560 RELATION TO ENVIRONMENT. 



and here also occur the globose spiny forms called "pin-cush- 

 ions," strawberry cacti, etc., of the genus Cereus, according to 

 Bray, numerous yuccas (yucca formations), agaves, also coarse 

 succulents, and coarse rough shrubby growths, like the mesquite 

 (Prosopis juliflora), acacias, shrubby junipers, dwarf oaks, etc., 

 which take the place largely of the bunch grasses and sage- 

 brush of the Northern Plain's area, the shrubs and dwarfed 

 trees often forming open or dense thickets known as "chap- 

 arral." 



IV. Edaphic Formations in the Prairie Region. 



The edaphic formations in the prairie region may be represented 

 by some of those in the prairies and Great Plains of the western 

 United States, as follows: the forest formations along the river 

 courses, the meadows, the sand-hills, the alkali areas, and the 

 Bad Lands of Nebraska and South Dakota. 



1047. The forest formations along the courses of the streams 

 are in some cases continuations of the great forest formations of 

 the eastern United States, which reach far up the fertile and 

 moist valleys of the larger rivers. In some cases the forests are 

 more or less interrupted and scattered, where the valleys are nar- 

 row, or along smaller streams where they take on the form of 

 "gallery woods" or "park- like" formations. The relation to 

 the forest formations of the east is shown by the presence of 

 many eastern species, as the black walnut, several species of oak, 

 hickories, elms, paper-birch, etc. In the lower moist valleys 

 broad-leaved deciduous trees prevail, while, in the higher and 

 drier bluffs, is the xerophytic coniferous forest. While many 

 species represent the invasion of elements from the eastern 

 forest, others represent the eastern extension of trees from the 

 Rocky Mountain region and the foot-hills. For example, the 

 yellow pine (Pinus scopularum) in Nebraska along the Niobrara 

 River meets and mingles with trees from the eastern forest (min- 

 gles with the black walnut, according to Bessey), or forms pure 

 forests. 



