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POPULAR SCIENCE MONTHLY 



estimate. Of course, tins rate for the crumbling away of the JSiagara 

 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 jSTiagara limestone at tlie mouth of the gorge can never 



Fig. 5. — Showing extent of erosiuu at \>-mv oC tin- Nin 

 (See description in the text.) 



since 1^54. 



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 Avith every successive 

 century. The actual progress of the enlargement would thus be 

 periodic, and not capable of measurement by decades; bnt after cen- 

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

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

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



