Prairie- and Sand Hill District. 543 
courses'). They do not form true forest growth but represent the tongue-like 
extensions of the eastern forested areas. Tracing the western limits ofthe 
species of trees derived from the east we find the following extend farthest 
west in the north central states: Araxinus viridis (= F. lanceolata), Celtis 
occidentalis, Ulmus americana, Funiperus virginiana, Ouercus macrocarpa, Po- 
pulus montlifera (= P. deltoides) and Negundo aceroides (= Acer Negundb). 
The last two can exist in the prairies because they grow on the river banks 
where the supply of water is abundant. In the south central states Ouercus 
obtusiloba (= O. minor) and O. marylandica (= O. nigra) replace Ouercus 
macrocarpa. Celtis occidentalis, Ulmus americana and Juniperus virginiana 
extend farthest west into the Texas plateau, meeting and mingling there 
with Sapindus Drummondii, Fuglans rupestris, Prosopis juliflora and the other 
southern and western species. 
The evergreen forests characteristic of the Rocky Mountains have not 
extended eastward as rapidly as the eastern forests have extended westward, 
but are confined wholly to the foot-hills and to the country. Eight species 
of trees have entered the plains of Nebraska from the Rocky Mountains 
by way of Pine Ridge and Cheyenne Ridge. These are at first Pinus 
ponderosa var. scopulorum, Shepherdia (Lepargyraea) argentea. The other trees 
which have entered from the west have followed one or the other of these 
two ridges. Thus Populus tremuloides, P. angustifolia, Acer glabrum, Betula 
occidentalis follow Pine Ridge. On Cheyenne Ridge, Populus acuminata has 
pushed in from the Wyoming foothills and the mountain mahogany Cerco- 
carpus parvifolius has pursued the same course ?). 
It remains shortly to distinguish the associations of the forests in these 
territories, following POUND and CLEMENTS for Nebraska, in considering firstly 
the River Bluff Formation. This is most extensive and characteristic. It is 
typic of the bluffs of the Missouri and other large streams. Two types are 
known, the red oak- and the bur oak association. 
The Red oak-Hickory Association is constituted by the following trees 
Quercus rubra, Carya alba (= Hicoria ovata), Ulmus americana, Fraxinus 
viridis, Fuglans nigra and as other species occur this type is both variable 
and heterogeneous. 
In the lower Missouri Valley Quercus macrocarpa, Q. coccinea, = alba, Carya poreina 
(= Hicoria glabra), C. amara (= Hicoria minima) are occasional intruders In places the elm- 
i es 
Gymnocladus canadensis (= G. ven Crataegus ee Zanthoxylum americanum, Cereis can- 
adensis, Ostrya virginiana. mus fulva is often associated with its congener U. americana which 
it may rarely replace. Gleditschia triacanthos, Juniperus virginiana, Platanus occidentalis are 
scattered here and there. The role of lianes is an important one. Smilax hispida grows in the 
ı) Kerroce, Rovaı $.: Forest Belts of western Kansas and Nebraska, U. S. Forest Service, 
Bulletin 66. (1905). 
2) See Bessev, CHARLES E.: Are the Trees Receding from the Nebraska Plains. Garden. 
and Forest X: 456. 
