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POPULAR SCIENCE MONTHLY. 



estimate. Of course, this 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 Magara 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 fol- 

 low 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 car- 

 ried 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 cen- 

 turies the progress would be clearly marked, and especially when- 

 ever there was a falling away of the lower stratum of compact Medina 

 sandstone, which is about two hundred feet below the top, would 



