16 POPULAR SCIENCE MONTHLY. 



of the upper cascade, prolonged the episode to six thousand years, 

 when Foster's Flats were passed. By this time the waters of 

 Lake Huron were probably turned into the Niagara by way of 

 Lake Erie. With the increased magnitude of the river the falls 

 are thought to have receded as far as the head of the whirlpool, 

 requiring four thousand years. By the close of this stage there 

 were some important changes in the river, as when the narrows 

 of the whirlpool rapids were excavated, and the three cataracts of 

 the second episode appear to have been united into one great fall. 

 With the increased volume of water at the maximum height of 

 the falls, as at the apex of the modern horseshoe cataract, the 

 recession was very rapid, so that the falls receded to above the 

 railroad bridges in eight hundred years more. It was in the next 

 episode that the descent of the river was reduced from four hun- 

 dred and twenty feet to three hundred and twenty-six feet, which 

 is that of the present day. These changes of height of falls and 

 volume of the river must not be supposed to have been sudden, 

 and, although they were secular, yet there were long periods of 

 rest, as shown by the landmarks, which are mostly obliterated 

 where they were imperfectly engraved during short epochs of 

 repose. The first stage of the last episode is characterized by the 

 retreat of the falls through the great rocky barrier (Johnson's 

 Ridge, e e, Fig. 9) on the northern side of the buried Tonawanda 

 Valley. Beyond this barrier the river speedily removed some 

 ninety feet of drift for a distance of a mile and a half to the head 

 of the rapids above the horseshoe cataract, and the recession across 

 the buried valley has been the last stage of the present episode 

 of the falls. Here the necessary time for the retreat of the falls 

 since passing the railway bridges has been three thousand years. 



Adding the duration of the various stages of the river to- 

 gether, the age of the falls is computed at thirty-one thousand 

 years, or of the river thirty-two thousand years. These figures 

 are based upon the severest analytical methods at present attain- 

 able, but the discoveries in the physics of the river cover most 

 of the doubtful points ; yet in the determination of the amount of 

 work performed in the middle episodes some points are open to 

 revision, but the errors they cover form only a small portion of 

 the life of the cataract, and a little time, more or less, would not 

 greatly change the results given. No general guesses or objec- 

 tions have been found worthy of consideration; The determina- 

 tions had to be attempted in parts, and the aggregate results have 

 been confirmed by two other sets of investigations : one on the 

 relative amount of tilting of the deserted shores, and the other 

 upon the rate of the rising of the land in the Niagara district, 

 which has been found to be about one foot and a quarter a cen- 

 tury, but much more rapidly to the north and east. 



