And such has been the origin of the deep clay, which, with its included and accompanying boulders, covers so large a part of our country. When this arctic condition of things began, the land must have been slowly sinking beneath the sea; and so, as years rolled past, higher and yet higher zones of land were brought down to the sea-level, where floating ice, coming from the north-west, stranded upon the rocks, and scored them all over as it grated along. This period of submergence may have continued until even the highest peak of the Grampians disappeared, and, after suffering from the grinding action of ice-freighted rocks, eventually lay buried in mud far down beneath a wide expanse of sea, over which there voyaged whole argosies of bergs. When the process of elevation began, the action of waves and currents would tend greatly to modify the surface of the glacial deposit of mud and boulders, as the ocean-bed slowly rose to the level of the coast line. In some places the muddy envelope was removed, and the subjacent rock laid bare, all polished and grooved. In other localities, currents brought in a continual supply of sand, or washed off the boulder mud and sand, and then re-deposited them in irregular beds; hence resulted those local deposits of stratified sand and gravel so frequently to be seen resting over the boulder clay. At length, by degrees, the land emerged from the sea, yet glaciers still capped its hills and choked its valleys; but eventually a warmer and more genial climate arose, plants and animals, such as those at present amongst us, and some, such as the wolf, no longer extant, were ere long introduced; and eventually, as lord of the whole, man took his place upon the scene.[7] [Footnote 7: The reader who wishes to enter more fully into the geological effects of icebergs, should consult the suggestive section on that subject in De la Beche's _Geological Observer_; also the _Principles_ and _Visit to the United Stales_ of Sir Charles Lyell, with the various authorities referred to by these writers.]