378 DYNAMICAL GEOLOGY. [Boor IIT. 
the length of time required for the excavation of the whole Niagara 
ravine at 35,000 years.’ — | ass 
A feature of interest in the future history of the Niagara river 
deserves to be noticed here. It is evident that if the structure of — 
the gorge continued the same from the falls to Lake Erie, the 
recession of the falls would eventually tap the lake, and reduce it to 
the level of the bottom of the ravine. Successive stages in this 
retreat of the falls are shown in Fig. 112, by the letters f to n, and in 
the consequent lowering of the lake by the letters a, b toe. It is 
believed, however, that a slight inclination of the strata carries the 
soft underlying shale out of possible reach of the fall, which will 
retard indefinitely the lowering of the lake. 

Fic, 112.—SEorioN TO ILLUSTRATE THE LOWERING OF LAKE ERIE BY THE 
RECESSION OF NIAGARA F'ALLs. 
A waterfall may occasionally be observed to have been produced 
by the existence of a harder and more resisting band or barrier of 
rock crossing the course of the stream, as, for instance, where the 
rocks have been cut by an intrusive dyke or mass of basalt, or where, 
as in the case of the Rhine at Schaffhausen, and possibly in that of 
the Niagara, the stream has been diverted out of its ancient course 
by glacial or other deposits, so as to be forced to carve out a new 
channel, and rejoin its older one by a fall.2_ In these and all other 
cases the removal of the harder mass destroys the waterfall, which, 
after passing into a series of rapids, is finally lost in the. general 
abrasion of the river-channel. | 
The resemblance of a deep narrow river-gorge to a rent opened 
in the ground by subterranean agency, has often led to a mistaken 
belief that such marked superficial features could only have arisen 
from actual violent dislocation. Hven where something is conceded 
to the river, there is a natural tendency to assume that there must 
have been a line of fault and displacement as in Fig. 118, or at least 
a line of crack, and consequent weakness (Fig. 114). But the 
existence of an actual fracture is not necessary for the formation 
of a ravine of the first magnitude. The gorge of the Niagara, 
for example, has not been determined by any dislocation. Still 
more impressive proof of the same fact is furnished by the most 
marvellous river-gorges in the world—those of the Colorado region 
* Lyell, “Travels in North America,” i. p. 32; ii. p.93. ‘ Principles,” i. p.358. Com- 
pare Lesley’s “ Coal and its Topography” (1856), p. 169. On recent changes at the Falls, 
see Marcou, Bull. Soc. Géol. France (2), xxii. p. 290. The Falls of St. Anthony on the 
Mississippi show, according to Winchell, a rate of recession varying from 3°49 to 6°73 
feet per annum, the whole recession since the discovery of the falls in 1680 to the 
present time being 906 feet. Q. J. Geol. Soc. xxxiv. p. 899. 
* Wirtenberger, Neues Jahrb. 1871, p. 582. 
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