The growing interest in the evolution of the continent now calls for more accurate information than formerly, regarding the changes of level of land and sea in recent geological times. As these oscillations constituted some of the most important factors in the building of the Great Lakes, the study of their history has contributed to our knowledge of the changing relations of the continent and the sea.
From investigation of the submerged channels along the American coast, it has been shown that the continent was greatly elevated during some epoch or epochs intervening between the middle Miocene and the early Pleistocene periods. The elevation of the land was over 3,000 feet higher than now, and probably reached for a short time to over 5,000 feet.*
This elevated condition of the continent was followed by a depression of the land to far below the present altitude before the . . .