FOSSILS 91 
are met with in all states of preservation. Exceptionally, the 
entire organism has been preserved with little or practically 
no change of the original substance—the bodies having been 
protected from decomposition by the nature of the materials in 
which they have been entombed. As examples may be cited 
the carcasses of the extinct mammoth and woolly rhinoceros 
which, long ages ago, were sealed up in the frozen earths and 
ice of Northern Siberia, so that when in recent times they 
became exposed, owing to the gradual dissolution of the 
medium in which they had been buried, their bodies were in 
so fresh a state that dogs devoured the flesh. Insects, spiders, 
and plants, have similarly been completely preserved in 
amber (fossil gum or resin); but, in most cases, these would 
appear to be more or less carbonised. The more common 
methods of preservation, however, are as follows :— 
Incrustation.—The organism under certain conditions is 
enveloped in a covering of mineral matter. Calcareous tufa, 
for example, is often precipitated upon plants growing near 
springs containing much calcium-carbonate. In the case of 
thermal waters siliceous sinter may be the incrusting substance. 
Vegetable and insect remains preserved in this manner are 
often more or less carbonised, or they may be entirely 
decomposed and dissipated, leaving merely hollow moulds 
behind them. 
Carbonisation.—Plant-remains and chitinous animal struc- 
tures, without having been previously incrusted, frequently 
undergo carbonisation—a deoxydising process which takes 
place under conditions permitting of only a limited access of 
air. Thus plants accumulated in marshy ground, or on the 
floor of lake or estuary, or buried in mud, etc., tend to under- 
go a kind of distillation whereby the oxygen and other gases 
are gradually eliminated—the carbon in this way becoming 
concentrated. 
Moulds and Casts.—The substance of a buried organic 
body may be entirely dissipated, and only a mould of it 
remain. Should this mould be subsequently filled with 
mineral matter, a cast showing the external form of the 
original will be produced. This isa common kind of fossilisa- 
tion. Many fossil shells, for example, are simply casts, and 
do not contain a particle of the original substance. When 
