[1] Compare results for the thorium content of such rocks (appearing in a paper by the author Cong. Int. _de Radiologie et d'Electricite_, vol. i., 1910, p. 373), and those for the radium content, as collected in _Phil. Mag._, October, 1912, p. 697. Also A. L. Fletcher, _Phil. Mag._, July, 1910; January, 1911, and June, 1911. J. H. J. Poole, _Phil. Mag._, April, 1915 26 atoms of lead and light atoms of uranium generating light atoms of lead. This assumption seems to be involved in the figures upon, which we are going. Still relying on these figures, we find, however, that existing uranium cannot give rise to lead of normal atomic weight. We can only conclude that the heavier atoms of uranium have decayed more rapidly than the lighter ones. In this connection it is of interest to note the complexity of uranium as recently established by Geiger, although in this case it is assumed that the shorter-lived isotope bears the relation of offspring to the longer-lived and largely preponderating constituent. However, there does not seem to be any direct proof of this as yet. From these considerations it would seem that unless the atomic weight of lead in uraninites, etc., is 206, the former complexity and more accelerated decay of uranium are indicated in the data respecting the atomic weights of radium and lead[1]. As an alternative view, we may assume, as in our first hypothesis, that some elementally different but genetically connected substance, decaying along branching lines of descent at a rate sufficient to practically remove the whole of it during geological time, formerly existed. Whichever hypothesis we adopt [1] Later investigation has shown that the atomic weight of lead in uranium-bearing ores is about 206.6 (see Richards and Lembert, _Journ. of Am. Claem. Soc._, July, 1914). This result gives support to the view expressed above. 27 we are confronted by probabilities which invalidate time-measurements based on the lead and helium ratio in minerals. We have, in short, grave reason to question the measure of uniformitarianism postulated in finding the age by any of the known radioactive methods. That we have much to learn respecting our assumptions, whether we pursue the geological or the radioactive methods of approaching the age of our era, is, indeed, probable. Whatever the issue it is certain that the reconciling facts will leave us with much more light than we at present possess either as respects the Earth's history or the history of the radioactive elements. With this necessary admission we leave our study of the Birth-Time of the World.