are remains of organisms or activity track of organisms
which lived in remote times.When alive beings die, the
soft parts rot fastly, and the hard ones dessapear in
more or less time. This is the normal process.
Nevertheles, if the being is fastly covered by sediments
which have suitable physicochemical properties, those
sediments prevent the oxigen providing, avoid the mechanical
destruction and stop the rotting.
The solid shells and skeletons use then to remain, while
the soft parts are integrated in the sediments and they
get the modification that transform them in stone.
When the organic material rots, the hard parts become
porouses and allow the mineral solutions, which flow
in the stone, to pass through.
most frequent fossilizations are in calcium carbonate,
calcium phosphate, silica, pyrites, marsh ore, hematite,
Vegetables and chitinous shells normaly appear carbonized,
because if there is not oxigen, cellulose transforms
in carbonic gas and methane, that use to leave, remaining
only the carbon.