NATURAL HISTORY, TORONTO REGION 
visit of Sir Charles Lyell in 1842, and many papers 
have been written on the subject. The most impor- 
tant contributions to the literature have been made 
by Gilbert, Spencer and Taylor, who do not entirely 
agree as to the succession of events. 
All agree that the falls began at the Niagara 
escarpment near Queenston on the Canadian side and 
Lewiston on the American when the ice of the 
Ontario lobe had so far withdrawn as to set free a 
large part of the basin. It is probable that the falls 
began as three separate plunges over the three harder 
rock layers of the escarpment, the Cataract sand- 
stone, the Clinton limestone and the Niagara (Lock- 
port) limestone. The upper limestone is the thick- 
est and most resistant of the three beds, so that ulti- 
mately the two lower falls worked their way back 
to the upper fall, probably at Niagara Glen; since 
then there has been only one fall. 
The Niagara gorge is 300 feet deep at Queenston 
Heights, but diminishes to 158 feet at the Canadian 
or Horseshoe Fall. Dr. Spencer has sounded below 
the falls, finding a depth of water of 192 feet, making 
a total depth of 340 feet for the chasm. 
The width of the gorge varies to an important 
degree, having two stretches which are narrower 
than the rest, at Niagara Glen and just below the 
railway bridges, both accompanied by powerful 
rapids. These narrower portions may be accounted 
for by supposing the greater part of the drainage of 
the Upper Lakes to be diverted to the Trent Valley 
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