1024 HISTORICAL GEOLOGY. 



vation was estimated by R. Bakewell, Jr. (son of the English geologist), in 

 1829, on the basis of facts received from a 40-years' resident. His estimate 

 was about three feet a year. Lyell, who was at the Falls in 1841 with 

 James Hall, reduced the rate to one foot a year, making the elapsed time 

 about 31,000 years. 



The question has since been considered by other geologists. G. K. Gilbert, 

 taking as data (1) a map made in 1842, after a careful survey by Blackwell 

 in 1841, and pubHshed in the N. Y. Geological Report of J. Hall (1842), 

 (2) another, made 33 years later, by the U. S. Army Engineers, and (3) a 

 third, made in 1886 by R. S. Woodward, concluded that the rate of cutting, 

 supposing the conditions to have been uniform, was about five feet a year, 

 and the length of time about 7000 years; but he observes, that instead of 

 uniform conditions, there have been great variations in the height of the 

 fall, and in the amount of water, and that the deduced rate cannot be safely 

 accepted {A7171. Rep. Smithsonian Inst, for 1890). 



That the investigation is beset with doubt is evident from the remarks 

 on page 987, and also by the various conclusions of recent writers on the 

 subject. W. Upham, on the grounds stated in his various papers, makes 

 the time 6000 to 10,000 years. J. "VV. Spencer, in his latest interpretation 

 of the facts (1894), arrives at the conclusion that the excavation of the 

 channel required 32,000 years. But, as already shown, the amount of water 

 now discharged by the river is no measure of that during the Champlain 

 period of moist climate and expanded but gradually diminishing lakes, and 

 no other safe basis for an estimate is known. As the amount of water then 

 was almost certainly much larger than now, the lower estimates are probably 

 nearest the truth. 



A similar estimate has been made by N. H. Winchell {Final Rep. Geol. 

 Minnesota) for the rate of recession of the Falls of St. Anthony on the 

 Mississippi at Minneapolis (page 973), with the result that the elapsed time 

 was probably about 8000 years. 



The rate of progress in a peat-bed, and that in a thickening deposit of 

 stalagmite in limestone caverns, are other uncertain data that have been 

 employed for deducing the length of time since the Glacial period. The 

 amount of stalagmite is dependent on the amount of carbonic acid or organic 

 acids in the filtrating waters, and partly on the texture of the limestone. The 

 results of such calculations do not appear to have any geological or archaeo- 

 logical value. 



Length of geological time according to geological evidence. — The facts from 

 geology used as a basis for calculating the length of geological time since 

 the Archaean are : the rate of sedimentation or accumulation, and the rate of 

 erosion or denudation, the former usually made dependent on the latter. 

 The rate of denudation is generally based on the results obtained by 

 Humphreys and Abbot from the Mississippi and its drainage area, and 

 related results from the study of other rivers. The rate of sedimentation 

 obtained from the Mississippi gives an average of one foot from the whole 



