J. W. Spencer — Duration of Niagara Falls. 467 



at the close of the first episode, and its extension 12 miles 

 lakeward (the mean depth of shales removed from eight miles 

 was 180 feet, and from four miles, 60 feet), and applying the 

 laws of erosion, I have found that the first stage required 6,000 

 years and the second 4,000 years ; or the duration of the second 

 episode was 10,000 years.* 



Third Episode. — The narrowest portion of the gorge ex- 

 tends from the whirlpool for a distance of 4,000 feet as is 

 shown in figure 9 and on the map in fig. 1. The various sec- 

 tions (figs. 6, 7, 9, 10, 11) should be compared. 



Fig. 9. — Section across the narrows just north of the railway bridges (dd, fig. 1) 

 b, original bank of the river ; r, surface of the river ; L. O., level of lake ; floor 

 of canon 80 feet below lake level. 



This is the site of the whirlpool rapids. My explanation of 

 this narrow chasm, without any increased thickness of the 

 limestone capping over the shaly bed is that the whole force 

 of the falls descending 420 feet was concentrated in one cata- 

 ract with a rapid of an additional height of 25 feet descending 

 in front of Johnson's ridge. Thus the force engaged in under- 

 mining the limestones was exhausted in the recession of the 

 falls by deepening the gorge in place of broadening it, a pro- 

 cess more strongly brought out by contrast with the sections of 

 the canon, immediately above (fig. 10) and below (fig. 6) which 

 are half as wide again. Such result is in accordance with the 

 common observations that increased declivity causes the chan- 

 nels to be deepened, and decreased slope accelerates the widen- 

 ing of the channel as is shown in the section near the end of 

 the gorge (fig. 7). The computation of the time of the retreat 

 of the falls across this section is a simple problem, as the fall of 

 water amounted to 420 feet in place of 320 of the present day, 

 and the volume was the same. Under these conditions the 

 duration of this episode was 800 years. 



Fourth Episode. — This is characterized by the rising of the 

 waters in the Ontario basin so as to bring the lake to the pres- 



* One method considers only the recession of the upper one of the retreating 

 falls (descending 150) feet during the two stages of this episode. Owing to the 

 prevalence of limestones in the upper section, the computation would appear to 

 be an under estimate. Another process is based upon the excavation of the new 

 portions of the chasm to the full depth of 420 feet, and adding to the components 

 the time required to deepen the gorge of the first episode and extend the canon 

 to the lake — the amount of work being considered in terms compared with the 

 full depth of excavation in the chasm. 



Am. Jour. Sci.— Third Series, Vol. XLVIII, No. 288.— Dec, 1894. 

 30 



