estimate. Of course, tins rate for the crumbling away of the Niagara shale on its fresh exposure is much in excess of the average rate for a long period of time; but it is clear that the rate of erosion at the base of the Niagara limestone at the mouth of the gorge can never
Fig. 5.—Showing extent of erosion at base of the Niagara shale since 1854.
(See description in the text.)
have been sufficiently slow to reduce the total average much below the assumed rate of a quarter of an inch a year.
To impress the truth of this statement it is only necessary to follow the progress, in imagination, of the crumbling process which has brought the side of the gorge to its present condition. At first the face of the gorge was perpendicular, the plunging water making the gorge as wide at the bottom as at the top. At successive stages the strata of shale on the side would crumble away, as is shown in our photograph, and undermine the strata of hard rock. The large fragments would fall to the bottom, and, being too large to be carried away by the current, would form the talus to which we have already referred, which would grow in height with every successive century. The actual progress of the enlargement would thus be periodic, and not capable of measurement by decades; but after centuries the progress would be clearly marked, and especially whenever there was a falling away of the lower stratum of compact Medina sandstone, which is about two hundred foot below the top, would
