Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/139

Rh of the bands of loam, that the retreating margin of the ice passed Schonen 12,000 years ago, but still lay at Meklenburg 14,000 years ago. The agreement between these figures is quite sufficient for our purpose.

For the older periods the thickness of the sediments yields an idea of the length of time of their deposition, and thereby, for example, a period of the magnitude of 1 to 10 million years for the Tertiary has been obtained. But the greatest authority certainly pertains to the somewhat similar values derived from the estimation of the age of the rocks by physical means from their helium content. The helium is formed by the disintegration of radioactive substances. The measurements are carried out on zircon crystals, the helium content of which is produced by the breaking down of uranium. Strutt, who developed this method, found for the Oligocene 8.4, for the Eocene 31, for the Carboniferous 150, and the Archæan 710 million years. Königsberger has revised the measurements of Strutt and determined other ages for some of the strata examined. The following periods of time are obtained from his and earlier data:—

By the aid of these figures and the routes taken by