Notices of Memoirs — The British Association. 563 



6. — Notes on Niagara. 

 By P. Hallett, M.A. 



THESE notes may be expressed in abstract in the following pro- 

 portion, and are submitted to the Section as questions for its 

 consideration. 



1. That, assuming the principle of the gradual formation of the 

 cataract, the condition of existence of the present overhanging 

 precipice is the superimposition of the hard Niagara limestone — 

 corresponding to the Wenlock limestone — upon the friable Niagara 

 shale, the latter being undermined, and the former overhanging; 

 that the condition of existence of the rapids above the precipice is 

 the succession of hard rocks simply, and that these differences of 

 condition probably differentiate overhanging Falls from Eapids 

 generally. 



2. Hence, in case of the precipice receding to a point above the 

 shale, the fall would disappear and become a rapid. 



3. That the form of the water in the rapid in consequence of its 

 increasing velocity is convergent to mid-channel ; and hence the 

 rapids, instead of being a source of danger to Goat Island and the 

 small islands in their current, are actually a protection to them by 

 determining the water from their banks. 



4. That the water of the Fall undergoes a continuous disintegra- 

 tion from summit to base, and breaking up into smaller and smaller 

 masses and spreading out as it descends. The "continuous roar" 

 of Niagara is really a succession of impulses. 



5. That this disintegration is a consequence of the collision be- 

 tween the falling water and the column of air beneath it ; and that 

 the compressed air in its descent is propelled inwards and outwai'ds ; 

 inwards to form the well-known rush of winds behind the Fall, and 

 driving the heading of excavation in the shale ; outwards sending 

 up the cloud mist that continuously hangs over the Falls. 



6. That this collision between the air and falling water is really 

 a conservative influence, distributing the direct force of the fall and 

 partly transmitting it both directly and by reaction along the 

 currents of the gorge. 



7. That besides the force of the air propelled against the shale 

 face of the precipice as a cause of its excavation, attention is also 

 to be directed to the continuous drainage as evidenced by springs, 

 etc., from the shale. Even along the gorge where there are no falls 

 this appears to determine an undermining action or recession of the 

 banks below, with overhanging rocks above. 



8. That retrocession of the Falls, evident as it is, is not to be 

 regarded as the operation of a mechanical force necessarily con- 

 tinuous, but as a movement to equilibrium. Hardly any retro- 

 cession has occurred in parts of the American Fall during the time 

 that the Canadian has gone back some 500 yards. Retrocession 

 would follow from any cause increasing the amount or force of the 

 water, elevation of the land, increased rainfall, etc., and each retro- 

 cession would increase the disfharging power of the river, thus 

 tending to carry off the increased water supply. The relation 



