During the brief time observations have been made, great fragments of rocks have been repeatedly carried down by the current pouring over Niagara, the frosts assisting disintegration. This caused not only a recession but decided changes in appearance. Since 1842 the New York State geologists, who then made a careful and accurate topographical map, have been closely watching these changes, and the average rate of recession is estimated at slightly over two feet annually. In Father Hennepin's sketch of 1678 there was a striking feature, since entirely disappeared, a third fall on the Canadian side facing the line of the main cataract, and caused by a large rock turning the diverted fall in this direction, this rock falling, however, in the eighteenth century. The rate at which recessions occur is not uniform. No change may be apparent for several years, and the soft underlying strata being gradually worn away, great masses of the upper and harder formations then tumble down, causing in a brief period marked changes. At the present location of the cataract, sheets of hard limestone cover the surface of the country, and from the top of the falls to eighty or ninety feet depth. Shaly layers are under these. All the strata slope gently downward against the river current at the rate of about twenty-five feet to the mile. Above the falls, in the rapids, the limestone strata are piled upon each other, until about fifty feet more are added to the formation, when they all disappear under the outcropping edges of the next series above, composed of marls and shales. Through these piles of strata the cataract has worked its way back, receding probably most rapidly in cases where, as at present, the lower portion of the cutting was composed of soft beds of rock, which being hollowed out and removed by frost and water, let down the harder strata above. The effect of continual recession must be to diminish the height of the falls, both by raising the river level at their base and by the sloping of the surmounting limestone strata to a lower level. A recession of two miles farther, the geologists say, will cut away both the hard and the soft layers, and then the cataract will become almost stationary on the lower sandstone formation, with its height reduced to about eighty feet. This diminution in the Niagara attractions might be startling were it not estimated that it can hardly be accomplished for some twelve thousand years. APPEARANCE OF NIAGARA.