92 CARNEGIE INSTITUTION OF WASHINGTON. 



A slice of the travertine extending across four lines of a complex picto- 

 graph and down to the granite base was cut out and the surfaces of the 

 sample are now being polished and prepared for critical examination by 

 skilled lapidaries in London under the personal supervision of Mr. Sykes. 

 It is now clear that the carvings were not made in the granite, but in 

 the travertine, and extended study may be necessary to determine the 

 depth at which the figures were made and what deposition and weath- 

 ering has since taken place. This fact favors the presumption that 

 Blake Sea was a fluctuating body of water and not a continuously 

 receding one. The final proof of the matter will rest chiefl}^ upon 

 biological evidence concerning the activities of organisms in connection 

 with the deposition of tufas to which the botanist may be expected to 

 contribute. The whole body of evidence to be obtained by the study 

 of this material promises to be of prime importance in determining 

 the climatic cycles and geological successions of plants in the Mohave 

 Desert region. 



General Features of Vegetation in the Mohave Desert, by Forrest Shreve. 



In the autumn of 1914 and the spring of 1915 a general reconnaissance 

 was made of the vegetistic features of the Mohave Desert and of the 

 region which lies between it and southern Arizona. The Mohave 

 Desert is dominated by an open stand of microphyllous shrubs, and 

 the principal differentiating feature of its chmate is the occurrence of 

 rain only in the late winter and early spring. The area is sharply 

 contrasted with the Tucson region, where the succulents form such a 

 large element in the vegetation and the chief climatic feature is the 

 bi-seasonal rainfall. The more elevated portions of the Mohave 

 Desert adjacent to the fringing mountains of its southern and western 

 boundaries are characterized by a richly diversified assemblage of 

 shrubs, by the arborescent Yucca, and by a small number of cacti. 

 Throughout the remaining portion of the area, below an elevation of 

 4,000 feet, the dominant plants are Covillea tridentata and Franseria 

 dumosa, and over extended areas these are often the only perennials 

 to be found. Covillea occurs in all topographic situations and in a 

 wide range of soils, in marked contrast to its more restricted occur- 

 rence in the vicinity of Tucson. Franseria is not wholly coextensive 

 with Covillea, being very uncommon on slopes which have rock in situ. 



It is true of the Mohave Desert in general that the minor topographic 

 features are without a marked influence on the character of the vegeta- 

 tion. For example, the open stand of Covillea on a bajada is usually 

 found to extend without any modification onto the adjacent slopes of 

 the mother mountain; the shrubbery found along small streamways 

 is no more dense than it is elsewhere and contains no distinctive species ; 

 the edges of dry lakes can be approached without change in composi- 

 tion or density of vegetation up to the lake bed itself; the north and 



