370 
GEOLOGY OF THE FOURTH DISTRICT. 
This table or platform is composed of a firm sandstone less than two feet thick, resting on 
softer strata beneath. A slight depression had been worn between a and a', over which a 
larger portion of water flowed than over the space between a and a . This depression in¬ 
creased in depth by the wearing action of the water and the effects of freezing, so that long 
since, the great body of water has flowed through this recent channel. During freshets, and 
at the breaking up of the ice in the spring, the narrow channel on the right is filled, and it 
then flows over the table above to the depth of a few inches. 
The following ground plan of the river at this place will illustrate its condition at the period 
before the projecting mass of rock was worn through : 
a, a. The table or platform, which originally extended 
across the whole width of the river’s bed. 
d, d. An elevated projecting mass of rock, standing in 
the direct course of the current. 
The first operation by which this change was effected, seems to have been a deflection of the 
current to the right side, caused by a bend in the river above. This force diminished towards 
the edge of the fall, and the projecting portion of the cliff d, cl, was thus protected for a long 
time, until the channel on the right side, becoming deepened, drew off nearly all the water in 
that direction, when it was gradually worn through. When first known, this isolated mass 
was joined to the main cliff; and when subsequently a passage was formed at c, the upper 
part still remained connected with it. This arch was afterwards broken down by the action 
of freezing water and its own weight, leaving it in its present condition. 
The principal modern effect illustrated in this example, is the formation of the narrow 
channel on the eastern side of the river bed, which now extends back from the fall about one- 
eighth of a mile, being in its greatest depth about eighty feet, and nearly the same in width. 
For the whole of this distance it forms a violent rapid, and the action of water and ice is con- 
