604 METASPERMAE OF THE MINNESOTA VALLEY. 
regions where Tertiary deposits are found, and quite certainly 
therefore in the valley, several varieties of palms. The archi- 
chlamydeous arboreal types reached a high degree of expan- 
sion and some of the noblest trees—such as the tulip-tree 
(Liriodendron tulipifera), for example—which are now of re- 
stricted North American range, were then widely distributed 
around the northern hemisphere. This middle Tertiary time 
might be called the Age of Archichlamydece just as the present 
age, succeeding the glacial epoch might be termed very appro- 
priately the Age of Metachiamydee. In North America the 
Tertiary movement extended from the polar regions at least 
to southern California and probably to Georgia. Under the 
competition and tensions of such a wide-spreading southward 
movement the development of many of our modern genera 
took place and even of several of the more common modern 
species of Monocotyledones and Archichlamydez. In this 
period, or more probably earlier, the newer types of the 
Metachlamydeze with their highly modified flowers and fruits 
began to emerge. 
In a general sense then the monocotyledonous and archi- 
chlamydeous plants of the Minnesota valley derive considerable 
explanation from the consideration of Tertiary comminglings 
just as do the metachlamydeous forms from interglacial and 
post-glacial comminglings. Even in Tertiary times the 
monocotyledonous trees must have been sharply attacked by 
the robustly developed archichlamydeous forms, but it was 
not until the glacial epoch that their hold on the region of the 
Minnesota valley was finally destroyed. 
The post-Tertiary movement. After Tertiary time the 
elevation of the northern part of the North American conti- 
nent and of the western part of the European continent, 
together, very probably, with the secular inclination of the 
earth’s axis, brought about the gradual glaciation of these 
areas. Not only once did the glacier plow its course south- 
ward in the northern hemisphere, but certainly several times. 
Two principal epochs of glaciation are recognised by Ameri- 
can glacialists—the earlier one in which the terminal moraines 
reached as far south as 39° N. lat. and the second, during which 
the ice moved to a much less distance and piled up the morainic 
area of the lake region in Minnesota near lat. 45° N. This 
morainic area forms the northern boundary of the Minnesota 
valley. Under the rigorous conditions of the advancing conti- 
nental ice-sheet it was necessary for plants either to migrate 
