Page:The American Cyclopædia (1879) Volume XII.djvu/432

418 NIAGARA Three miles below the falls the river, bending toward the Canadian side and contracted to a width of about 220 ft., rushes violently into a deep depression in the steep cliff on that side, from which it emerges, turning back almost at a right angle into the American side. This depression presents the appearance of having been hollowed out by a great eddy or whirl of the waters, and is known as u the whirlpool." It seems to be a portion of the bed of an an- cient channel, now filled with diluvium. The surface of the country, which at the foot of Lake Erie is low, scarcely rising above the level of its waters, gradually becomes more elevated toward the north, till near Lewiston it is 38 ft. higher than Lake Erie. The course of the Niagara river is thus in the direction of the ascent of this inclined plane, and must originally have been induced by a depression in the surface. Beyond this there occurs a sud- den descent toward Lake Ontario of 250 ft., down to the plateau upon which stands on the American side of the river the village of Lew- iston, and on the Canadian that of Queenston. The high land forms a bold terrace looking out upon Lake Ontario, from which it is 7 m. dis- tant, and from its foot the surface descends so gently to the lake that the fall, amounting to 120 ft., is hardly perceptible. The fall made by the river in this lowest portion of its course is only 4 ft., not impeding navigation. The gorge through which the Niagara river flows be- low the falls, amounting at the terrace to about 366 ft. in depth, bears evidence of having been excavated by the river itself. In the short period, hardly reaching back into the last cen- tury, during which observations other than those of passing travellers have been made and preserved, changes have taken place by the falling down of masses of rock, the effect of which has been to cause a slight recession of the cataract, and extend the gorge to the same amount upward toward Lake Erie. Thus in 1818 great fragments descended at the Amer- ican fall, in 1828 at the Horse-shoe fall, and since 1855 several others, which have mate- rially changed the aspect of the falls. Table rock, once a striking feature, has wholly dis- appeared. Previous to the careful trigono- metrical survey made in 1842 under the direc- tion of Prof. James Hall, for the state geologi- cal survey, no marks nor monuments had been fixed, by which the rate of recession could be ascertained. The results of this survey, with the map of the falls, are presented in the state geological report of Prof. Hall. In this report is a facsimile of a view of the falls by Father Hennepin, made in 1678. This sketch and the accompanying description present a striking feature in the falls which has now entirely disappeared ; this is a third fall from the Cana- dian side toward the east, facing the line of the main fall, and caused by a great rock that turned the divided current in this direction. In 1750 the falls were visited by Kalm, a Swe- dish naturalist, whose description and view were published in the " Gentleman's Maga- zine" in 1751; he alludes to the rock having fallen down a few years previous, and indicates the spot in his sketch. From such evidences it is certain that changes have been and are taking place, which have excavated the great gorge, and will carry it further up toward Lake Erie. But the rate at which these changes take place is not uniform. For several succes- sive years there will be no apparent change ; and then, the soft underlying strata having been gradually worn away, great masses of the upper and harder ones fall down, causing a very noticeable change in a very brief time. Taking long periods, the estimate of Lyell that the retrocession of the falls has been about a foot a year appears to be approximately correct. From the variable nature of the strata over which the river flows, the circumstances that affect the rapidity of the cutting action dif- fer all along its course. At the present site of the falls sheets of a hard limestone rock, of the formation known as the Niagara limestone, cover the surface of the country, and form the edge of the cataract to the depth of between 80 and 90 ft. Under this, extending to the foot of the fall, are the shaly layers of the same formation. All these strata slope downward against the current of the river at the rate of about 25 ft. in a mile ; and in the rapids above the fall the uppermost layers of the Niagara limestone succeed, one stratum above another, till about 50 ft. more is added to the thickness of the formation, when all disappear beneath the outcropping edges of the next series above, which is that of the shales and marls of the Onondaga salt group. In the other direction, toward Lake Ontario, these strata gradually rise to higher levels, till along the great terrace the capping is of the lower 20 ft. of the Niag- ara limestone, below which the shaly strata form the next 80 ft. of the steep slope ; and next appears a succession of calcareous layers, shales, and sandstones, belonging successively to the Clinton and Medina formations. Through these piles of strata the river has worked its way back, receding probably most rapidly where, as in its present position, the lower portion of the cutting was composed of soft beds, which being hollowed out let down the harder strata above ; and less rapidly where the strata near the base were hard sandstones, such as occur in some of the lower groups. The effect of continued recession must be to grad- ually diminish the height of the falls, both by the rising of the bed of the river at their base and by the slope of the massive limestone to a lower level. The thin-bedded limestones above being swept off, the succeeding shales and marls of the Onondaga salt group must immediately follow, and the falls may then become almost stationary, when their base is at the base of the massive sandstone and their upper line is as now over its upper edge. This, as shown by Prof. Hall, is likely to be the case after a further recession of about 2 m., and the height