It is evident, therefore, that the phenomena presented by the varying polar snows of Mars are in no way opposed to that modification of Dr. Croll's theory of the conditions which brought about the glacial epochs of our northern hemisphere, which is here advocated; but are perfectly explicable on the same general principles, if we keep in mind the distinction between an ice-sheet--which a summer's sun cannot materially diminish, but may even increase by bringing vapour to be condensed into snow--and a thin snowy covering which may be annually melted and annually renewed, with great rapidity and over large areas. Except within the small circles of perpetual polar snow there can at the present time be no ice-sheets in Mars; and the reason why this permanent snowy area is more extensive around the northern than around the southern pole may be partly due to higher land at the north, but is perhaps sufficiently explained by the diminished power of the summer sun, owing to its greatly increased distance at that season in the northern hemisphere, so that it is not able to melt so much of the snow which has accumulated during the long night of winter. * * * * * {58} CHAPTER IX ANCIENT GLACIAL EPOCHS, AND MILD CLIMATES IN THE ARCTIC REGIONS Dr. Croll's Views on Ancient Glacial Epochs--Effects of Denudation in Destroying the Evidence of Remote Glacial Epochs--Rise of Sea-level Connected with Glacial Epochs a Cause of Further Denudation--What Evidence of Early Glacial Epochs may be Expected--Evidences of Ice-action During the Tertiary Period--The Weight of the Negative Evidence--Temperate Climates in the Arctic Regions--The Miocene Arctic Flora--Mild Arctic Climates of the Cretaceous Period--Stratigraphical Evidence of Long-continued Mild Arctic Conditions--The Causes of Mild Arctic Climates--Geographical Conditions Favouring Mild Northern Climates in Tertiary Times--The Indian Ocean as a Source of Heat in Tertiary Times--Condition of North America During the Tertiary Period--Effect of High Excentricity on Warm Polar Climates--Evidences as to Climate in the Secondary and Palaeozoic Epochs--Warm Arctic Climates in Early Secondary and Palaeozoic Times--Conclusions as to the Climates of Secondary and Tertiary Periods--General View of Geological Climates as Dependent on the Physical Features of the Earth's Surface--Estimate of the Comparative Effects of Geographical and Physical Causes in Producing Changes of Climate.