Niagara Gorge and Postglacial Time. — Upluvii. 243 
ing ceased, 2,000 years were probably adequate for the comple- 
tion of the gorge to the Whirlpool, the work having been 
greatlv lessened by preglacial erosion. Similarly, on account of 
the old St. David's ravine, 1,000 years, or less, would suffice for 
the erosion along the Whirlpool rapids. Afterward, under 
the present conditions of gorge lengthening, 3,000 years were 
needed for the last two miles, next to the present falls. The 
whole history would thus comprise about 7,000 years. 
This measure, which (not to be too exact in figures de- 
pending on the many varying conditions of the Niagara his- 
tory) we may place in round numbers as between 5,000 and 
10,000 years, is at the same time the duration of the period 
since the end of the Ice age, or, speaking more definitely, since 
the retreat of the continental glacier from the northern United 
States and Canada. It may be accepted with confidence, for it 
agrees with the estimates and computations independently 
made for the same period by Prof. N. H. Winchell, from the re- 
cession of the falls of St. Anthony ; by Dr. Andrews, and re- 
cently also by Leverett. from the shore erosion of lake Michi- 
igan and the accumulation of sand at its south end ; by 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 Etirope, likewise, ntmierous estimates 
of the lapse of time since the Glacial period, as collated by 
Hansen, are found to be comprised between the limits of 
5,000 and 12,000 years, being thus well harmonious with the 
measure given us by Niagara falls. 
In accordance with the ratios of the relative duration of 
preceding geologic periods and eras, having now found the 
approximate measure of the latest term in the series, namelv, 
this Postglacial period, as about 7,000 years, we mav well 
estimate the whole Quaternary era, including the Ice age, as 
about twenty times longer, giving to this era some 150,000 
years. The Tertiary era, with erosion of the stupendous Col- 
orado canon and baseleveling of the plains between the Red 
River of the North and the Rocky mountains, appears by the 
changes of its marine molluscan faunas to have been vastly 
longer, having com]:)rised probably three to five million years ; 
and the very long preceding eras since life began on the earth 
