Here, then, is the composition and structure of our dam. It is thirty feet high, with a base two and a half miles certainly, and probably five, in width. How to break through it is the problem to be solved by the great inland sea which laves it, so that the water may flow onward and downward to the Atlantic. Fortunately we have, all along the shores of our inland lakes, an annual demonstration of the method by which such problems are solved. A constant abrasion of their banks is produced by the action of water, frost, and ice. And these are the resistless elements which, by their persistent and powerful action during the lapse of ages, excavated a channel for the waters of the Niagara. The gradual upward slope of the rock and the thick upper drift broke the force of the huge waves that were occasionally dashed upon them. Their position could not have been more favorable to resist attack. It was a Malakoff of earth on a foundation of rock. Little by little the refluent waves carried back portions of the crumbled mass, and deposited them in the neighboring depressions. Slowly, wearily, desultorily, the erosion and desquamation went on. At last the upper drift was broken down, and its crumbled remains were swept from the rock. Then the insidious forces of heat and cold, sun and frost became potent. The thin laminæ of limestone were loosened by the frost, broken up and disintegrated. At last a thin sheet of water was driven through the gorge by some fierce gale. The steep declivity of the counterscarp was then fatally attacked, and after a time its perpendicular face was laid bare. Thenceforth the elements had the top and one end of the rocky mass to work on, and they worked at a tremendous advantage. The breaking up and disintegration of the rock went on. It was gradually crumbled into sand, which was washed off by the rains or swept away by the winds. Finally a channel was excavated, of which the bottom was lower than the surface of the great lake above; the sparkling waters rushed in, dashed over the precipice, and Niagara was born. As the water worked its way over the precipice gradually, so it would gradually excavate its channel to Lake Ontario, and it is not probable that any great inundation of the lower terrace could have occurred. CHAPTER VII. Composition of the terrace cut through--Why retrocession is possible--Three sections from Lewiston to the Falls--Devil's Hole--The Medina group--Recession long checked--The Whirlpool--The narrowest part of the river--The mirror--Depth of the water in the chasm--Former grand Fall. The water having laid bare the face of the mountain barrier from top to bottom, we are enabled to examine the composition of the mass through which it slowly cut its way. After removing the thin plates of the upper stratum, as we descend, according to Professor Hall, we find: 1. Niagara limestone--compact and geodiferous. 2. Soft argillo-calcareous shale. 3. Compact gray limestone.