A revised geological time-scale

Abstract

The time-scale constructed in 1947 was based on certain assumptions that have recently been shown to be wrong. Appalachian pegmatites dated at 350 million years (m.y.) and thought to be Taconic (Ordovician) are now found to be Acadian (late Devonian), while others, dated at 255 m.y. and thought to be Acadian can now be referred to the Permian. Other recent evidence consistently leads to an extension of the 1947 scale that carries the beginning of the Cambrian back to about 600 m.y. ago. The scale now constructed from the data available up to October, 1959, is as follows (in m.y.): One of the more significant consequences of this revision is that many dated rocks from Africa and the other “Gondwanaland” continents which were formerly ascribed to the late Precambrian now become Cambrian. Important orogenic and plutonic phases of a major geological cycle, implying by analogy an extensive system of geosynclines, occurred at about the close of the Precambrian and early in the Ordovician.