THE HISTORY OF THE NIAGARA RIVER. 247 



little further until tlie subject was studied by Dr. Julius Pohlman.* 

 He pointed out that the upper course of the ancient gorge could not 

 have lain outside the modern gorge. If the course of one gorge lay 

 athwart the course of the other, we should have two breaks in the con- 

 tinuity of the strata, instead of the single one at the Whirlpool. The 

 upper part of the ancient gorge necessarily coincides with a part of the 

 modern gorge; and so when the cataract, in the progressive excava- 

 tion of the caiion, reached a point at the Whirl])ool where it had no 

 firm rock to erode, it had only to clear out the incoherent earth aiul 

 bowlders of glacial drift. To whatever distance the gorge of the 

 earlier stream extended, the modern river found its laborious task per- 

 formed in advance. 



Let us put together what we have learned of the Niagara history. 

 The river began its existence during the final retreat of the great ice 

 sheet, or, in other words, during the series of events that closed the age 

 of the ice in North America. If we consider as a geologic period the 

 entire time that has elapsed since the beginning of the age of ice, then 

 the history of the Niagara Eiver covers only a portion of that period. 

 In the judgment of most students of glacial geology, and, I may add, 

 in my own judgment, it covers only a small portion of that period. 



During the course of its history the length of the river has suffered 

 some variation by reason of the sucessive fall and rise of the level of 

 Lake Ontario. It was at first a few miles shorter than now ; then it 

 became suddenly a few miles longer, and its present length was gradu- 

 ally acquired. 



With the change in the position of its mouth there went a change in 

 the height of its mouth ; and the rate at which it eroded its channel 

 was affected thereby. The influence on the rate of erosion was felt 

 chiefly along the lower course of the river, between Lewiston and Fort 

 Niagara. 



The volume of the river has likewise been inconstant. In early days, 

 when the lakes levied a large tribute on the melting glacier, the 

 Niagara may have been a larger river than now ; but there was a time 

 when the discharge from the upper lakes avoided the route by Lake 

 Erie, and then the Niagara was a relatively small stream. 



The great life work of the river has been the digging of the gorge 

 through which it runs from the cataract to Lewiston. The beginning 

 of its life was the beginning of that task. The length of the gorge is in 

 some sense a measure of the river's age. In the main the material dug- 

 has been hard limestone and sandstone, interbedded with a coherent 

 though softer shale; but for a part of the distance the material was 

 incoherent drift. 



The geologic age of the earth — the time during which its surface has 

 been somewhat as now, divided into land and ocean, subject to endless 

 waste on the land and to endless accumulation of sediment in the 



* Proceedings Am. Assoc. Adv. Sd., 35th meeting (Buffalo), pp. 221-222. 



