84 BOTANICAL GAZETTE [aucusT 
steep and high, but pass back into a complex of gently rounded, — 
semi-detached hills, which present a sinuate or undulating sky line 
of jumbled peaks; it is a vast mountain system in miniature. Backa 
few miles from the bluffs these hills pass imperceptibly into the low, 
rolling prairie hills, which extend with scarcely a variation in either 
direction. So perfectly is this tributary drainage system established 
that upon the prairie swamps and “sloughs” rarely occur. The 
small streams which have threaded the upland are characterized by 
ravines of depth and precipitousness, especially in the loess, where 
they usually end abruptly in a bluff, again dividing the upland into a 
series of ridges, intricately related, which pass back into the low, 
rolling hills of gentle profile. 
Origin of the prairie 
The uplift of the Rocky Mountains, which terminated the Cre- 
taceous, introduced a modifying element which exercised an evel 
increasing influence upon the climate of the Great Plains region. 
Intercepting the eastward-moving moisture-laden winds from, the 
Pacific, a decrease in the annual precipitation to the east of the range 
must of necessity have followed. The greatest reduction would have 
been nearest the mountains, decreasing to the eastward. When this 
interior continental land was finally left by the interior sea and opened 
to migration, invasion must have been in large degree controlled by 
this graduated distribution of rainfall. The subsequent origin of 
the Cascades could have served only to accentuate this distributional 
difference and reduction in precipitation. Under such conditions the 
Tertiary phytogeographical distribution of this central region must 
have been adjusted. Whatever the source, it would seem highly 
probable that while the entire Mississippi valley was occupied by the 
rich Tertiary forest, the region lying to the west and bounded by the 
Rocky Mountains and extending northward into Assinaboia Was: on 
account of the low precipitation, denied to tree invasion and a” | 
to be occupied by a prairie formation, increasing in its xerophytis™ 
westward just as it does today. 
Toward the close of the Tertiary (late Pliocene) fossil evident’ 
points conclusively to climatic change. A retrogressive successio? 
floral waves swept southward under the influence of the 
