Orhjiii and Af/c of fJte Laiircnl !(i n Ldkes. — '"plKnn. 177 
of average annual recession of the falls (nearly tive feet), we 
have approximately 7,000 years, as announced b}' Gilbert at 
the meeting of the American Association in Bult'alo in 1886, 
as the probable time re(iuiretl for the erosion of the gorge. 
This measure, which (not to be too exact in figures depend- 
ing on somewhat varying conditions of the Niagara history) 
we may place in round numbers as between 5,000 and 10,000 
years, is of great interest to geologists because it is at the 
same time the duration of the period since the end of the Ice 
Age, or. speaking more definitel}^ since the retreat of the con- 
tinental glacier from the northern United States and southern 
Canada. It may be so accepted with confidence, for it agrees 
with the estimates and computations independently made for 
the same period by Prof. N. H. Winchell, from tiie recession 
of the Falls of St. Anthony; by Prof. G. Frederick Wright, 
from the filling of depressions among kames and eskers, and 
from erosion by streams tributary to lake Erie: and by Prof. 
B. K. Emerson, from postglacial deposition in the valley of 
the Connecticut river. In Europe, likewise, numerous esti- 
mates of the lapse of time since the Glacial ]>eriod. as collated 
by Hansen, are found to be coni])rised between the limits of 
5,000 and 12,000 years, thus being well harmonious with the 
measure given us by Niagara falls. 
From this unit of measurement and its ratios of comparison 
with the preceding geologic periods and eras, according to 
Dana, Walcott, and others, we derive an approximate duration 
of 100,000 years for the whole Quaternary era : of j^erhaps 
;{, 000. 000 years for the Tertiary era ; some ten million years 
for the Mesozoic era : and probably forty to eighty million 
years for the Paleozoic era. In total, the time since life began 
on the earth is thus shown to be probably a hundred million 
years, compared with which the duration of Niagara, though 
a hundred times longer than a man's lifetime, seems geologi- 
<'ally like a span or a iiand's breadth beside the long vista of 
the past ages. 
