This work has gone on year after year and century after century, without being affected by either the volume of the river's waters or the shifting in the elevation of the land. The work of the atmospheric agencies in enlarging the mouth of the Gorge has had the effect of changing its shape from that of a rectangle, whose perpendicular sides were 340 feet, to a figure with a level base formed by the river, whose sides slope off at the same angle on each side. Now if it were possible to measure the rate at which this enlargement is taking place, the problem of determining the age of the river would be a more simple one. The relative thickness of the different layers of material forming the walls of the Gorge is not the same throughout; at the escarpment at Lewiston, the summit is found to consist of a stratum of Niagara limestone, about twenty-five feet thick. Beneath this layer of lime is to be found about seventy feet of Niagara shale. The Niagara shale rests upon a twenty foot layer of hard Clinton limestone, which in turn is supported by a shale seventy feet thick. Forming the base is twenty feet of hard Medina sandstone, beneath which is another sandstone which is much softer and much more susceptible to erosion and the disintegrating forces of the atmosphere. These thick layers of shale form the part upon which the atmospheric powers exert their energies, undermining the strata composed of material which with much more effect resists the attempt of any agency to break it down. As the shale is removed from beneath the harder layers immense masses of the latter fall and form a talus along the lower part of the cliff. This in brief is the manner in which the mouth of the Gorge is growing wider. The present width of the mouth of the Gorge at the water's level is 770 feet. It is not likely that the river was ever any wider than now at this point, since its narrowest portion is over 600 feet, and this where the hard layer of Niagara limestone is much thicker than at the mouth. The current here is comparatively weak, so that there has been little erosion due to it. On the contrary the falling masses of sandstone and limestone have probably encroached somewhat upon the ancient margin of the stream, its weak current being unable to sweep out these obstructions which have formed an effectual protection to the bank.