1894.] Quaternary Time Divisible in Three Periods. 983 
much longer preglacial time of gradual uplift of the continent, 
and the Postglacial or Recent period in which we live, to form 
together the three suecessive parts of the Quaternary era. How 
long the early part comprising the epeirogenic uplift, repre- 
sented by the deposition and erosion of the Lafayette forma- 
tion, may have been, we can only vaguely or perhaps approx- 
imately estimate. During the beginning of the uplift its effect 
would be probably to increase the transportation and deposi- 
tion of gravel and sand by the rivers many times beyond 
their present action. The rate of average land erosion now 
prevailing throughout the drainage area of the Mississippi is 
supposed by McGee to be competent to supply in about 120,000 
years a volume of river gravel, sand, and silt equal to the 
original Lafayette formation in the Mississippi Valley. With 
the greater altitude and increasing slopes of the land during 
the deposition of the Lafayette beds it may have required a 
third or a sixth of the time here mentioned, that is, some 
40,000 or 20,000 years. As the elevation continued, however, 
rapid fluvial erosion of these deposits and of the underlying 
strata ensued, which was extended over so long and broad an 
area of the lower Mississippi Valley, and to such depth, that, 
even with the high continental elevation of 2000 to 3000 feet, 
known from submerged valleys off both the Atlantic and 
Pacifie coasts, it must have required a long epoch. Perhaps 
it may be reasonably estimated twice as long as the time of the 
deposition, or somewhere between 40,000 and 80,000 years. 
The Lafayette period thus comprised two parts or epochs, the 
first characterized by deposition of the formation, the second 
by its extensive erosion and the culmination of the continental 
uplift. 
THE GrACIAL PERIOD. 
Comparison of the work of the glaciers and ice-sheets of the 
present time with those of Pleistocene time seems to me best 
accordant with a reference of all our glacial drift to a single 
continuous period of glaciation, which, though occupying 
probably 20,000 years or more, was yet brief as compared with 
the duration of most other recognized geologic periods or 
