NO. 



GENTRY : LAND PLANTS 93 



No permanent source of fresh water is known. Physiographlcally it is 

 young and the arid climate tends to perpetuate the youthful appearance. 

 Sedimentary rocks appear to overlie basic igneous and are in part over- 

 lapped by extensive lavas. Very little appears to be on record describ- 

 ing the island. W. H. Burt of the University of Michigan in his notes 

 generously loaned to me described the island as follows: "A range of 

 mountains, attaining a height of 4315 feet in the northern part, traverses 

 the entire length of the island. This mountain range is highest at the 

 two ends and there is a low pass near the center of the island. The west 

 shore is for the most part precipitous but there are several landing places 

 on the east shore and at the north and south ends. The mammalian 

 fauna which is represented by three species, a pocket mouse {Perog- 

 naihus)j rock mouse (Peromyscus) , and a wood rat (Neotoma), seems 

 very small for an island of this size which is only eight miles from the 

 mainland shore at its nearest point." 



Vegetation is sparse, particularly at the northern end of the island. 

 Both Slevin (1923:69) and Johnston (1924) state that they found 

 more vegetation at the southern end. It is densest on the low gentle 

 slopes in the valleys, and along washes. Judging from the flora (Table 7 

 and Plate 8), it appears to be a microphyllous, succulent tree and suf- 

 frutescent shrub desert, not essentially different from that common to 

 the low and middle elevations of the adjacent mid-peninsula. The suc- 

 culent or sarcophytic tree forms consist of Pachycereus Pringlei, Le- 

 maireocereus Thurberi, Pachycormus discolor pubescens, Bursera ?nicro- 

 phylla, and are accompanied, at least in the washes and valleys, by such 

 nonsucculent microphyllous trees and shrubs as Prosopis juli flora, Acacia 

 Greggii, Cercidium micro phy Hum, and Olneya tesota. A similar mixture 

 of succulent and microphyllous species occurs in the shrub populations 

 also. Low shrubby suffrutescents are well represented in Dalea, Erra- 

 zurizia, Frankenia, A triplex, Petalonyx, Franseria, and Encelia. 



The flora also shows a strong relationship to the central peninsula, 

 although some of the more striking peninsular species are apparently 

 lacking, e.g., Idria, palms and yuccas. Many of the plants of the upper 

 gulf region are here near their southern limits. The 96 species known 

 to the island are enumerated in Table 7. The known endemics are only 

 6, surprisingly few for the extent and nature of the area. But here again 

 only the spring flora is known. Additional field work is necessary before 

 we can be satisfied of an adequate showing of the flora and before prob- 

 lems of distribution and speciation can be evaluated. The Rempel and 

 Dawson collections are enumerated below in the general catalogue of 



