The Principle of Faunal Succession was later added by
William Smith in the late 1700's who observed and studied fossils
embedded in rock layers. This principle states that the oldest fossils
in a series of sedimentary rock layers will be found in the lowest layer
(layer A). Progressively younger fossils occur in higher layers (layer
B). This is the same concept as superposition, but it helped geologists
realize that you can look at the age of these layers and assign relative
dates. This parallels evolution. Younger organisms replace older
organisms as the older ones become extinct.
Since organisms change through time, it allows correlation of
beds far apart. If the layers have similar fossils, one can deduce that
the layers are the same age.