LIFE ON THE EARTH. 141 



ter. This example is found in the recession of the 

 Falls of Niagara 1 . 



The river St Lawrence, in traversing the space 

 of 32 miles between Lake Erie and Lake Ontario, 

 has a fall of 330 feet. The general floor of the whole 

 country is limestone resting on shale ; the limestone 

 appears in the stream, covered in the banks on 

 each side by alluvial sand from 10 to 140 feet thick 

 for the first 25 miles. At this point the great Falls 

 occur; the river being precipitated over the solid 

 and projecting rock of limestone, in one tremendous 

 cascade 158 to 164 feet deep, into the subjacent 

 shale, which is deeply excavated below the general 

 level of the channel, and also worn into a recess 

 forty feet within, a perpendicular line dropped from 

 the limestone edge of the cataract. Below this point 

 the river flows in the deep chasm which it has 

 worn for itself, seven miles, to Queenstown and Lake 

 Ontario. 



The Falls recede, not regularly, but by sudden 

 steps, in proportion as the subjacent shale is worn 

 away, and leaves the crown of limestone unsupported. 

 In the course of forty years they have thus receded 

 fifty yards/ Adopting this as the rate of recession 

 for the whole of the channel below the Falls, we 

 have 9856 years for the time which has elapsed 

 1 Lyell, Principles of Geology, I. 277. 



