Nicolas Steno

Nicolas Steno was a Danish scientist and, in his final years, a Catholic bishop, who is now regarded as one of the founders of modern geology and stratigraphy. In his 1669 treatise Dissertationis prodromus, Steno set out four principles for interpreting layered rock, now called Steno's laws: the law of superposition, holding that in an undisturbed sequence of rock layers the oldest layer lies at the bottom; the principle of original horizontality; the principle of lateral continuity; and the principle of cross-cutting relationships. Steno also argued, against the common view of his time, that fossil shark teeth called glossopetrae were the mineralized remains of once-living sharks' teeth rather than stones that had grown in the rock, an insight central to the modern understanding of fossils. In anatomy, Steno discovered and described the parotid salivary gland duct, still called Stensen's duct in his honor, and in crystallography he formulated Steno's law of the constancy of interfacial angles. Steno converted from Lutheranism to Catholicism in 1667, was ordained a priest in 1675, and was later beatified by the Catholic Church.