waves was disturbed by the bottom near the edge of the deep water marking the ancient boundary of the lake, sand-bars would be deposited there as we find them, and these would stretch across the mouths of the submerged river-valleys, and on further uplifting they would separate the waters occupying them from those of the great lake, which, meanwhile, would go on adding more sand to them from without. This is the condition of things existing at present. The changes of level that have brought it about have not been uniform and constant; they may have consisted of a single sinking and rising, but more probably there were many. Even yet we see that the solid seeming earth is sinking and swelling there in a most capricious manner. It is hard to tell to what the present movements are tending even—whether for a long period the land is to remain substantially at its present level, whether it is to rise until the river-lakes are drained and the Western Michigan lake-ports are left stranded inland, or whether the country is to be again submerged. We see, within the memory of those now living there, a variation of level to the extent of six feet at least, and in both directions. Forty years ago the land seems to have been at a higher level than it is at present, and to have continued so long enough to permit the growth of large trees on land since submerged. Then there was a subsidence to an extent of several feet, then an uplift until the waters were below their present level, and at last accounts another subsidence seemed to be in progress. Who can tell us its limits, either as to time of continuance, rapidity, or extent? What is the nature of this movement? There are difficulties in the way of accounting for it that would not exist if Lake Michigan were the ocean. A rising and falling of the land as a whole would include the bed of the lake, and would not produce these changes of relative level. To lift the bed of Lake Michigan, might pour out a part of its contents, and so cause an enormous increase in the volume of the St. Clair, Detroit, and St. Lawrence Rivers, with a corresponding diminution when a subsidence was taking place, the rivers rising as the lake was going down, and falling as the waters of the lake were rising; but this, we believe, has not taken place. Is it a shrinking and swelling of the upper strata of Western Michigan, leaving the deeper strata in which the bed of the lake rests comparatively undisturbed? Is it a rocking of the lake-bed from side to side, one part sinking as another rises? What is the extent of the country through which these movements are felt? These questions, and others relating to the matter, would seem to be of interest. Perhaps, if the Government would take the subject in hand and cause a record to be kept of the waterlevel at all light-houses and life-saving stations, a few years might throw light upon it.
Page:Popular Science Monthly Volume 24.djvu/850
THE POPULAR SCIENCE MONTHLY.