Page:Popular Science Monthly Volume 49.djvu/24

12 —The modern rate of retreat of the cataract during forty-eight years has been determined by comparing the crest of the falls, carefully mapped by four surveys—the first by Prof. James Hall in 1842 and the last by Mr.



Augustus S. Kibbe in 1890, Between the times of these surveys not merely the historic Table Rock, but six acres of rocks forming the floor of the river fell away by the undermining action of the falls, and the center of the cataract moved upward for a distance of two hundred and twenty feet. From the 275,400 square feet thus removed it is found that the mean annual recession has been four feet and a sixth a year for the Canadian falls and two thirds of a foot for the American cataract. The recession is shown in Fig. 12.

The work of the falls is not uniform, for there are years of rapid central recession and slow lateral expansion, followed by even a total central rest and rapid lateral enlargement of the curve. From an approximate estimate of the variation in the amount of work due to the physical and geological structure of the district, the mean rate of recession of the falls under existing volume of water and descent of the river may perhaps be reduced to 3·75 feet a year, which factor alone would indicate the age of the cataract to be ten thousand years. But this simple story would leave out of consideration the variability of the volume of Niagara River and the descent of the cascades.

—At the close of the ice age, and after the geological