278 WALKS AND TALKS. 



was bounded throughout all Csenozoic and Mesozoic time. 

 There was an epoch when the excavation of this gorge began. 

 The great tide swept along at the high level of the land. But 

 the stream has also scored deeper than at present. The rocky 

 bottom of its c*hannel is everywhere many feet below the 

 present bed of sediment. The country stood higher once than 

 now ; the descent to the sea was sharper, and the erosion more 

 profound. 



Of course, during the melting epoch, of the great glacier, 

 all the streams were flooded. Not only did the dissolving ice 

 supply enormous quantities of water; evaporation must have 

 been increased by the extension of evaporating surface, and 

 condensation must have been promoted by the large amount of 

 ice-cold surface. ^It was an epoch of rains and floods.) All the 

 rivers have left records of their ancient altitudes. These are 

 ' the terraces. On the Connecticut we find them from one 

 hundred and eighty to two hundred feet above present flood 

 level ; on the Hudson and Mohawk, three hundred and thirty 

 feet; on the Genesee, two hundred and thirty-five feet; on 

 the lower Ohio, fifty to one hundred and eighty feet ; on the 

 Missouri, two hundred and fifty feet ; and so throughout North 

 America, all the rivers at this epoch were flooded. 

 \ The Niagara river had been at work on a vast gorge ever 

 since the Devonian Age. Probably none of the great lakes 

 except Superior, then existed. From Lake Superior sprang 

 a river which flowed along a valley in which the basins of 

 Lake Huron and Lake Erie have since been excavated. At 

 a point west of the present mouth of the Niagara, it made a 

 fall, and flowed as a river along the Ontario valley, and thus 

 to the sea. In the course of ages, the stream excavated a 

 gorge as far as the whirlpool perhaps even farther. On the 

 re-establishment of the river, it did not find its ancient gorge, 

 but precipitated itself over the escarpment at Lewiston. 

 Here it began a new gorge, and dug back four miles, when 

 it struck the old gorge. Of course the falls now continued 

 their recession rapidly as far back as the head of the old 

 gorge. Since that was reached, the work has been contin- 



