MAMMALS OP THE MEXICAN BOUNDARY. 31 



Yuma, and Colorado deserts are dry, rocky, and extremely barren. 

 When the Coast Range is reached (highest monument 1,371 meters 

 or 4,498 feet), the sterility lessens during the ascent, and the fer- 

 tility of their west slopes appears in striking contrast, moist verdure 

 everywhere meeting eyes long weary of the sight of glistening 

 desert sands and mountains of bare rock. 



Deserts. — The Mexican Boundary Line -bisects two great interior 

 deserts, separated from each other by a more fertile elevated central 

 tract, and from the coasts by tracts of forested or chaparral lands. 

 The Eastern Desert Tract is high, averaging over 1,000 meters or 

 3,280 feet in altitude ; the Western Desert Tract being low, that por- 

 tion known as the Colorado Desert including an immense depression, 

 below the level of the sea. The Eastern Desert Tract is formed by 

 a series of ancient lake basins, many of which are connected by the 

 Rio Grande and its tributaries. The Western Desert Tract is divided 

 by the Great Colorado River. 



Geology. — The Rio Grande embayment corresponds to the Tamau- 

 lipan Tropical Tract." Between this and the Quitman Mountains of 

 western Texas is what geologists designate the Texan Region, in this 

 work spoken of as the Middle Texan Tract. The Eastern Desert 

 comprises a series of ancient lake basins, bounded on the east by the 

 Quitman Mountains and on the west by the mountains of the Elevated 

 Central Tract ; and the Western Desert occupies the site of an ancient 

 sea, situated between the Elevated Central Tract and the Coast Range 

 of California. The Elevated Central Tract lies between the Eastern 

 and Western Desert tracts, and corresponds to the irregular upfolding 

 of the region between the high table-land of Mexico and the great 

 Colorado Plateau, which latter includes the northeastern third of 

 Arizona and the adjacent portion of New Mexico. The lowest trans- 

 verse depression in the Elevated Central Tract is crossed by the 

 Southern Pacific Railroad. On the south the final break in the Colo- 

 rado Plateau probably corresponds to the Guadalupe Mountains on 

 the Mexican Boundary Line, where the head tributaries of the Rio 

 Yaqui have cut to the lowest level on this portion of the boundary 

 region. Across northeastern Arizona the Colorado Plateau breaks 

 off sharply, often precipitously, affording a clear view of its geolog- 

 ical structure, which is still better shown at the Grand Canyon of the 

 Colorado River to the northward, where a vertical section of the 

 earth's crust a mile in thickness may be plainly viewed. In general, 

 this whole region is of limestone above and sandstone below, over- 

 laid by volcanic rock. The desert ranges of mountains between the 

 Colorado Plateau and the Coast Range of California are mainly of 

 coarse granite and other intrusive rock, more or less covered by and 



For a description of tlie faunal " tracts " and life areas of the Mexican 

 Boundary strip see page 70. 



