Science, Geology and Physics 



This is at the site of the whirlpool rapids. My explanation 1895 

 of this narrow chasm, without any increased thickness of the pencei 

 limestone capping over the shaly bed is that the whole force of 

 the falls descending 420 feet was concentrated in one cataract 

 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 process more 

 strongly brought out by contrast with the sections of the canon, 

 immediately above (fig. 26) and below (fig. 22) which are 

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

 common observations that increased declivity causes the channels 

 to be deepened, and decreased slope accelerates the widening of 

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

 (fig. 23). 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 present 

 level, 320 feet below the rapids above the falls. The com- 

 mencement of the work of this epoch was taken where the canon 

 suddenly became broad at the head of the whirlpool rapids, a 

 phenomena explained by the force of the river being vertically 

 diminished and latterly increased — the converse to the con- 

 ditions of those of the third episode. At first the rocks in John- 

 son's ridge offered great resistance on account of the increased 

 thickness of limestones nevertheless the lateral erosion gained the 

 ascendancy over the vertical. The section through Johnson's 

 ridge is 5,500 feet long, and with the laws of erosion the time 

 necessary for the falls to retreat through it would be about 1 ,500 

 years — thus would end the first stage of the last episode. The 

 last stage is the modern, or that since the cataract reached the 



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