This type of preservation takes place when the original hard parts of
organisms are removed after being dissolved by underground water. This
is accompanied by almost simultaneous deposition of other substances in
the resulting voids. Some replaced fossils will have the original
structure destroyed by the replacing minerals. Others, as in the case of
certain silicified tree trunks, may be preserved in minute detail.
Although more than 50 minerals have been known to replace original
organic structures, the most frequent replacing substances are calcite,
dolomite (a calcium magnesium carbonate), silica, and certain iron
compounds.