Reviews of the Ice Age. 229 
caused to halt or re-advance, forming marginal moraines. 
The relationship of the moraines to the glacial lake Agassiz 
demonstrates that only a few decades of years were enough 
for the accumulation of the highest and broadest belts of mo- 
rainal drift; and the growing forests on the Malaspina ice- 
sheet in Alaska show how only moderate advances of the 
principally waning ice-sheet would envelop forest trees and a 
rich flora of temperate species between deposits of boulder- 
clay or till. All the records of both the European and Amer- 
ican glacial and modified drift seem to be explained by the 
climatic vicissitudes of one general accumulation and advance 
of the ice-sheets, followed by a fluctuating recession. The 
Glacial period was short, as shown by the continuance of all 
its marine mollusks ; but many mammals of that time are now 
extinct, while man, a "living soul," brought into existence 
during the Quaternary era, gives it a second title, Psycho- 
zoic. 
Mr. Frank Leverett exhibited a map of Illinois, showing 
the moraines and divisions of the drift which have been found 
by him in that state, and spoke of the two formations of till, 
one above the other, and each bearing a soil and leached sub- 
soil, with an eroded surface, preceding the deposition of the 
loess. These older drift deposits bear testimony of a great 
lapse of time, with movements of moderate uplift and sub- 
sidence, previous to the chief interglacial epoch, which ap- 
pears to have been long, for the valleys then cut by the 
streams are larger than those channelled in the later drift. 
In the discussion following these papers, Prof. G. F. 
Wright urged that the rock erosion of the rivers flowing from 
the glaciated area in the coastal and Appalachian region was 
done during a preglacial time of higher altitude. The early 
gravel terraces are to be regarded, he thinks, as remnants of 
valley drift flood plains that filled the rock gorges to their 
tops and overspread the rock terraces and loops of the 
old river courses left during the preglacial erosion. In gen- 
eral there is a remarkable parallelism of the extreme glacial 
boundary and the retreatal moraines, implying, as he believes, 
that the extra-morainic drift was produced in the same 
glacial epoch with the moraines. The glacial retreat was 
geologically rapid, but was many times somewhat interrupted. 
