102 On Different States of Fossil Vegetables. 
case, the cellular tissue is replaced by amorphous chalcedony 
and the ligneous and vascular tissues are alone petrified, so 
as to preserve the forms which characterise them. Some- 
times, though more rarely, the reverse of this takes place ; 
the cellular tissue is silicified and preserves its organisation, 
and the denser tissues have disappeared during petrifaction, 
leaving cavities in their place ; whether it be that these tis- 
sues had never been silicified, or that, transformed into a 
more alterable substance, they were destroyed at a later 
period. Thus, [ have seen many examples of the wood of 
silicified palms, in which the place of the fibrous bundles 
was, at leastin great part, represented by empty cavities, the 
remains of the tissue being silicified. 
2. Sometimes tissues of the same nature are differently 
preserved in different parts of the same specimen. In some 
cases, a kind of partial maceration has destroyed the struc- 
ture in certain places, while it is well preserved in the neigh- 
bouring parts ; but there are other instances where the tissue 
is petrified in a distinct and regular manner at one point, and 
destroyed at the side. This is particularly obvious in a re- 
markable fossil wood, described by Mr Witham under the 
name of Anabathra pulcherrima ; and I have likewise seen it 
in some other specimens. The siliceous petrifaction appears 
to have taken place at first in certain zones very distinctly 
defined, and most frequently in the form of insulated spheres. 
In all these parts the tissue is perfectly preserved ; but 
around it, in the intermediate spaces, this tissue is entirely 
destroyed, and has been replaced by amorphous silica. At 
first sight, and in a transverse section, the silicified parts 
would seem so many distinct ligneous fascicles, and give to 
these stems a very anomalous structure ; but an attentive 
examination shews that the medullary rays and ligneous zones 
are continued from one part to another, and we may re- 
establish, so to speak, the tissue throughout. Besides, we per- 
ceive that these fascicles are not continued in length; they 
are insulated spheres, results of a partial petrifaction, en- 
veloped in an amorphous siliceous mass. 
3. It very often happens that, during silicification, the 
vegetable has been compressed, broken, and deformed, fis- 
sures filled with crystallised or amorphous silica run across 
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