Stratigraphy, a branch of geology, studies rock layers and layering (stratification). It's 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 Rock layers were studied since the time of Avicenna (Ibn Sina), a Persian geologist who wrote The Book of Healing in 1027. He was the first to outline the law of superposition of strata:… (More on Stratigraphy)