264 The American Naturalist. [Mareh, 
showed by a comparison of the shore erosion and accumulation of 
beach gravel and sand by the waves of Lake Agassiz with those of 
Lake Michigan that the existence of the former might be estimated at 
not more than 1000 years; the moraines belonging to the area of the 
later drift were probably formed in twice that time; the recession of 
the ice from its outermost limit to the first of these moraines a similar 
length of time, or perhaps, longer. In these conclusions the author 
agrees with Prestwich, who estimates the epoch of extreme cold at 
15,000 to 25,000 years, and the melting of the ice-sheet to from 8,000 
to 10,000 years or less. 
In order to show that his conclusion as to the age of Lake Agassiz 
is consistent with the known records and inferred conditions of the 
Ice age upon the central belt of the North American continent, Mr. 
Upham reviews the series of formations in the Mississippi and Nelson 
river basins which belong to the times immediately preceding, during 
and following the Glacial period, especially considering the changes in 
the altitude and slopes of the land and the probable measures of time 
demanded by the processes of drift transportation and deposition, by 
subsequent weathering with soil formation, and stream erosion. Asa 
result of his investigations, he gives the following estimates of the dur- 
ation of the three parts of the Cenozoic period under study, arranged 
in chronological order : 
“The time of preglacial epeirogenic elevation, with the deposition 
and erosion of the Lafayette beds, some 60,000 to 120,000 years; the 
Glacial period, regarded as continuous, without interglacial epochs, at- 
tending the culmination of the uplift, but terminating after the subsi- 
dence of the glaciated region, 20,000 to 30,000 years, and the Post- 
glacial or recent period, extending to the present time, 6000 to 10,000 
years. In total the Plistocene era in North America, therefore, has 
comprised probably about 100,000 or 150,000 years, its latest third or 
fourth part being the Ice age and subsequent time. The pre-plisto- 
cenic Cenozoic era appears by changes of its marine molluscan faunas 
to have been vastly longer, having comprised, perhaps, between hie 
and four million years, of which the Pliocene period would be a sixth 
or eighth part, thus exceeding the whole of the ensuing era of great 
epeirogenic movements and resulting glaciation.” 
In the discussion which followed the reading of Mr. Upham’s pape 
Mr. McGee called attention to the unmistakable unconformity betwee 
the Columbia and Lafayette formations in the Coastal plain series. 
unconformity represents erosion approaching 1000 feet in depth in pe 
Lower Mississippi region and from 300 to 500 or more feet in depth 1m 
