In his memoir on “Modern Denudation,” Dr. A. Geikie determined approximately the number of years required by the following seven rivers to lower the general surface of their basins by one foot. He found that the Danube would take 6846 years, the Mississippi 6000, the Nith 4723, the Upper Ganges 2358, the Rhone 1528, the Hoang Ho 1464, and the Po 729, years. The average of these figures is 3378, and “this,” says Dr. Croll, in his recent interesting work on “Stellar Evolution” (p. 41), “gives a mean of 3378 years to remove one foot, or a little over one-half the time taken by the Mississippi.” As the mean rate of subaërial denudation is an important element in physical geology, it may be worth while to point out a slight mistake in this determination of its value, and also to show how a more correct estimate may, with our present data, be obtained.