The geological time-scale

Abstract

SINCE my communication on this subject was written1, much has happened. Prof. J. L. Kulp2 has just published a time-scale in which the duration of the geological periods is given as follows: Ordovician, 490-430 m.y.; Silurian 430-410 m.y.; Devonian 410-355 m.y.; Mississippian 355-330 m.y.; Pennsylvanian 330-275 m.y.; Permian 275-220 m.y.; Triassic 220-180 m.y.; Jurassic 180-135 m.y.; Cretaceous 135-70 m.y. An almost identical geochronology (±5-10 m.y.), awaiting publication, was communicated to me privately by Prof. A. Holmes3 a month ago. The Oxford age of 305 m.y. for Dartmoor granite (by potassium/argon assay on biotite) has been confirmed by Dr. H. Faul4 of U.S. Geological Survey (292 m.y. and 288 m.y. on the same rock by the same method). A full isotopic assay of Hercynian pitchblende from Geevor, Cornwall, has yielded to Dr. A. G. Darnley5 of the Geological Survey of Great Britain a comparable date of 288 m.y. From all this it is confirmed that an extension of the Holmes B time-scale is called for, and that the best values are, as one would expect, a little higher than the averages of the experimental results published previously1. There is however, no support from recent researches for the conclusion that the Lower Cambrian is older than 600 m.y. © 1959 Nature Publishing Group.