Helium age measurements on magnetite specimens of various geological ages show a grouping and sequence in accordance with geological knowledge and a spacing compatible with ideas derived from sedimentational history. Ages in millions of years are given as follows: mean Miocene, 19; mean Laramide, 57; mean Nevadan, 118; mean late Triassic, 155; mean late Appalachian, 215; mean Devonian, 348; pre-Cambrian ranging from 500 to 1650. As a result the scale is considered suitable as an index for preliminary investigation of the helium retentivity of minerals and application of the method to igneous rocks.
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