The essay on The Birth-time of the World is based on a lecture delivered before the Royal Dublin Society.  The subject has attracted much attention within recent years.  The age of the Earth is, indeed, of primary importance in our conception of the longevity of planetary systems.  The essay deals with the evidence, derived from the investigation of purely terrestrial phenomena, as to the period which has elapsed since the ocean condensed upon the Earth’s surface.  Dr. Decker’s recent addition to the subject appeared too late for inclusion in it.  He finds that the movements (termed isostatic) which geologists recognise as taking place deep in the Earth’s crust, indicate an age of the same order of magnitude

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as that which is inferred from the statistics of denudative history.[1]

The subject of Denudation naturally arises from the first essay.  In thinking over the method of finding the age of the ocean by the accumulation of sodium therein, I perceived so long ago as 1899, when my first paper was published, that this method afforded a means of ascertaining the grand total of denudative work effected on the Earth’s surface since the beginning of geological time; the resulting knowledge in no way involving any assumption as to the duration of the period comprising the denudative actions.  This idea has been elaborated in various publications since then, both by myself and by others.  “Denudation,” while including a survey of the subject generally, is mainly a popular account of this method and its results.  It closes with a reference to the fascinating problems presented by the inner nature of sedimentation:  a branch of science to which I endeavoured to contribute some years ago.

Mountain Genesis first brings in the subject of the geological intervention of radioactivity.  There can, I believe, be no doubt as to the influence of transforming elements upon the developments of the surface features of the Earth; and, if I am right, this source of thermal energy is mainly responsible for that local accumulation of wrinkling which we term mountain chains.  The

[1] Bull.  Geol.  Soc.  America, vol. xxvi, March 1915.

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paper on Alpine Structure is a reprint from “Radioactivity and Geology,” which for the sake of completeness is here included.  It is directed to the elucidation of a detail of mountain genesis:  a detail which enters into recent theories of Alpine development.  The weakness of the theory of the “horst” is manifest, however, in many of its other applications; if not, indeed, in all.