I have said the Niagara River commenced its present gorge during the Champlain Epoch. In reality there was no Niagara River when this work commenced. Lake Erie stretched down the valley of the existing river, and the overflow of its basin wore the notch in the rocky rim which was the beginning of the Niagara River.

Lake Erie stands at present three hundred and thirtyfour feet above Lake Ontario. At the time of which I am speaking it stood three hundred and seventy-two feet above Lake Ontario, and filled the valley of Niagara River as far as the heights above Lewiston (Fig. 84).* Indeed, there are clear evidences, in the form of beaches containing freshwater shells, that the level of the river was once forty feet above the present summit of the falls. No barrier has ever existed to dam the water to this height except the escarpment at Lewiston. This is one hundred and five feet above the summit of the falls, and thirty-eight feet above Lake Erie. The indications seem to be conclusive that the waters of Lake Erie stood thirty-eight feet higher than at present, and poured over the bluff at Lewiston, in a series. of cascades, three hundred and seventy-two feet, to the sea, which at this time filled the basin of Lake Ontario. During the subsequent ages, the mighty stream has dug a gorge in the solid rock, which is seven miles long, two hundred and fifty feet deep, and, on an average, about one thousand feet wide. The material transported from this gorge into Lake Ontario is over three hundred and forty millions

* Explanation of Fig. 84.-The diagram on the following page is intended to illustrate the geological position of Niagara River and Falls, and the ancient lake levels from Lake Ontario to Chicago. The vertical scale is 560 feet to the inch; the horizontal scale is irregular. The diagram is merely a series of sections around the lakes, placed end to end. The dips of the strata are much exaggerated. The two portions of the diagram join each other along the line a, b, c, d, etc. The figures against the vertical dotted lines show the heights in feet above the sea of the points to which the lines extend.

of cubic yards, and weighed nearly seven billions of tons. The time consumed in the execution of this stupendous piece of engineering may be roughly calculated from the observed rate of recession of the falls. In 1842 Professor Hall executed a careful trigonometrical survey of the shorelines and landmarks of the falls. In 1855, twenty-three years later, M. Marcou made careful re-examinations, which he reported to the Geological Society of France. From these data it appears that the Canadian Fall, over which the largest body of water is discharged, has receded, by the wearing of the rocks, to the extent of twelve feet, or a little more than six inches a year. With this clew, we determine that the time required for the excavation of the entire distance from Lewiston is over seventy thousand years. This presumes the rate of recession has always been the same. The more I consider this subject the more I am impressed with a conviction that the rate of recession was formerly more rapid than during the last one hundred years. I am willing to reduce the time consumed to twenty thousand, or even to ten thousand years. Geologists most greedy of time ought to be satisfied with this when it is considered that this interval is but the unit in the arithmetic which calculates the time consumed in the revolutions of the globe. Before the beginning of the excavation of the great gorge, geological agencies had strewed the surface with drift-deposits, some of which had been transported hundreds of miles. Before the transportation of the drift, the basin of Lake Ontario had been scooped out, and the vast erosion of the escarpment at Lewiston had been effected. Before the period of the erosion was that of the solidification of the sediments; and still farther back, the incalculable intervals during which the sediments were accumulating five miles of thickness. At the commencement of the excavation of the gorge, the fauna which populated

the region was essentially the same as now. If, in an interval of twenty or even of ten thousand years, little perceptible change has taken place in the populations of the globe, how vast a period must have elapsed during the progress of organic mutations which have twenty times resulted in the almost complete extinction of existing forms, and their replacement by beings of other types!

more.

I said that the level of Lake Erie was once at the top of the heights of Lewiston, thirty-eight feet above its present altitude. This elevation submerged the flats to the east and west of the Detroit and St. Clair Rivers, and united Lake Erie with Lake Huron by a shallow expanse of water, which in some places possessed a breadth of fifty miles or Still farther, the level of Lakes Huron and Michigan was raised twenty-five feet above their present altitude, and a portion of the waters of the upper lakes found an outlet from Lake Michigan into the Des Plaines River, and thence into the Illinois and the Mississippi-if, indeed, a large portion of the prairie region of Illinois was not submerged by such an altitude of the lakes. At the same time, Saginaw Bay of Lake Huron stretched into the centre of the peninsula of Michigan.

This is not the highest altitude at which the waters of the lakes have stood, though the barriers which dammed them have long since disappeared. Along the southern. borders of Lakes Erie and Ontario, the rocks arise from their more southern depressions, and face the lakes in bold escarpments three hundred and fifty feet above the respective levels of the waters. These bluffs have been the rocky shores of the lower lakes. For unnumbered ages the furious north wind has rolled mad waves against those adamantine walls, and battlement after battlement has tumbled down and been ground to powder by the tireless beating of the stormy surge. Between the foot of the mural

escarpment and the present margin of the lake is a series. of parallel terraces, each showing the altitude at which the receding waters have made a pause. These terraces along the southern shore of Lake Erie range from ninety to one hundred and twenty feet above the present level of the water. In Huron and Sandusky Counties the subsidence of the escarpment permits them to diverge a greater distance from the lake. Curving northward into Michigan, they pass through Monroe, Wayne, and Oakland Counties, and continue in that direction. They are recognized again. on the shores of Lake Michigan.

At Mackinac Island are deeply engraved records of a higher level of the waters. The island itself is but a vestige of an ancient formation which once filled the straits, and joined the highlands on the west and south. It towers, a monument of the destructive agencies of geogony, three hundred and eighteen feet above the lake. The principal plateau of the island is one hundred and fifty feet above the lake. In the midst of this rises "Sugar Loaf," one

hundred and thirty

four feet higher (Fig.

85). This is a remnant of the rocks forming the upper plateau which once extended over the whole island. It is a conical mass of brecciated limestone, all whose sides bear the marks of the longcontinued action of the waves. On one side an ancient grotto

Fig. 85. View of Sugar Loaf, Mackinac Island. has thus been exca