CHAPTER VIII. 



THE CAUSES OF GLACIAL EPOCHS. 



Various suggested causes — Astronomical causes of changes of Climate — ■ 

 Difference of Temperature caused by varying distance of the Sun — 

 Properties of air and water, snow and ice, in relation to Climate — 

 Effects of snow on Climate — High land and great moisture essential to 

 the initiation of a Glacial Epoch — Perpetual snow nowhere exists on 

 lowlands — Conditions determining the presence or absence of perpetual 

 Snow — Efficiency of Astronomical causes in producing Glaciation — 

 Action of meteorological causes in intensifying Glaciation — Summary 

 of causes of Glaciation — Effect of clouds and fog in cutting off the 

 Sun's heat — South Temperate America as illustrating the influence of 

 Astronomical causes on Climate — Geographical changes how far a 

 cause of Glaciation — Land acting as a barrier to ocean-currents — The 

 theory of interglacial periods and their probable character — Probable 

 effect of winter in aphelion on the climate of Britain — The essential 

 principle of climatal change restated — Probable date of the last 

 Glacial Epoch — Changes of the sea-level dependent on Glaciation — The 

 planet Mars as bearing on the theory of excentricity as a cause of 

 Glacial Epochs. 



No less than seven different causes have been at various times 

 advanced to account for the glacial epoch and other changes of 

 climate which the geological record proves to have taken place. 

 These, as enumerated by Mr. Searles V. Wood, Jun., are as 

 follows : — 



1. A decrease in the original heat of our planet. 



2. Changes in the obliquity of the ecliptic. 



3. The combined effect of the precession of the equinoxes 

 and of the excentricity of the earth's orbit. 



