PLATE XXIII. HALOES IN MICA; CO. CARLOW. HALO IN BIOTITE CONTAINED IN GRANITE - to face p. 224 PLATE XXIV. RADIUM HALO, MUCH ENLARGED. THORIUM HALO AND RADIUM HALO IN MICA - to face p. 228 PLATE XXV. HALO ROUND CAPILLARY IN GLASS TUBE. HALOES ROUND TUBULAR PASSAGES IN MICA - to face p. 230 PLATE XXVI. ALETSCH GLACIER, SWITZERLAND - to face p. 282 PLATE XXVII. THE MIDDLE ALETSCH GLACIER JOINING THE GREAT ALETSCH GLACIER. GLACIERS OF THE LAUTERBRUNNEN THAL - to face p. 285 PLATE XXVIII. PERCHED BLOCK ON THE ALETSCH GLACIER. GRANITE ERRATIC NEAR ROUNDWOOD, CO. WICKLOW; NOW BROKEN UP AND REMOVED - to face p. 286 And Fifteen Illustrations in the Text. x PREFACE 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.