51 
PA Belge. 
CHAPTER IV. 
FOSSIL BOTANY. 
Fosstu VecETaBLEs.—The remains of the vegetable king- 
dom are presented to the notice of the geologist in various 
conditions; in some instances these relics are but little 
changed in their aspect, as, for example, in the recent accu- 
mulations of mud and silt, at the bottoms of lakes and 
rivers, and in morasses, and peat-bogs. In tufaceous incrus- 
tations, the imprints of wood, and of leaves and stems, are 
often sharply defined on the solid masses of concretionary 
and crystalline limestone. 
In the ancient deposits, vegetables are found in two dif- 
ferent states. In the one their substance is completely 
permeated by mineral matter ; it may be calcareous (lime), 
siliceous (flint), ferruginous (iron), or pyritous (sulphuret of 
tron); and yet both the external characters, and the in- 
ternal structure, may be preserved. Such are the fossil 
trees of the Isle of Portland, fragments of which so closely 
resemble decayed wood, as to deceive the. casual observer, 
until by close examination of their texture and substance he 
finds that they possess the weight and hardness of stone. 
In the silicified wood which abounds in many of the tertiary 
strata, the most delicate tissues of the original are pre- 
served, and by microscopical examination (see Pl. V.) may 
