172 BOTANICAL GAZETTE [September 



sandstone the soil is more porous, with muclf sand; and plants 

 of an assemblage typical of sandy soil are seen (17). Sand hills are 

 present locally, usually to the leeward of larger streams. 



Near the mountains the debris-cover, if present, considerably 

 modifies topography, soil conditions, and vegetation. It may 

 extend a long way into the plains, or may have been removed very 

 near the beginning of the graded slope from the foothills. 



Saline or alkaline areas are locally present. The valleys of the 

 Arkansas and its tributaries (wet- weather streams, many of them, 

 with trenched flood-channels) are in many places alkaline, and show 

 prominent stands of Sarcohatus-Chrysothamnus vegetation. 



Woody vegetation from the foothills extends locally far into the 

 plains in rock outcrops, and along stony crests of stream-blufifs or 

 terraces. The larger streams are bordered for many miles from 

 the mountains by cottonwoods, usually scattered. 



Climate 



The region has a continental climate, semi-arid, less so at the 

 base of the mountains and in the foothills, with most of the rainfall 

 in the warmer months. Wind movement, proportion of sunshine, 

 and evaporating power of the air are high in the plains, with wide 

 extremes of temperature; all of these features are less marked in 

 the foothills. 



The southern part of the region is warmer and drier than the 

 northern, and with different distribution of rainfall. The rapid 

 east-west change in elevation and topography at and near the 

 mountain-front is accompanied by more or less considerable climatic 

 variation; this with the local peculiarities occasioned by the ele- 

 vated Platte-Arkansas divide, and the differences between areas 

 north and south of the divide, may be seen in the summaries of 

 climatic data for the particular subregions. These data have been 

 taken from the summary of Climatological Data for eastern Colo- 

 rado, southeastern Wyoming, and northeastern Colorado."* The 

 facts shown in table II should be considered in the light of their 



'» Section 6, northeastern New Mexico, by C. E. Linney. Section 7, region drained 

 by the Arkansas in Colorado, and section 8, region drained by the Platte in Colorado, 

 by F. H. Brandenburg. Section 24, southeastern Wyoming, by W. S. Palmer. 



