604 METASPERMAE OF THE MINNESOTA VALLEY. 



regions where Tertiary deposits are found, and quite certainly 

 therefore in the valley, several varieties of palms. Thearchi- 

 chlamydeous arboreal types reached a high degree of expan- 

 sion and some of the noblest trees — such as the tulip- tree 

 {Liriodenclron 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 Ai'chichlamydece just as the present 

 age, succeeding the glacial epoch might be termed very appro- 

 priately the Age of Metachlamydece. 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 Archichlamydese. In this 

 period, or more probably earlier, the newer types of the 

 Metachlamydese 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 moTement. 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 



