28G NORTH AMERICAN FAUNA. [No. 7. 



they will not be considered here. It should be mentioned, however, 

 that each of the life zones is subdivisible both latitudinally and longitu- 

 dinally, and that while the former divisions are clearly dependent on 

 temperature, the causes controlling- the latter are not always well under- 

 stood. Such local factors as soil and slope are not here referred to. 

 The most marked longitudinal divisions, so far as the Great Basin is con- 

 cerned, are those of the Lower Sonorau Zone, which may be designated 

 the Larrea belt and the Grayia belt. The creosote bush (Larrea triden- 

 tata) is the most conspicuous, most widely distributed, and best-known 

 bush of the torrid deserts of the southwest, where it covers the gravel 

 soils up to a certain line, which probably marks the southern limit 

 of killing frost. The Larrea belt is the most important of all from the 

 horticultural standpoint, because it is suited to the requirements of the 

 citrus fruits, the olive, almond, fig, and raisin grape. Associated with 

 the Larrea, and coinciding with it in distribution, is the inconspicuous 

 Franseria dumosa. Another species occupying the same gravel soils, 

 but less generally distributed, is the beautiful and fragrant Krameria 

 parvifolia. The alkali soils of the same belt are covered with grease- 

 woods of the genus Atriplex, of which A.polyearpa is the most charac- 

 teristic. The Grayia belt, named from its most distinctive and wide- 

 spread bush {Grayia spinosa), occupies the strip between the upper 

 limit of Larrea and the lower border of the true sage brush (Artemisia 

 tridentata), which latter indicates the beginning of the Upper Sonoran 

 Zone. Other shrubs of the Grayia belt are the dark Goleogyne ramosis- 

 sima, which resembles Krameria parvifolia in general appearance, but 

 belongs to a different order and has yellow flowers; the handsome 

 Tetradymia spinosa and T. glabrata; the fetid Thamiiosma montana; 

 the stunted Menodora spinosa, whose conspicuous green berries always 

 grow in pairs; and the singular Salazaria mexicana, whose inflated cap- 

 sules are borne away by the wind and lodge in great numbers upon the 

 spiny cactuses. Certain shrubs range over the whole breadth of the 

 Lower Sonoran Zone, occurring alike iu the Larrea and Grayia belts. 

 The most noticeable members of this category are the olive- colored 

 Ephedra nevadensis, which has no apparent foliage and is used as a 

 medicine by the Indians and miners; the handsome Daleas, with their 

 blue and purple flowers, and Lycium andersoni, which bears a small 

 edible fruit. 



The true sage brush (Artemisia tridentata) begins with a solid front 

 along the southern border of the Upper Sonoran Zone and spreads 

 northward over the Great Basin like a monstrous sheet, covering almost 

 without a break hundreds of thousands of square miles. It is not 

 only the most striking and widely diffused plant of the Upper Sono- 

 ran and Transition zones, but as a social plant has few equals, often 

 occupying immense areas to the exclusion of all but the humblest and 

 least conspicuous forms. Wherever one travels in this vast region, 

 the aromatic odor of the sage brush is always present, and some- 



