KRRE 
546 Part IV. Chapter 3. 
 Salır cordata and Populus monilifera, both fast growing trees, are prevailing. 
In the lower stretches of the Missouri, Populus montilifera prevails; at the 
mouth of the Niobrara Salir cordata is most prominent and exclusive; else- 
where the two trees may be in association. The secondary layers are almost 
entirely lacking. Salir fluviatilis grows at the water’s edge, with Amorpha 
Fruticosa; Negundo occurs in open places while back in the formation grow 
Cornus shrubs and Ribes gracile with swampy vegetation of Cicuta maculata, 
Seirpus atrovirens, Asclepias incarnata and Lythrum alatum. A loose grass 
covering exists on many of the islands consisting of Elymus virginicus, Muhlen- 
 bergia racemosa and Poa pratensis. ; 
2. Rocky Mountain Region. ö 
The flora of the Stony or Rocky Mountains is a complex one largely 
derivative. The principal forest trees as we have shown (pages 244—250) are 
Pacific coast species that have migrated from a center located somewhere 
between northern central of California and Puget Sound. The alpine species 
show a strong northern character, and in all probability, they reached the region 
during, or after, the final close of the glacial period. Other species have 
wandered in, as elements of the Mexican flora, while still others (a small 
percentage), are endemic to the mountains and probably originated in them. 
Two districets may be distinguished, a northern, or Dominion District, 
situated above a point represented by the headwaters of the Saskatchewan 
River and Milk River (a tributary of the Missouri) on the east, and Clarke’s 
River, a tributary of the Columbia, on the west; and a southern (Park 
Komiiin District) extending south from this point into northern New 
Mexico. An eastern outlobe is the Black Hills Territory. 
The flora of the Park Mountains is continued southward into the mount- 
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other by considerable stretches of arid country with a flora characteristic of the 
Great Basin, or the Mexican tableland. These areas are shown on the ge- 
neral map and a district might be established to include this Aora. As these 
mountains are separated by wide intervals of desert country from the main 
Rocky Mountain mass and as the Rocky Mountain vegetation is likewise broken 
into more or less disconnected masses found only on the higher mountain 
slopes surrounded by a sea of Great Basin, or Mexican arid plant types, it 
has been thought best to treat each‘ of these separated mountainfloras, 48 
well, as those of Rocky Mountain affınity (as delimited on the map) in central 
Nevada with the phytogeography of the Great Basin and Mexican foristic 
regions. : 
A. Northern Dominion District‘). Se 
Although the Selkirks and the Rocky Mountains are orographically 
of the same system, yet floristically there is a considerable difference, because 
ı) See plate I: Mr. STEPHEN in the Rocky Mts.. of Canada, 
