The study of the age of rocks.
Chronostratigraphic divisions of the geologic timescale are in units of geological material, while geochronologic divisions are periods of time. It is important to make this distinction because while you can visit a deposit of fossils from a certain time period (the geological material; chronostratigraphy), you can visit the actual time period (geochronology).
Biostratigraphic correlation is a method used in geology to correlate and date rock layers (strata) based on the fossils they contain. By identifying and comparing the distribution of specific fossil species across different geographic locations, geologists can establish relative ages and chronostratigraphic relationships between those layers. This technique is crucial for reconstructing past environments and understanding the geological history of an area. It relies on the principle of faunal succession, which states that fossil assemblages succeed one another in a recognizable order.