460 



J. W. Spencer — Duration of Niagara Falls. 



aries in its higher portion, which is an antique structure. The 

 depression is so obstructed with drift, that gives rise to landslides 

 that the old topography is much obscured. Yet a little stream 

 has removed the fallen earth and exposed a natural section of 

 Clinton limestones, which cross the valley at an elevation of 

 115 feet above the surface of the whirlpool, or 160 feet above 

 Lake Ontario, with Niagara shales showing for at least 20 feet 

 higher. Thus the rocky barrier across the ravine is not less 

 than 240 feet above the bottom of the canon in the whirlpool. 

 This barrier in the ravine is illustrated in tig. 3, which should 

 be compared with figures 9 and 10, in order to appreciate the 

 insignificance of the whirlpool ravine.* 



Fig. 3. — Section across the whirlpool ravine, located at bb, fig. 2 ; W, well ; 

 R, stream. 



The form of the whirlpool cauldron requires explanation. 

 At Mr. Shepherd's house, a short distance west of the whirl- 

 pool, there is a well 90 feet deep without reaching rock (w, tig. 

 3) and this shows the absence of Niagara limestones to a depth of 

 more than 50 feet below the surface rocks of the western wall of 

 the whirlpool. At that point the limestones rise 40 feet higher on 

 the eastern side of the river than on the western, but the depres- 

 sion was leveled up with drift. Thus it appears that at this 

 point the Niagara river took possession of the eastern side of a 

 drift-filled valley (Tonawanda — St. David's), and the whirlpool 

 ravine was a little tributary to it. When the falls had receded 

 to the whirlpool and penetrated the rocky barrier, the currents 

 were able to remove the filling of the buried ravine, and this 

 gave rise to the form of the cauldron, which deepened its 

 basin to lower levels by the currents of the river acting upon 

 the underlying soft shales, with the landslides obscuring the 

 older features. It is evident that there was no preglacial 

 Niagara river. 



The Niagara river crossed the broad shallow depression of 

 the Tonawanda drainage, at the falls and that adjacent to the 

 whirlpool on a basement of drift, but elsewhere generally on 

 hard limestones. Out of both of these materials, terraces were 

 carved thus marking the old river level, before it sunk within 

 the chasm. 



*In Rept. of meeting of Am. As. Ad. Sc. in Science, Sept., 1886, it is noted 

 that Prof E. W. Claypole found rocks in the ravine, without giving any details in 

 explanation. Since this paper has been in type. Prof. James Hall informed me 

 that Prof. J. W. Powell and himself had also seen the occurrence of the rocks, 

 but no notice has been printed. The error has been even recently repeated by a 

 writer in "Nature." 



