252 GROVE KAEL GILBERT— DAVIS [i: " I0Ui m,r™xi; 



part on facts observed during a joint excursion which Gilbert had arranged with him in the 

 hope of reaching an agreement in the field, the comment made by the noncombative one of 

 the two upon what he believed to be an altogether erroneous interpretation of the facts was 

 not phrased in such a way as to throw all the blame on the other side, but rather in such a 

 way as to share it : He remarked that, although the excursion had not brought the two observers 

 into agreement, it had "served to prove that we differ widely as to the criteria by which shore 

 ridges and shore terraces are distinguished from ridges and terraces of other origin." e Geology 

 gained immensely from his method and manner as well as from his observations and inferences. 



LATEST WOKE AND WORDS ON NIAGARA 



Gilbert's latest field study and report and his latest written article on Niagara both followed 

 the essays and lectures above analyzed by about 10 years. The field study 7 was undertaken in 

 connection with a new survey of the falls executed in 1905 at Gilbert's instance, and involved a 

 critical comparison of its outline with those of all earlier surveys. The report in which the 

 results of this study are presented calculates a recession of 5.3 feet a year for the central part of 

 the -Canadian or Horseshoe falls. This brief numerical value should be quoted as a standard 

 until it is modified by later surveys ; but the report in which it is announced has a greater and 

 more enduring value as an illustration of the manner in which a scientific inquiry should be 

 conducted. It deserves to be analyzed as to its method even more than to be quoted as to its 

 result; it is a model of carefid, critical, and impartial procedure. It opens with a thorough his- 

 torical review of the problem under discussion. The relative accuracy of various surveys is 

 nicely evaluated, and a curious error in the famous survey of 1842 is brought to light by means 

 of a most ingenious series of tests. Geology is indeed a curious science inasmuch as it here 

 demands that the position of the verandah of a hotel, of which only the foundations nowremain, 

 should be identified in order to erect a scaffolding in the same position from which a photograph 

 of the falls might be taken — official authority and an engine had to be secured to open the desired 

 prospect by removing an obstructive freight car from a near-by railroad siding — and thus dupli- 

 cate the record of a camera-lucida drawing made in 1827 by Capt. Basil Hall, an English traveler, 

 so as to measure the change in the vertex of the Horseshoe falls with respect to a line of the same 

 bearing in two views. And yet there are persons who think that geology deals chiefly with 

 rocks! A profitable essay might be written on the various mental qualities and personal activi- 

 ties that entered into this simple investigation. 



Gilbert's last words on Niagara appeared in a review written in 1908 of a work on the 

 "Evolution of Niagara Falls," 7a and considered especially the interpretation there given to the 

 variation in the rate of the falls' retreat in response to the variation in the volume of water that 

 passed over them. In the work in question, the retreat of the falls had been taken as propor- 

 tional to the volume of falling water; and the age of the falls had been calculated on that basis 

 after estimating the volume to which the river was reduced in the two epochs when it drained 

 only the Erie Basin, and the length of the two parts of the gorge that were eroded in those 

 epochs. Gilbert's view was that the rate of the falls' recession was reduced much more rapidly 

 than in direct proportion to the diminution of river volume, and that the calculated age of the 

 falls was therefore much too small. His reasons for this view were chiefly that, considering the 

 river as an eroding engine, the greater its energy, the greater the share of energy would be 

 applied to its work; but he also pointed out that various other factors would enter the problem. 

 For example, the greater the thickness of the capping limestone, the more abundant would be 

 the fallen blocks which could be used as grinding pestles by a river that was large enough to 

 churn them about, but which would act as obstructions to erosion in a river so small that it 

 could not move them; also, that even with a constant volume of river discharge, the recession 

 of the falls would vary in some manner inversely with their breadth. But the chief factor being 

 variation in volume, his inclination was to assign a much greater age to the falls than the 39,000 

 years that had been calculated on the assumption of a direct relation between volume and 

 recession. 



• Bull. Geol. Soc. Amer., iii, 1892, 493. 



' Rate of recession of Niagara Falls. Bull. 306, U. S. Geol. Survey, 1907. 



'■ Science, uviii 1908, 148-151. 



