DEPARTMENT OF BOTANICAL RESEARCH. 93 



south slopes of hills and low mountains are also without differences of 

 vegetation. These statements are not true of the more elevated parts 

 of the Mohave, but each of them holds true below about 4,000 feet, 

 and in all of the features mentioned the Mohave area differs greatly 

 from southern Arizona, where all slight topographic differences are 

 accompanied by dissimilarities in the vegetation. It is only in the 

 largest mountains that the vegetation exhibits a differentiation due to 

 altitudinal climatic changes. Many mountains which rise 2,000 to 

 3,000 feet above the desert floor are strikingly similar to the desert 

 itself in their vegetation. The bajadal slopes are also identical through 

 a vertical range of 3,000 feet. The principal vegetistic dift^erences 

 from an elevation of 4,000 feet down to the level of the Colorado River 

 lie in (1) the dissimilarity of the texture and perhaps of the soluble 

 mineral content of the soils which lie nearest the dry lake-basins and 

 those which lie nearer the surrounding hills or mountains, and in (2) the 

 special features of the sandy areas. The fine alkaUne soils which lie 

 nearest the dry lakes are covered by Atriplex, while the coarser and 

 presumably less alkaline soils of the bajadas are dominated by Covillea 

 and Franseria. Areas with sandy soil always exhibit differences from 

 adjacent non-sandy areas, even when the two are topographically 

 equivalent. The most striking features of the sand are the low and 

 open stands of Covillea and Franseria, the presence of perennial and 

 annual grasses, and the great abundance of ephemeral herbaceous 

 plants. 



The most striking feature of the vegetation of the Mohave Desert, 

 so far as investigated, is the slight amount of habital differentiation 

 which it exhibits. This condition is closely related to the severity of 

 the physical environment and to the highly specialized behavior of the 

 few species of dominant plants involved in the vegetation, each of 

 which is capable of enduring a wide range of conditions. 



Climatic Changes, by Ellsworth Huntington. 



A preliminary reconnaisance of the principal basins in the Mohave 

 Desert region and to the northv/ard was made during the earlier part 

 of the year. The purpose of this work was to determine whether more 

 detailed study another year will furnish an adequate basis for a climatic 

 scale extending from the Tertiary to the present time, and sufficiently 

 accurate to serve as a standard of reference for other parts of the world. 

 Such a scale has been prepared by Penck, for example, on the basis of 

 glaciation in the Alps and elsewhere. Glaciation, however, tends to 

 destroy the records of past events more than to preserve them. Hence 

 the attempt to use it as the basis for determining the number, nature, 

 and probable duration of periods having one kind of climate or another 

 is fraught with great difficulty. In a desert region such as that which 

 drains to Death Valley, on the contrary, the records of past events 



