"Secondly, a period of submergence, by which the land north of the Thames and Bristol Channel, and that of Ireland, was gradually reduced to an archipelago; and finally to such a general prevalence of sea as is seen in map, Fig. 39." (This map is intended to represent the British Isles as they appeared above water when Scotland was submerged to two thousand feet and other parts of the isles to one thousand three hundred feet.) "This was the period of submergence and of floating ice, when the Scandinavian flora, which occupied the lower grounds during the first continental period, may have obtained exclusive possession of the only lands not covered with perpetual snow. "Thirdly, a second continental period, when the bed of the glacial sea, with its marine shells and erratic blocks, was laid dry, and when the quantity of land equalled that of the first period.... During this period there were glaciers in the higher mountains of Scotland and Wales.... "The submergence of Wales to the extent of one thousand four hundred feet, as proved by glacial shells, would require fifty-six thousand years, at the rate of two and a half feet per century; but taking Professor Ramsay's estimate of eight hundred feet more, that depression being required for the deposition of some of the stratified drift, we must demand an additional period of thirty-two thousand years, amounting in all to eighty-eight thousand; and the same time would be required for the reëlevation of the tract to its present height. But if the land rose in the second continental period no more than six hundred above the present level ... this ... would have taken another twenty-six thousand years; the whole of the grand oscillation, comprising the submergence and reëmergence, having taken, in round numbers, two hundred and twenty-four thousand years for its completion; and this, even if there were no pause or stationary period, when the downward movement ceased, and before it was converted into an upward one."[9] Lyell admits that the average rate of two and a half feet per century is a purely arbitrary and conjectural one, and there are cases where the change is even six feet a century, yet the average rate of motion, he thinks, will not exceed that above proposed. With this opinion, Lubbock believes most geologists will agree.[10]