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The region embracing the States of Utah and Nevada is much diversified as to both topography and plant covering. Though lying wholly within the arid portion of the United States, it contains high mountains separated one from another by dry desert valleys or by table-lands. The region affords a highly varied topography, and its flora ranges from arctic to subtropical and from truly desert elements to the humid elements of the Rocky Mountains and the Sierra Nevada. With the one hundred and ninth meridian for its eastern boundary and the one hundred and twentieth for its western, the greatest breadth is about 890 kilometers (554 miles). The forty-second parallel borders the region on the north and the thirty-fifth on the south, making its greatest length about 780 kilometers (485 miles). Within this region the flora typical of the western United States meets the flora typical of northern Mexico. The line of demarcation between these floras is conspicuous in southwestern Utah and southern Nevada, where it coincides with the northern limit of Covillea tridentata, the creosote-bush, and to some extent with that of Clistoyucca brevifolia, the Joshua-tree .