How this first great glacial period came on, or how long it lasted, we do not know, unless Croll's astronomical theory, which will be considered later, affords a clue. But we know generally that it must have lasted for an immense time from the amount of work done and the changes which took place. The ice, which covered so great a portion of the northern hemisphere, was not a polar ice-cap, but spread outwards in all directions from great masses of elevated land, as is proved conclusively from the direction of the striæ which were engraved by it on the subjacent rocks. This land must have been more elevated than at present, so as to rise, like Greenland, far into the region of perpetual snow, where all rain falls and accumulates in the solid form; and also to supply the enormous mass of _débris_ which the ice-caps and glaciers left behind them. It is not too much to say that a million of square miles in Europe, and more in North America, were covered by the _débris_ of rocks ground down by these glaciers, and often to great depths. Most of the _débris_ of the first glaciation have been removed by denudation, or ploughed out by the second great advance of the ice, leaving only the larger and harder boulders to testify to their extent; but enough remains to show that the first series of boulder-clays and drifts must have been on a scale larger than those of the second and subsequent glaciations, which now form the superficial stratum of so much of the earth's surface, and often attain a depth of several hundred feet. Wright, in his _Ice Age in North America_, estimates that "not less than 1,000,000 square miles of territory in North America is still covered with an average depth of fifty feet of glacial _débris_."