982 The American Naturalist. [December, 
afterward the land was much higher than now, to permit the 
extensive and deep erosion of that time. A simpler view of 
the epeirogenic movements, closing the Tertiary era and 
inaugurating the Quaternary, seems to me to be found in 
ascribing these beds to deposition on land areas by flooded 
rivers descending from the Appalachian mountain region and 
from the Mississippi basin, spreading gravel, sand and loam 
over the coastal plain and along the great valley during the 
early part of a time of continental elevation. The land 
had lain during the long Tertiary periods at lower altitudes, 
and its surface was largely enveloped by residual clays and 
by alluvial sand and gravel. With the elevation of the con- 
tinent, increased rainfall and snowfall and resulting river 
floods swept away these superficial materials from the higher 
lands and spread them on the coastal plain and along the 
Mississippi Valley, where the streams expanded over broad 
areas with shallow and slackened currents. As the elevation 
increased, however, the rivers would attain steeper slopes and 
finally erode much of the deposits which they had previously 
made. During the culmination of the uplift, which the writer 
believes to have been the chief cause of the Ice age, Chesa- 
peake and Delaware Bays were excavated and erosion was in 
progress at a far more rapid rate than with the present low 
altitude of this region. 
The Lafayette formation seems to me more closely related 
to the Glacial period and the conditions producing the ice- 
sheets than tothe preceding very long Tertiary era, and for the 
same reasons which have been well stated by Hilgard and 
Spencer, namely, their dependence alike on the epeirogenic 
elevation? With the Ice age we should unite this probably 
*That epeirogenic movements of land elevation caused the accumulation of the 
Pleistocene ice-sheets, and conversely, that the end of the Glacial period was 
due to land depression, I have shown in an appendix of Wright's * Ice Age in 
North America," 1889, pp. 573-595; the Am. Geologist, Vol. vi, pp. 327-339, 
Dec., 1890; and the Am. Journal of Science, ITI, Vol. xli, pp. 33-52, Jan., 1891; 
and same, Vol. xlvi, pp. 114-121, Aug., 1893. This view, which may be ‘called 
the epeirogenic theory of the causes of the Ice age, has been gradually thought 
out in America by Dana, LeConte, Hilgard, Wright and others, and in Scotland 
by Jamieson. Its earliest announcement was in 1855, by Dana in his Presidential 
Address before this Association (Proc. A. A. A. S., Vol. ix, for 1855, pp. 28, 29; 
. Am. Jour. Si., IT, Vol. xxii, pp. 328, 329, Nov., 1856). 
DM M c e a MEE E C QU LL d LM cM c M lab diia a a 
