Stratigraphy, a branch of geology, studies rock layers and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy includes two related subfields: lithologic or lithostratigraphy and biologic stratigraphy or biostratigraphy. Historical development The theoretical basis for the subject was established by Nicholas Steno who introduced the law of superposition, the principle of original horizontality, and the principle of lateral continu… (More on Stratigraphy)