The Actinium Series and the Lead Ratios in Rocks

Abstract

PROF. JOLY in his beautiful work on pleochroic haloes in mica has stressed the importance of a small discrepancy in the observed position of the first halo ring. The discrepancy was shown to vary with the age of the rock. It was recognised that the influence of actinium in the formation of these uranium haloes had not been taken into account, but it did not seem possible that the actinium series could have any appreciable effect since it is regarded as only a 3 per cent, off-shoot from the main uranium series. Certainly it could not account for a shifting inner ring, and Prof. Joly suggested that the type of thing needed was a variable decay-constant for uranium—such as would arise if uranium had an isotope of considerably shorter life than uranium I. But it can be shown that more than one additional α-ray is needed to account for the observed shifts.