The Birth-Time of the World and Other Scientific Essays. John Joly

      "

      PREFACE

       Table of Contents

      Tins volume contains twelve essays written at various times

       during recent years. Many of them are studies contributed to

       Scientific Reviews or delivered as popular lectures. Some are

       expositions of views the scientific basis of which may be

       regarded as established. Others—the greater number—may be

       described as attempting the solution of problems which cannot be

       approached by direct observation.

      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

      xi

      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.

      xii

      paper on _Alpine Structure_ is a reprint from "Radioactivity and

       Geology," which for

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