as in the familiar example of wooden piles or 
other buried timber. Now, if as fast as each particle 
is set free by putrefaction in a fluid or gaseous state 
a particle equally minute of carbonate of lime, flint, or 
other mineral is at hand and ready to bo precipitated, 
we may imagine this inorganic matter to take tho place 
just before left unoccupied by the organic molecule. 
In this manner a cast of the interior of certain vessels 
may first be taken, and afterwards the moro solid walls 
of the same may decay and suffer a like transmutation.' ” 
Silicified wood appeals directly to tho observor, its 
original woody condition being so apparent. Specimens 
are common enough in basaltic country, being evidently 
derived from the drifts underlying the basalt. Blocks 
of silicified wood are also strewn in great abundance 
about Milparinka, and from there across to the South 
Australian border. 
The divisions of geological history are founded 
entirely on the animals and plants which characterize 
