The estimate is that nine hundred millions of cubic feet of water pour over Niagara every hour, and great as this mass is, there is a belief that half the water passing into Lake Erie from the upper lakes does not go over the falls, but finds its way into Ontario through a subterranean channel. Nothing demonstrates this theory, but it is advanced to account for the difference between the amount of water accumulated in the upper lakes and that going over the falls. The actual current is sufficiently enormous, however, and steadily wearing away the rocks over which it descends, it has during the past ages excavated the gorge of the lower rapids. The land surface, which is low at Lake Erie, scarcely rising above the level of its waters, gradually becomes more elevated towards the north, till near Lewiston it is about forty feet above Erie. The Niagara River thus flows in the direction of the ascent of this moderately inclined plane. Beyond this the surface makes a sudden descent towards Lake Ontario of about two hundred and fifty feet down to a plateau, upon which stands Lewiston on the American side and Queenston on the Canadian side of the river. There thus is formed a bold terrace looking out upon Ontario, from which that lake is seven miles away, and from the foot of the terrace the surface descends gently one hundred and twenty feet farther to the lake shore. The gorge through which the river flows is three hundred and sixty-six feet deep at this terrace. There is no doubt the first location of the great cataract was on the face of the terrace near Lewiston, and it has gradually retired by the eating away, year after year, of the rocky ledges over which the waters pour. This, however, has not been done in a hurry, for the geologists studying the subject estimate that it has required nearly thirty-seven thousand years to bring the falls from Lewiston back to their present location. In fact, from the stratification, Professor Agassiz expressed the opinion that at one time there were three distinct cataracts in Niagara River.