FOSSIL PLANTS. 351 
noticed above as marked by horizontal splits, are of the same com- 
pound in their whole length. 
The silicified wood of the Coal Measures of Ohio, as that also 
of more recent formations of our continent, furnish us some valu- 
able data for the examination of another vexed question : concern- 
ing their mode of fossilization, or rather the origin of the silica 
which has produced their transformation. Two opinions, above 
all, have been advanced on this subject. Prof. Goppert thinks 
that the process of petrification has been very slow, of long dura- 
tion, and that to explain it, it is not necessary to suppose that the 
water in which the vegetable substance has been transformed, was 
richer in silica than it may be now in its normal state. Prof. 
Schimper, on the contrary, asserts that the water in which wood 
has been silicified should have been of a higher temperature, more 
abundantly saturated with silica, and therefore, he concludes that 
the kind of mineralization has happened in a much shorter time 
than is generally supposed, and by volcanic agency, as is now the 
case in the vicinity of the Geysers of Iceland.* To sustain this 
assertion, the celebrated professor says: that the progress of 
the fossilizing process should have been rapid enough to reach the 
whole substance of the wood before its decomposition by putrefac- 
tion. But the woody tissue, when entombed and protected against 
atmospheric influence, is unalterable for a considerable period of 
time, and slowly passes, by emerecausis, into coal. It is, there- 
fore, conceivable, that in the first stage of this slow burning, when 
the whole vegetable has been reduced to a soft matter, it may be 
penetrated by mineral fluids which, by crystallization, transform it 
into stone. In the valley of Locle, Switzerland, large prostrate 
trunks, more than fifty feet long, were discovered some years ago 
in a bed of sandy clay of the upper Tertiary. These trees, most 
of them Dicotyledonous, had their bark still in a good state of 
preservation, their woody tissue admirably preserved, and looked, 
- indeed, as if they had been recently buried. Yet their wood was 
soft enough to be cut through with the knife without effort, like 
butter. Beds of lignites, in Germany, where the emerecausis is 
in a more advanced stage, contain large trunks of wood, softened 
in the same degree, and already blackened. In that state, the 
woody tissues are easily impregnated by dissolved mineral sub- 
*Traite de Pal. Veget., p. 38 and 39. 
