442 J. W. SPENCER — ^RELATIVE WORK OF TWO FALLS OF NIAGARA 



little attention to this topic, which iinder other conditions -wo-iild have 

 deserved more prominence. Between the time of the survey by Prof. 

 James Hall, in 18-1:2, and that by Mr. Aug. S. Kibbe, in 1890, there had 

 been only a small recession, and since then apparently so little change had 

 occurred that 1 did not make a resurvey of them in 1905. My impressions 

 were correct, for by other surveys of 1905^ and 1906* it has been found 

 that scarcely any appreciable amount of rock has fallen. The measure- 

 ments showed a recession from 1842 to 1890 of 0.60 foot a year, but the 

 average amount from 1842 to 1906 is now found to be 0.45 foot. Origi- 

 nally I stated that since 600 years ago (now corrected to 550 years) ^'the 

 American Falls do not seem to have retreated more than 110 feet, if so 

 much, in excess of the unknown widening of the gorge from frost action" 

 (page 38). This would represent, if correct, only 0.20 foot a year. This 

 might have left a wide correction for the determination by future genera- 

 tions, but now it can never be done, as the power diversion has already 

 rendered it impossible. Let us put on record how the recession of the 

 smaller falls really compares with that of the main cataract. 



While the American cataract suffered a mean recession of only 29 feet 

 between 1842 and 1906 (by actual measurement), the main, or Canadian, 

 Falls retreated 265 feet for a mean breadth of 1,200 feet. Fragments of 

 the original banks are still preserved. As the gross errors of observation 

 are equal in the two cases, these are reduced to a relatively small amount 

 in the latter instance, even though the undercutting before 1842 had con- 

 siderably progressed. The undercutting in the case of the fallen Table 

 Eock reached from 40 to 50 feet, and nearly the same amount is still to 

 be seen overhanging in front of Wintergreen Flats, some miles down the 

 gorge. 



Under these conditions, especially as the superior ledges of hard rocks 

 at the American Falls now project but little, and as there are huge masses 

 of rocks fallen at their foot, it would seem that the undercutting had 

 progressed far when Professor Hall made his survey. This undercutting 

 has caused an excessive amount of collapse of the upper layers of rock 

 since his time, especially as it does not appear how the falls could have 

 receded more than 200 feet since the separation of the two cataracts. All 

 these considerations show that absolute measurements of the recession of 

 the American Falls are not obtainable for calculating the age of the 

 gorge ; yet, on the other hand, close estimates of their rate of retreat can 

 be made. 



As the diameter of the main falls is 1,200 feet and of the smaller one 



* P.y Mr. BasH ITall for the U. S. Geological Survey. 



* U. S. Lake Survey. 



