All of North America, to the north and northeast of this line, must have been covered by one vast sea of ice.5 Doubtless, as in Greenland to-day, there was no hill or patch of earth to be seen, simply one great field of ice. The ice was thick enough to cover from sight Mt. Washington, in New Hampshire, and must have been at least a mile thick over a large portion of this area,6 and even at its southern border it must in places have been from two hundred to two thousand feet thick.7 This, as we have seen, is a picture very similar to what must have been presented by Europe at this time.8 Antarctic Ice Sheet. The Northern Atlantic Ocean must have presented a dreary aspect. Its shores were walls of ice, from which ever and anon great masses sailed away as icebergs. These are startling conclusions. Yet, in the Southern Hemisphere to-day is to be seen nearly the same state of things. It is well-known that all the lands around the South Pole are covered by a layer of ice of enormous thickness. Sir J. A. Ross, in attempting to reach high southern latitudes, while yet one thousand four hundred miles from the pole, found his further progress impeded by a perpendicular wall of ice one hundred and eighty feet thick. He sailed along that barrier four hundred and fifty miles, and then gave up the attempt. Only at one point in all that distance did the ice wall sink low enough to allow of its upper surface being seen from the mast-head. He describes the upper surface as an immense plain shining like frosted silver, and stretching away as far as eye could reach into the illimitable distance.9 The foregoing makes plain to us one phase of the Glacial Age. Though it may not be quite clear what this has to do with the antiquity of man, yet we will see, in the sequel, that it has considerable. As to the periods of mild climate that are thought by some to have broken up the reign of cold, we do not feel that we can say any thing in addition to what has been said in a former chapter.10 We might, however, say, that the sequences of mild and cold climate are not as well made out in America as they seem to be in Europe; or at least our geologists are more cautious as to accepting the evidence as sufficient. And yet such evidences are not wanting: as in Europe, at various places, are found layers of land surfaces with remains of animals and plants, but both above and below such surface soil are found beds of bowlder clay. These offer undeniable evidence that animals and plants occupied the land during temperate inter-glacial epochs, preceded and followed by an Arctic climate, and ice-sheets like those now covering the interior of Greenland, and the Antarctic Continent.11