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 Grreensburg, 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- 



