486 
PALAEONTOLOGY OF ILLINOIS. 
fracture is generally observable on the silicified trunks so abundantly found in 
some parts of Southern Ohio, especially in the bed of Shade river, near Athens. 
They are, most of them, pieces of stems of fern trees (Psaronius ), varying in 
diameter from three to twelve inches, broken in disks from two to fourteen 
inches long. A few of these pieces of silicified wood are irregularly broken 
and disfigured on the outside by maceration ; but generally they preserve their 
cylindrical form, and when of some length show here and there, at various dis¬ 
tances, horizontal splits, uninterrupted all around the trunk, where a disruption 
is easily produced by a hard stroke. From the great bed of sandstone overly¬ 
ing the Pittsburg coal, near Greensburg, I have received, from Rev. W. D. 
Moore, large specimens of fossil wood, most of them long, irregularly broken, 
much decayed pieces, evidently representing sections of trunks broken length¬ 
wise. These were found in various positions in the sandstone, and were mostly 
broken before they were imbedded in it. But among them there is one which 
bears, attached to a short stem, three diverging branches of its roots, a proof 
that it has been buried in its original standing position; and this one has its 
top horizontally broken and flat. 
From these data and a number of others, which it is useless to mention, being 
all of the same kind, and bearing the same evidence, it appears that the frac¬ 
ture of the fossil wood is of two kinds : irregular, for trunks fossilized after 
prostration or in a decaying state, as they are generally found in our Tertiary 
and Cretaceous strata; and horizontal, by splits perpendicular to the natural 
direction of the stems and the roots. If the cause of fracture in the first case 
is, without doubt, essentially due to atmospheric agency, that of the second, 
which has acted upon the vegetable while it was still subjected to the process 
of petrification, is certainly different, and can be explained, I think, by the 
difference of density of both the surrounding mineral matter and the imbedded 
vegetable. Evidently, all the stems in the process of fossilization have been 
subjected to a softening process of their whole mass. The outside pressure of 
the surrounding mineral matter must have been felt, and can have acted only 
in one way, that is, vertically, as it happens in the forcing of a body of less 
density out of water; and the result of that action cannot but have been a ten¬ 
dency to dislocation, and therefore to splitting of the trunks in a horizontal 
direction, It might be supposed, perhaps, that a gradual accumulation of sand 
or other mineral matter around standing trees, in burying them, has formed 
layers of different density, whose action may have produced, in the fossil vege¬ 
table, zones of petrification also varying in density, tending, therefore to cleave 
from each other, and horizontally separable. But the roots of fossilized trees 
which tend downwards in an inclined direction, or even are nearly horizontal, 
should be split in an inclined plane and not perpendicularly to their axis, as 
they are, at least, on all the roots of standing trees which I have had opportu- 
