It is not always easy to determine the position of each individual phase of the two glacial and the inter-glacial periods, for they must often be intermixed, and the results of the last glaciation and of subsequent denudation have to a great extent obscured those of the earlier periods. But taking a general view of the glacial period as a whole, there are a few leading facts which testify conclusively to its immense antiquity. First, there is the amount of elevation and depression. We have seen that marine Arctic shells have been found on the top of Moel-Tryfen, 1300 feet above the present sea-level. Nor is this an isolated instance, for marine drifts apparently of the same character have been traced on the mountains of Scotland, Wales, and Ireland to a height of between 2000 and 3000 feet. In Norway, also, old sea beaches are found up to a height of 800 feet. Nor are these great movements confined to the Old World or to limited localities. According to Professor Le Conte, at the last meeting of the Geological Congress at Washington, a great continental movement, commencing in the later tertiary and terminating in the beginning of the quaternary, caused changes of level amounting to 2500 or 3000 feet on both sides of the continent of North America. Now elevation and depression of large masses of land are, as far as we know anything certain about them, very slow processes, especially in countries unaffected by recent volcanic action, which is the case with nearly all the regions in North America and Europe which were covered by the great ice-sheets. There has been little or no perceptible change anywhere since the commencement of history, and the only accurate measurements of changes now going on are those made in Sweden, where it appears that in some cases elevation, and in others depression, is taking place at the rate of about two and a half feet in a century. In volcanic regions earthquakes have occasionally caused movements of greater amount in limited areas, but there is no trace of anything of the sort in these movements of the glacial period which have apparently gone on by slight secular changes in the earth's crust as they are now doing in Scandinavia. But in this case a depression of 2000 feet, followed by an elevation of equal amount, at Lyell's rate of two and a half feet per century, would require 160,000 years, without allowing for any pauses during the process. And this only embraces part of the whole glacial period, for the depression did not begin until after the climax of the first great glaciation, when the land probably stood higher than at present. Of course the actual movements may have been more rapid, but unless we resort to the exploded theories of cataclysms and catastrophes, the time for such movements must have been very great.