PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE GEOLOGICAL SOCIETY OF WASHINGTON 



The 293d meeting was held in the lecture room of the Cosmos Club, 

 on March 10, 1915. 



INFORMAL COMMUNICATIONS 



J. W. Spencer: Results of recent soundings at Niagara, and their in- 

 terpretation. In October, 1914, the speaker made a re-survey of the 

 crest-line of the Falls and made additional soundings. In the light of 

 these and of previous work an analysis of certain conclusions in the 

 Niagara Folio of the U. S. Geological' Survey was given. The folio 

 adopts, in f].nding the rate of recession, a condition where most of the 

 water temporarily passes over about two-thirds the width of the Falls. 

 This is used as the mean rate for the full breadth. This factor under- 

 lies all calculations. The Falls were lately higher than now, as shown 

 by soundings, by borings at the Whirlpool Rapids Narrows, and from 

 the historic records (of Kalm in 1750) ; and this increased the rate of 

 recession. While the errors made by neglecting these factors are op- 

 posing, they do not lead to the correct determination of the age of the 

 Falls. A vertical section under the Falls is given in the folio, showing 

 a depth of water in the gorge of about 200 feet, although the evidence 

 mentioned proves it to be less than 100 feet. Nearly the same amount 

 of work should be performed by the same volume of water, acting on 

 the same set of rocks (height and breadth of the Falls being the same) 

 in the five sections adopted. Yet from the speaker's interpretation, 

 one-quarter the volume (in descending the same height) is made to dig 

 four times as deeply at one point as the full volume at another — a vari- 

 ation in efficiency of IGGO per cent. Between other sections, similar 

 variations appear, while not more than 10 to 25 per cent would be ad- 

 missible. The age of the Falls as estimated by the speaker is believed 

 to be based on an excessive rate of recession, modified by unsupported 

 opinions to avoid implication of attempts at precision. Thus if we ap- 

 ply calculations alone to his data, the age would be 26,100 years, yet 

 the speaker gives as the sum of his components 19,500 to 30,000 years. 

 The effects of the variable height of the Falls are not considered by 

 him; if these be taken into account the time would be reduced to 14,700 

 years. The great importance of the age of Niagara lies in its being a 

 chronometer of many geological events, and therefore it should be de- 

 termined with the greatest precision. The re-survey of the crest-line 



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