The first serious attempts to formulate a time scale of historical geology that could be applied anywhere on Earth happened in the late 18th century. The most influential of those early attempts was led by Abraham Werner and others. They divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a period of Earth history. It was possible to speak of a "Tertiary period" as well as of "Tertiary rocks". Indeed, "Tertiary" (now Palaeocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) remained in use as names of geological periods well into the 21th century.