PREFACE

It is perhaps a little indelicate to ask of our Mother Earth her age, but Science acknowledges no shame and from time to time has boldly attempted to wrest from her a secret which is proverbially well guarded. On January 30th, 1911, I placed before the Natural History Society of the Royal College of Science a brief account of some of these attempts, and out of that paper this little book has gradually developed. In the present survey of the subject—the measurement of geological time—I have endeavoured to give as full and complete an account of the chief methods of attack as space would allow. My particular object has been to draw attention to their respective limitations and to test the validity of the various assumptions on which they are based. So far, the most remarkable feature of the problem lies in the extraordinary discrepancy between the conclusions drawn from the two most prominent methods of dealing with it. I have sought to mitigate the severity of this disagreement by discussing the possibility and consequences of reconciliation and by suggesting a path along which reconciliation may be found. If my treatment should stimulate a greater interest in the time problem, or provide material for further discussion, it cannot fail to bring nearer the ultimate solution of a question which for more than a century has had an unrivalled notoriety for provoking controversy.

I desire to acknowledge my gratitude to Prof. J. Joly and the Council of the Royal Dublin Society for permission to use the admirable series of micro-photographs which constitute the Frontispiece; to Prof. P. Lowell for his photographs of the Polar Caps of Mars; and to Sir Wm. Crookes and the Council of the Royal Society for the photograph and radiograph of a piece of pitchblende.

To a number of friends who have helped me in revising the proof-sheets, and to others who have turned my rough sketches into finished diagrams, I wish to tender my thanks.

Finally, I have a special pleasure in expressing my gratitude for the valuable assistance which has been afforded me by my friend Mr. R. W. Lawson, of Armstrong College, Newcastle-upon-Tyne. While the manuscript was in preparation I had the benefit of his suggestions and friendly criticism, and at a later stage he kindly verified many of my calculations. I am indebted to him in particular for the mathematical treatment of the final section of Appendix A (dealing with the uranium time-average) and for the two diagrams with which it is illustrated.

Arthur Holmes.

Geological Department, Imperial College, S.W. December 1st, 1912.

CONTENTS

page

Preface ix

List of Illustrations xii

chap.

I. The Time Problem and its History 1

II. Time and Tide in the Solar System 22

III. Astronomical Considerations 32

IV. The Work of Denudation 48

V. The Salinity and Age of the Oceans 61

VI. Sedimentation and Geological Time 76

VII. Radioactivity 91

VIII. The Thermal Energy of the Sun 110

IX. The Thermal Energy of the Earth 122

X. Radioactive Minerals and their Ages 137

XI. Review of the Evidence 166

Appendix A 177

Appendix B 184

Index 191

LIST OF ILLUSTRATIONS

Figures printed as plates

Fig. 1 Pleochroic Haloes Frontispiece

Fig. 2

Fig. 3

Fig. 4

facing page

Fig. 5 Spiral Nebulæ 28

Fig. 6

Fig. 7 Polar Caps of Mars 39

Fig. 8

Fig. 12 Photographs of Pitchblende 92

Fig. 13

Figures printed in text

page

Fig. 9. Glacial Movement in S. Scandinavia 40

Fig. 10. Upper Cretaceous Strata of Colorado 45

Fig. 11. Section illustrating the Formation of

Sediments on Continental Shelf 83

Fig. 14. Apparatus for Estimating Radium 106

Fig. 15. Pleochroic Haloes of the Uranium Family 108

Fig. 16. Temperature Gradients 127

Fig. 17. Geological Time Scales 172

Appendix A—

Graph I 180

Graph II 181

Table of Radioactive Elements 190

THE AGE OF THE EARTH