492 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



jack-rabblts, of which the northern species, the common jack-rabbit 

 or prairie hare, ranges from Saskatchewan to southern Colorado, 

 i. e., a little south of the Arkansas Divide, and the southern 

 species, the black-tailed jack-rabbit, from Mexico and Texas on 

 the eastern side to northern Colorado, a little north of the divide. 

 The plains east of the Rockies may therefore be divided into 

 two life zones as far as the native flora is concerned. 



1. The Great Plains, north of the Arkansas Divide, and 

 belonging to the Subboreal or Transition Zone. 



2. The Staked Plains, south of the divide and belonging to the 

 Upper Sonoran division of the Austral Zone. 



9. Columbia Plains 

 The upper part of the Columbia Plains belongs also to the 

 Submontane Zone. For a description I shall only refer to Piper's 

 Flora of Washington.^ 



V. Upper Sonoran Zone 

 Just as the Submontane Zone runs across the mountains and 

 forms the bottom of the saddle between the Northern and South- 

 ern Rockies, so does the Upper Sonoran Zone in the saddle between 

 the Southern Rockies and Sierra Madre of northern Mexico. 

 Just as the Submontane Zone is made up of two principal divisions, 

 (i) The wooded hilly or broken country, and (2) The grass-covered 

 plains, so is also the Upper Sonoran region bordering the Southern 

 Rockies, but the plains are not always covered by grasslands. 

 This is especially the case on the western side of the mountains. 

 The Upper Sonoran division of the Rocky Mountain region may 

 be divided into the following divisions: 



1. Pi non-Cedar Belt. 



2. Staked Plains. 



3. Great Basin, including the basins of the Colorado and 

 Snake Rivers. 



4. Columbia Plains. 



I. Pifion-Cedar Belt 

 This has ahcadN- Ixcn nu'iitioned under the Foothills, It is 

 madi- up of hills or tablelands mostly covered by an open stand of 



' Contr. I'. S. Nat. Ilcrl). ii: 1900. 



