HISTORICAL 



35 



One of the striking features of the Chihuahua desert consists of the 

 great sand dunes, or " medanos," more than a hundred feet high, in 

 some instances forming great ridges that have almost the imposing ap- 

 pearance of mountain ranges ; these move across the floor of the desert 



Fig. 3. Western America showing conceptions of American deserts'current in 



1859. Copied from Warren's Physical Geography, published in 1S59. 

 plains with a sweep that obliterates minor features of topography. 

 These moving dunes bear a characteristic vegetation adapted to the 

 unusual physical conditions offered by the sand and lack of water and 

 nutrient material. The gypsum deposits forming the White Sands in 



