AT the estimated 2 × 1020 gm. uranium and 5 × 1020 gm. thorium in the lithosphere, helium should be generated radiogenically at a rate of about 3 × 109 gm./year. Moreover, the (secondary) cosmic-ray source of helium has been estimated to be of comparable magnitude. Apparently nearly all the helium from sedimentary rocks and, according to Keevil1 and Hurley2, about 0.8 of the radiogenic helium from igneous rocks, have been released into the atmosphere during geological times (currently taken to be about 5 × 109 yr.). Hence more than 1020 gm. of helium should have passed into the atmosphere since the `beginning'. Because the atmosphere contains only 3.5 × 1015 gm. helium-4, the common assumption is therefore that about 1020 gm. of helium-4 must also have passed out through the exosphere, and that its present rate of loss through the exosphere balances the rate of exudation from the lithosphere.