A cast has the same shape as the original organism. However, when it is broken open, the lack of inner structure shows it to be a cast. Mineral matter may seep into shells, wood or bone openings. The mineral may then preserve the fossil. Silica replaces cellulose to form petrified wood which preserves the tree rings, bark and cells.
Earth history is divided into time units based on changes in the fossil record.
Divisions of time are based on the appearance and disappearance of certain fossils. These divisions of geologic time are recognized on a worldwide basis. It is not always easy to realize the great amounts of time that are involved in the history of the earth. One way to do so is to diagram events on a time line. A time line has a scale in which a given length represents a certain number of years.