THE DESERT SCRUB CLIMAX. 163 



westward, where Larrea is the chief dominant, the general name is grease- 

 wood. None of these will serve as a desirable name for the formation as a 

 whole or for either of its associations. As previously suggested, it seems best 

 to restrict the use of the word chaparral to the Quercus-Ceanothua climax. 

 Mesquite is of too limited application in so far as the scrub community is 

 concerned, being applied only to Prosopia or to Prosopis with some other 

 related dominant, such as Acacia. The word has the further disadvantage of 

 being used for a number of grasses, Bouteloua, Bulhilis, and Hilaria, probably 

 because of their frequent association with the mesquite in grassy parks. 

 Greasewood is the designation of several shrubs, but the common usage 

 seems to agree with the scientific in confining the word to Sarcohatus vermi- 

 culatus. Hence, in seeking a readily usable name for this extensive formation, 

 it has appeared necessary to employ two words, i. e., desert scrub, the latter 

 referring to its nature, the former to its typical habitat. 



Range. ^The area characterized by the desert scrub climax is difficult to 

 delimit for two reasons. It shares with the sagebrush and chaparral forma- 

 tions the habit of breaking up along the line of contact with grassland or other 

 scrub communities, and thus forming a broad ecotone of mixed or alternating 

 conmiunities. In addition, it is especially given to forming parks or savannahs 

 with grassland, particularly along its northeastern edge and on the bajada 

 slopes of mountains. This is typical of Prosopis, but it is also true to a large 

 degree of Yucca and Flourensia, and, to a much smaller one, of Larrea. In 

 Texas, for example, while the three dominants have not been seen together 

 east of Ozona and Odessa, Prosopis is a regular feature in the grassland as far 

 north as Lubbock, and it occurs frequently farther north, finally disappearing 

 in southwestern Kansas. It also occurs generally, but more or less sparsely, 

 in the desert plains grassland, from the mountains of western Texas and 

 southern New Mexico to those of southern and central Arizona, where it is 

 frequently associated with Yucca radiosa. 



If the limits of the formation be determined by the presence of two of the 

 three dominants in more or less complete control, its area will comprise 

 southwestern Texas west of Odessa and Ozona, and the southern quarter of 

 New Mexico. In Arizona the formation is limited to the southern third of the 

 State, owing to the barrier of the central mountain ranges, and to the north- 

 western part beyond the Hualpai Mountains and the Grand Canyon. It is 

 typical in general of southeastern CaUfornia, east of the Laguna, San Jacinto, 

 San Bernardino and Tehachapi Mountains, and the Antelope Valley. It 

 occupies the southern portion of Nevada south and west of Caliente and also 

 the extreme southwestern part of Utah. Its range in Mexico is unknown, but 

 it is the typical formation of the northern part from the mouth of the Pecos 

 westward through Lower California (MacDougal, 1904, 1908; Goldman, 

 1916 : 334, 338). A related community of Prosopis and Acacia extends 

 eastward along the Rio Grande plain as far as the coast, but too little is known 

 of it to warrant assigning it definitely to the desert scrub formation. 



Unity of the formation. The geographical unity of the desert scrub is perhaps 

 greater than that of any other western formation. It constitutes a broad 

 band 500 to 1,000 miles wide from trans-Pecos Texas to southern California and 

 and Lower California. This suffers two great interruptions, one due to the Sierra 



