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 feet below the top, would a new cycle of rapid disintegrations in the superincumbent strata follow.
Fig. 6.—Section, drawn to equal vertical and horizontal scale, showing enlargement of Niagara gorge on the east side at its mouth at Lewiston: 1, Niagara limestone, 20 to 30 feet; 2, Niagara shale, 70 feet; 3, Clinton limestone, 20 to 30 feet; 4, Clinton and Medina shale, 70 feet; 5, Quartzose Medina sandstone, 20 to 30 feet; 6, softer Medina sandstone, 120 feet above water level.
An important point to be noticed, and which is evident from two of the reproduced photographs (Figs. 3 and 4), is that the talus has never reached up so high as to check the disintegration at the mouth of the gorge of the Niagara shale and limestone which form the upper one hundred feet of the face, and which exhibit the maximum amount of enlargement which has taken place. The thickness of the Niagara limestone is here so small that it has not been so important an element in forming the talus as it has been farther up the stream, where it is two or three times as thick. Now, while our original supposition was that one quarter of an inch annually was eroded from the upper two hundred feet, this would involve the erosion of a half inch per annum over the top of the gorge to bring the calculation within the limit of ten thousand years. It certainly is difficult for one who examines the facts upon the ground to believe that the crumbling away of this exposed Niagara shale could have been at any less rate than that; so that the estimate of about ten thousand years for the date of that stage of the Glacial period in which Niagara River first began its work of erosion at Lewiston (an estimate which is supported by a great variety of facts independent of those relating to the Niagara gorge) is strongly confirmed by this new line of evidence.
So far as I can see, the only question of serious doubt that can be raised respecting this calculation will arise from the possible supposition that, when the eastern drainage over the Niagara channel began, the land stood at such a relatively lower level as would reduce the height of the fall to about half that of the present escarpment at that point; when it might be supposed that a protecting talus had accumulated which would interrupt the lateral erosion for the indefinite period when the drainage was being drawn around by way of the recently opened Lake Nipissing and Mattawa outlet. Then, upon the resumption of the present line of drainage, with the land standing at nearly its present level, the talus may have been undercut, and so fallen down to leave the upper strata exposed as at present. But there does not seem to be sufficient warrant for such a supposition to make it necessary seriously to entertain it, while the objections to it are significant and serious. First, the present narrowness of the river at the water level is such that it does not give much opportunity for enlargement after the first formation of the gorge; secondly, the Niagara limestone at the mouth of the gorge is so thin (stated by Hall to be twenty feet thick) that it would not form a protecting talus, even at half its present height.
P. S.—Since the above was written there has been reported in the papers an immense fall of rock from the east side of the gorge, near the head of the Whirlpool rapids. The estimate made of the amount is one hundred thousand tons. If that estimate is correct, it is a very impressive illustration of how the average fall of material from the side of the gorge is occasionally increased by a single instance. In making our calculations above, the total amount of material annually falling off from the portion of the side of the gorge under consideration amounted only to 1,237 tons, while the amount of material was 611 cubic yards. But the 100,000 tons which came off in a single slide a few weeks ago would be equal to twenty inches in thickness from the whole face of the cliff, where our estimate was only a quarter of an inch.
N. B.—In the diagram (Fig. 6) extend the Niagara shale (2) up to occupy lower two layers of (1), thus making Niagara limestone (1) half as thick as now.
A piece of skin which the authors maintained to be of great antiquity and to have belonged to the extinct mylodon or ground sloth, found in a cave in Patagonia, was recently exhibited to the London Zoölogical Society by Mr. A. Smith Woodward and Dr. F. P. Moreno.