Correlation. What is the history of the Earth?. Unfortunately there is no place on the earth where rocks were formed and preserved for the entire length of the Earth’s history. As we have learned rocks are formed, changed, and disappear.
Correlating a rock type exposed on one side of a valley to a rock exposure on the other side of the valley is a simple correlation.
If rock exposures are numerous in a particular area, then the correlation can be a simple trace, such as the Principle of Lateral Continuity.
Replacement- can occur when some material of the dead organism is removed and replaced with mineral matter. A mould is created when a hard part, like a shell is buried in sediment and subsequently dissolved by water, leaving a mould behind. A cast is created if the mould becomes filled with mineral matter.
Index fossil overlap is not a total waste as it can still give a clue as to the age of a rock.