Even the continuous dropping of water will in time wear away the hardest rock. Nothing in nature is absolutely still. Motion seems an inherent property of matter. In some cases this motion which seems immovable is greater than we have any idea of. It is said that the recession of Niagara Falls proceeds at a rapid rate, the falls having receded no less than thirty-five feet in thirty three years, while the centre of the Horseshoe cataract has gone back about 160 feet in the same period. In Lyell's "Antiquity of Man," a very important argument bearing on the duration of man's existence on the earth is based on the rate of recession deducible from many observations.