154 POPULAR SCIENCE MONTHLY. 



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 supposi- 

 tion that, when the eastern drainage over the Niagara channel be- 

 gan, 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 in- 

 definite 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 mate- 

 rial 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 Zoological Society 

 by Mr. A. Smith Woodward and Dr. F. P. Moreno. 



