My thanks are due to Mr. John Murray for kindly consenting to the reissue of the essay on _The Birth-time of the World_ from the pages of _Science Progress_; to Messrs. Constable & Co. for leave to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make some extracts from _Radioactivity and Geology_; and to the Council of the Royal Dublin Society for permission to republish certain papers from the Proceedings of the Society. _Iveagh Geological Laboratory, Trinity College, Dublin._ July, 1915. xvi THE BIRTH-TIME OF THE WORLD [1] LONG ago Lucretius wrote: "For lack of power to solve the question troubles the mind with doubts, whether there was ever a birth-time of the world and whether likewise there is to be any end." "And if" (he says in answer) "there was no birth-time of earth and heaven and they have been from everlasting, why before the Theban war and the destruction of Troy have not other poets as well sung other themes? Whither have so many deeds of men so often passed away, why live they nowhere embodied in lasting records of fame? The truth methinks is that the sum has but a recent date, and the nature of the world is new and has but lately had its commencement."[2] Thus spake Lucretius nearly 2,000 years ago. Since then we have attained another standpoint and found very different limitations. To Lucretius the world commenced with man, and the answer he would give to his questions was in accord with his philosophy: he would date the birth-time of the world from the time when [1] A lecture delivered before the Royal Dublin Society, February 6th, 1914. _Science Progress_, vol. ix., p. 37 [2] _De Rerum Natura_, translated by H. A. J. Munro (Cambridge, 1886). 1 poets first sang upon the earth. Modern Science has along with the theory that the Earth dated its beginning with the advent of man, swept utterly away this beautiful imagining. We can, indeed, find no beginning of the world. We trace back events and come to barriers which close our vista--barriers which, for all we know, may for ever close it. They stand like the gates of ivory and of horn; portals from which only dreams proceed; and Science cannot as yet say of this or that dream if it proceeds from the gate of horn or from that of ivory. In short, of the Earth's origin we have no certain knowledge; nor can we assign any date to it. Possibly its formation was an event so gradual that the beginning was spread over immense periods. We can only trace the history back to certain events which may with considerable certainty be regarded as ushering in our geological era.