Because of the principle of faunal succession, you know that B and D are of the same age. You also know that C and F are of the same age. The geologic time scale applies to the entire Earth. When scientists first attempted to assign ages to rock layers that were separated by large distances, they relied on fossils like these to correlate the layers. Correlation is the matching up of rocks of similar ages in different parts of the world. If rocks contain similar fossils, they are assumed to be of the same age. A group of rock layers that contains similar fossils of similar ages is called a stage. Index fossils are the best fossils for making rock correlations. Through correlation, rock strata can be matched up worldwide.