ILLUSTRATED BY FIGURES. 93 
bonaceous matter: others are filled up with sand, and other substances, the 
external part, or cortex, alone remaining; but it has been found that many 
retain their original structure, the interstices being filled up by calcareous or 
siliceous crystallizations. A method has lately been discovered, by which 
the organization of the latter may be satisfactorily examined. 
This method, which I have had the pleasure of recommending to the 
York and Newcastle Philosophical and Natural History Societies, and of 
which a particular account will be given elsewhere, may here be briefly de- 
scribed as follows :—A slice, or thin fragment, is obtained in the usual man- 
ner. One side of it is ground and polished, and is then applied to a piece 
of plate or other glass, by means of a transparent gum or resin. The other 
side is then ground down, parallel to the glass, and, on being brought to 
the necessary degree of thinness, polished. By this means, the internal 
structure may be as distinctly seen as in the slice of a recent vegetable. 
As might be supposed, the remains of the internal organization, thus 
displayed, are frequently distorted and disrupted in various ways, crushed 
into confused masses, or widely distended and separated. In examining 
them, therefore, caution is’ required, lest the regular be confounded with 
that which has been modified by various causes. The crystallization of the 
infiltrated substance has operated powerfully, among other causes, in pro- 
ducing these modifications. In general, the calcareous matter has crystal- 
lized in divergent prisms, and has thus given rise to a kind of cellular ap- 
pearance, very readily distinguishable from the true, but which a novice 
might easily interpret into the regular texture of a cellular or agamic plant ; 
while the siliceous or calcedonic has arranged itself in parallel undulations 
or series of curves. The appearances produced by the two modes of erys- 
tallization are very distinct, insomuch, that a person might in many cases 
pronounce with accuracy as to the infiltrated substance, without using any 
other means of detecting it than ocular inspection. 
The regularity of structure displayed by many fossil vegetables is truly 
astonishing. Many of them exhibit an internal texture as perfect as can be 
obtained from the nicest slice of a recent plant,—of which many examples 
will be seen in the Plates. The structure thus disclosed often corresponds 
