THE HISTORY OF THE NIAGARA RIVER. 249 



ocean, greeu with verdure and nourishing the varied forms of animal 

 life — this time is of immense duration. Even the units into which 

 geologists divide it, the periods and epochs of their chronology are 

 themselves of vast duration. Human history is relatively so short, 

 and its units of centuries and years are so exceedingly brief, that the 

 two orders of time are hardly commensurate. Over and over again 

 the attempt has been made to link together the two chronologies, to 

 obtain for the geologic units some satisfactory expression in the units 

 of human history. It can not in fairness be said that all these attempts 

 have failed, for some of them are novel and untested ; but, however 

 successful or unsuccessful they may have been, the interest in the 

 subject remains, and no discussion of the history of the Niagara River 

 would be complete without some allusion to its value as a geologic 

 chronometer. It is true we know but little of the ratio the river epoch 

 bears to the extent of the glacial period, or to any longer geologic unit; 

 but yet were we able to determine, even approximately, the time con- 

 sumed by the river in cutting its gorge, we should render less hazy and 

 vague our conception of the order of magnitude of the units of the 

 earth's geologic history. The problem has been attacked by numerous 

 writers, and the resulting estimates have ranged from three or four 

 thousand years to three or four million years. 



The method of reaching a time estimate has been, first, to estimate 

 the present rate of recession — the rate at which the cataract is increas- 

 ing the length of the gorge ; second, to compute, with the aid of this 

 estimate and the known length of the gorge, the time necessary for the 

 entire excavation ; and, third, some writers have modified their result 

 by giving consideration to various conditions affecting the rate of 

 erosion during earlier stages of the excavation. The enormous range 

 of the resulting estimates of time has depended chiefly upon the im- 

 perfection of data with reference to the present rate of recession of the 

 falls. It is but a few years since measurement of the rate of recession 

 was substituted for bald guessing. 



This measurement consists in making surveys and maps of the falls 

 at different times, so that the amount of change in tbe interval between 

 survi^ys can be ascertained by comparison of the maps. In 1842 Pro- 

 fessor Hall made a survey of the outlines of the falls, and he published, 

 for the use of future investigators, not only the map resulting from the 

 survey, but also the bearings taken with the surveying instrument in 

 determining the principal points of the map.* He likewise left upon 

 the ground a number of well-marked monuments to which future sur 

 veys could be referred. Thirty-three years later a second survey was 

 made by the United States Army Engineers, and they added still 

 further to the series of bench marks available for future reference. 

 Three years ago my colleage, Mr. R. S. Woodward, executed a third 

 survey.t 



* Natural History of New York, Geology, Part IV, pp. 402, 403. 

 t Science, vol. Viii, 1886, p. 205. 



