History of lake Agassis. — Upham. 225 
of Saint Anthony ; of Dr. Andrews, * that the erosion of the shores 
of lake Michigan, and the resulting accumulation of dune sand 
drifted to the southern end of that lake, cannot have occupied 
more than 7,500 years ; of professor Wright,! that streams trib- 
utary to lake Erie have taken a similar length of time to cut their 
valleys and the gorges below their water-falls ; of Mr. Gilbert, X 
that the gorge below Niagara falls has required only 7,000 years 
or less ; and of Prof. B. K. Emerson, || on the rate of deposition 
of modified drift in the Connecticut valley at Northampton, Mas- 
sachusetts, from which he believes that not more than 10,000 
years have elapsed since the glacial period. An equally small es- 
timate is also indicated by the studies of Gilbert! and Russell** 
for the time since the last great rise of lakes Bonneville and La- 
hontan. These measures of time, surprisingly short whether we 
compare them on the one hand with the period of authentic human 
history or on the other with the long record of geolog}', carry us 
back to the date when the ice-sheet of the last glacial epoch was 
melting away from the basins of the upper Mississippi, of the 
Bed river of the North, and of the Laurentian lakes. 
The entire departure of this ice-sheet therefore probably occu- 
pied at the most not more than two or three thousand years ; and 
half of this time ma} r measure the duration of lake Agassiz, with 
the formation of its beaches marking more than twenty-five suc- 
cessive stages in the concurrent subsidence of its surface and rise 
of the earth's crust, which amounted together to 700 feet on the 
latitude of the north part of Duck mountain and the middle of 
lake Winnipeg. But even these short estimates may be too long. 
The shores of lake Michigan, similar with those of lake Agassiz 
in the drift of which they are formed, in their north and south 
trends, and in the adjoining depths of water, have suffered an 
* Transactions of the Chicago Academy of Sciences, vol. ii. James 
C. Southall's Epoch of the Mammoth and the Apparition of Man upon 
the Earth, 1878, chapters xxii and xxiii. 
f Am. Jour. Sci., III, vol. xxi, pp; 120-123, Feb., 1881; The Ice Age in 
North America, 1889, chapter xx. 
% Proceedings, Am. Assoc, for Adv. of Science, vol. xxxv, for 1886, 
p. 222. "The History of the Niagara River," Sixth An. Rep. of Com- 
missioners of the State Reservation at Niagara, for 1889, pp. 61-84. 
|| Am. Jour. Sci., Ill, vol. xxiv, pp. 404-5, Nov., 1887. 
| U. S. Geological Survey, Second annual report, p. 188. 
** U. S. Geological Survey, Monograph XI, Geological History of Lake 
Lahontan, p. 273. 
17 
