CHAPTER IX.

THE GLACIAL PERIOD AND CROLL'S THEORY.

Causes of Glacial Periods--Actual Conditions of existing Glacial Regions--High Land in High Latitudes--Cold alone insufficient--Large Evaporation required--Formation of Glaciers--They flow like Rivers--Icebergs--Greenland and Antarctic Circle--Geographical and Cosmic Causes--Cooling of Earth and Sun, Cold Spaces in Space, and Change in Earth's Axis, reviewed and rejected--Precession alone insufficient--Unless with High Eccentricity--Geographical Causes, Elevation of Land--Aërial and Oceanic Currents--Gulf Stream and Trade Winds--Evidence for greater Elevation of Land in America, Europe, and Asia--Depression--Warmer Tertiary Climates--Alps and Himalayas--Wallace's _Island Life_--Lyell--Croll's Theory--Sir R. Ball--Former Glacial Periods--Correspondence with Croll's Theory--Length of the different Phases--Summary--Croll's Theory a Secondary Cause--Conclusions as to Man's Antiquity 293