( 99 ) 
On the Different States in which Fossil Vegetables are found. 
The vegetables which we find in a fossil state are scarcely 
ever (I believe it may be said that they are never) complete ; 
they are only portions or fragments of vegetables, stalks, 
branches, leaves, fruits, or rarely flowers, separated from the 
other organs of the plant. In this respect, we are in the 
same condition as in regard to actually existing vegetables, 
when we receive insulated and incomplete portions of an exotic 
plant, which we often find great difficulty in determining. 
But besides this, fossil vegetables, thus reduced to some of 
their insulated organs, scarcely ever present them in such a 
state of preservation, as enables us to study them in all their 
constituent parts. Thus the stalks often retain nothing more 
than their external form, or, in other cases, nothing more 
than their internal structure, frequently altered in many re- 
spects. The leaves, in many instances, retain nothing but 
the imperfect net-work of their nervures, and rarely can the 
epidermis and its details of structure be conveniently studied. 
As to the fruits, most frequently the external form alone is left 
to enable us to judge of their affinities, their internal structure 
being destroyed or greatly altered by compression or by 
petrifaction. 
The different modes in which vegetables are preserved in a 
fossil state, may, however, be referred to two principal classes. 
The impression or cast of the plant, accompanied with the 
complete destruction of the vegetable tissue, or the preserva- 
tion of few of its constituent parts; petrifaction or carbon- 
isation, which preserves, more or less completely, the struc- 
ture of the tissues of the vegetable organs, by changing com- 
pletely, or only modifying, their nature. 
The impression or cast, in a strict sense, that is, without 
the preservation of any portion of the organs of the vegetable 
more or less altered, is rather rare; yet it is the habitual 
_ State of fossil vegetables in the variegated sandstone (the 
Gris-ligarré) and tertiary limestones. 
The place once occupied by the vegetable is empty, or the 
vegetable is replaced only by a substance usually ferruginous, 
sometimes calcareous or earthy, which presents no organisa- 
tion, and which, consequently, is not the vegetable petrified. 
