376 -- DYNAMICAL GEOLOGY. —[Boox IIL. 
softer beds, presents a structure well adapted for showing the part 
played by waterfalls in river erosion. The waterfall acts with 
special. potency against the softer underlying strata at its base. 
These are hollowed out, and as the foundations of the superin- 
cumbent more solid beds are destroyed, slices of the latter from > 
time to time fall off into the boiling whirlpool, where they are 
reduced to fragments, and carried down the stream, Thus the 
waterfall cuts its way backward up the stream, and as it advances, - 
it prolongs the excavation of the ravine into which it descends. The 
student will frequently observe that in the recession of waterfalls and 
consequent erosion of ravines an im- 
portant part is taken by lines of joint 
in the rocks; that these lines have often 
determined the direction of the ravine, 
and that the vertical walls on either 
side depend for their precipitousness 
mainly upon these divisional planes in 
the rock. The gorge of the Niagara 
affords a magnificent and remarkably 
simple illustration of these features of 
river action. At its lower end, where © 
it enters the wide plain that extends 
to Lake Ontario, there stretches away, 
wenren | O01 either side of the river, a line of 
cliff and steep wooded bank, formed by 
the escarpment of the massive Niagara — 
limestone. Back from this line of 
cliff, through which it issues into,the © 
lacustrine plain, the gorge of the river — 
extends for about 7 miles, with a width 
of from 200 to 400 yards, and a depth _ 
of from 200 to 300 feet. At the upper 
end lie the world-renowned falls. The 
whole of this great ravine has un- 
questionably been cut out by the re- 
: cession of the falls. When the river 
hia Ladsprtidtsletole medi ak ay first began to flow, it may have found 
the escarpment running across its 
course, and may then have begun the excavation of its gorge, 
More probably, however, the escarpment and waterfall began 
to arise simultaneously and from the same geological structure. 
As the former grew in height, it receded from its starting 
point. The river-ravyine likewise crept backward, but at a more 
rapid rate, and the result has been that while at present the cliff, 
worn down by atmospheric disintegration, stands at Queenstown, the 
ravine dug by the river extends 7 miles further inland. The water- 
fall will continue to cut its way back as long as the structure of the 
gorge continues as it 1s now—thick beds of limestone resting hori- 

