GEOLOGY. 259 
but the point in which they deceive themselves is the 
explanation of the phenomenon. ‘lhey have attributed 
this to the force of the current of air; and there is the 
mistake. 
‘The fact is a very simple one, and easily explained in 
accordance with what has been evolved by the study of 
the effect of a sudden retardation on a current of matter. 
Through the velocity acquired, and the upward direction 
given to their movement, the stones, detaching themselves, 
are projected forward from the water by which they were 
borne along, 
‘We find that it is towards the contraction of a water- 
course, occasioned by a bridge, that the phenomenon 
manifests itself with most intensity. It is, moreover, at 
such points that it ought to be most easily observed. At 
the time of a great flood there are few spectators in the 
deserted gorges of the torrents. 
‘From the moment that we are in possession of prin- 
ciples, nothing is more easy than to account rationally for 
all the effects, and all the accidental incidents which they 
may produce.’ Such is the transporting power of water. © 
In the Falls of Imatra, as in the Falls of Trollhatten in 
Sweden, and the Falls of Clyde in Scotland, we have what 
more resembles a rapid than a waterfall, such as it is gene- 
rally pictured by the imagination. In these it seems that 
while smaller stones have been carried away, larger ones 
have been left in the channels, forming an irregular slope 
from the peaceful rapids above to the peaceful lake below, 
but the mass of stones which have been carried away must 
have been enormous,and thus our thoughts again are carried 
back to the glacial period, and we are prompted to imagine 
what must have been then the state of things here. 
Nowadays the flow both at Niagara and at Imatra is com- 
paratively small, but when the formation of these channels 
was begun, and when the operation was in its early energy 
it was probably very different. In the case both of the 
one and of the other there are indications of the depth 
