Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric Theory.—Helmholtz’s Condensation Theory.—Confusion of Ideas.—Gravitation not the chief Source of the Sun’s Heat.—Original Heat.—Source of Original Heat.—Original Heat derived from Motion in Space.—Conclusion as to Date of Glacial Epoch.—False Analogy.—Probable Date of Eocene and Miocene Periods 346 CHAPTER XXII. A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY ROCKS OF THE GLOBE. Prevailing Methods defective.—Maximum Thickness of British Rocks.—Three Elements in the Question.—Professor Huxley on the Rate of Deposition.—Thickness of Sedimentary Rocks enormously over-estimated.—Observed Thickness no Measure of mean Thickness.—Deposition of Sediment principally along Sea-margin.—Mistaken Inference regarding the Absence of a Formation.—Immense Antiquity of existing Oceans 360 CHAPTER XXIII. THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND DURING THE GLACIAL EPOCH. Displacement of the Earth’s Centre of Gravity by Polar Ice-cap.—Simple Method of estimating Amount of Displacement.—Note by Sir W. Thomson on foregoing Method.—Difference between Continental Ice and a Glacier.—Probable Thickness of the Antarctic Ice-cap.—Probable Thickness of Greenland Ice-sheet.—The Icebergs of the Southern Ocean.—Inadequate Conceptions regarding the Magnitude of Continental Ice 368 CHAPTER XXIV. THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND DURING THE GLACIAL EPOCH.—(_Continued._) Extent of Submergence from Displacement of Earth’s Centre of Gravity.—Circumstances which show that the Glacial Submergence resulted from Displacement of the Earth’s Centre of Gravity.—Agreement between Theory and Observed Facts.—Sir Charles Lyell on submerged Areas during Tertiary Period.—Oscillations of Sea-Level in Relation to Distribution.—Extent of Submergence on the Hypothesis that the Earth is fluid in the Interior 387 CHAPTER XXV. THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE AND ON THE LEVEL OF THE SEA.