MODERN ACTION OF RIVERS. 
369 
least perceptible change within the memory of man becomes a vast incomprehensible amount 
during the millions of ages through which the earth has arrived at its present condition. If 
a river channel can be widened and deepened to the amount of a few inches within the recol¬ 
lection of any individual, may not a gorge of one or two hundred feet be formed by the same 
process? Our theories of modern operations make provision for wide sweeping deluges, for 
immense excavating waves, for hemispheres of ice, the upheaval of mountain chains, and the 
transportation of ice floes from frozen islands in our own latitudes ; but we have almost for¬ 
gotten the quiet operation of running streams, and the freezing of water in fissures of hardened 
rocks. 
In the first place we may consider a few of these effects which are known to us, and after¬ 
wards compare them with similar ones which may have resulted from the same causes. By 
this method we shall be able to prove that rivers have worn their channels not only through 
preexisting beds of modern detritus, but also through the barriers of solid rocks. 
The sketch at the head of the chapter represents the lower falls of the Genesee at Portage. 
The bed of the stream is bounded on either side by cliffs three hundred feet high. Upon the 
left bank is a table of rock, which was formerly the river bed; and upon the right is a small 
conical island of rock, between which and the table on the other side, the stream now flows. 
Within the memory of the oldest observers, the river flowed almost wholly over this table 
rock, and the isolated mass was joined with the right bank of the river. The following dia¬ 
gram will enable the reader fully to comprehend its present and former condition : 
175. 
A, B, represents the width of the chasm at the top. 
a, a. The platform or bed of the stream, over which the water was originally precipitated ninety six feet to the level of the 
river below the falls. This platform a a was formerly continuous to a’. 
b. The narrow channel of recent excavation. 
d, d, and c. A recer.t gorge, separating the small island from the main bank. 
[Geol. 4th Dist.] 
47 
