Page:Popular Science Monthly Volume 57.djvu/449

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Scientific Literature.


In accordance with the general results of Mr. G. K. Gilbert's investigation of recent earth movements in the Great Lakes region—that the whole district is being lifted on one side or depressed on the other, so that its plane is bodily canted toward the south-southwest, and that the rate of change is such that the two ends of a line one hundred miles long, running in a south-southwest direction, are relatively displaced four tenths of a foot in one hundred years—certain general consequences ensue. The waters of each lake are gradually rising on the southern and western shores, or falling on the northern and eastern shores, or both. This change is not directly abvious, because masked by temporary changes due to inequalities of rainfall and evaporation and various other causes, but it affects the mean height of the lake surface. In Lake Ontario the water is advancing on all shores, the rate at any place being proportional to its distance from the isobase through the outlet. At Hamilton and Port Dalhousie it amounts to six inches in a century. The water also advances on all shores of Lake Erie, most rapidly at Toledo and Sandusky, where the change is eight or nine inches a century. All about Lake Huron the water is falling, most rapidly at the north and northeast; at Mackinac the rate is six inches, and at the mouth of French River ten inches a century. On Lake Superior the isobase of the outlet cuts the shore at the international boundary; the water is advancing on the American shore, and sinking on the Canadian. At Duluth the advance is six inches, and at Huron Bay the recession is five inches a century. The shores of Lake Michigan are divided by the Port Huron isobase. North of Oconto and Manistee the water is falling; south of these places it is rising, the rate at Milwaukee being five or six inches a century, and at Chicago nine or ten inches. Eventually, unless a dam is erected to prevent it, Lake Michigan will again overflow to the Illinois River, its discharge occupying the channel carved by the outlet of a Pleistocene glacial lake. The summit in that channel is now about eight feet above the mean level of the lake, and the time before it will be overtopped may be computed. For the mean lake stage such discharge will begin in about one thousand years, and after fifteen hundred years there will be no interruption. In about two thousand years the Illinois River and the Niagara will carry equal portions of the surplus water of the Great Lakes. In twenty-five hundred years the discharge of the Niagara will be intermittent, failing at low stages of the lake, and in thirty-five hundred years there will be no Niagara. The basin of Lake Erie will then be tributary to Lake Huron, the current being reversed in the Detroit and St. Clair channels.


Relating to the Royal Geographical Society the story of his exploration of the Bolivian Andes, Sir Martin Conway spoke of his journey by way of the Arequipa Railroad, Peru, to Lake Titicaca. That remarkable sheet of water is fourteen times the size of the Lake of Geneva and twelve thousand feet above the sea, and might be regarded as the remnant of a far greater inland sea, now shrunk away. Driving from Chililaya, he reached the snowy mountain called the Cordillera Real—the backbone of Bolivia—which he had come especially to visit, and in the region of which he spent four months. To the east the mountains fell very rapidly to a low