248 



D. T. MacDOUGAL 



comfiture. The camel is slow in all but two things. One of these 

 is rising from the ground; and no sooner do you touch the saddle 

 than he may go up to his full height with lightning-like rapidity, 

 which may mean a very bad spill ; and even veteran riders may ac- 

 cept the aid of a camel boy who holds the animal's head to the 

 ground until the rider is safely seated. 



The vegetation of arid regions is striking to the casual observer ; 

 but it is no less characteristic than the surface geological features. 

 Rocks crumble and the fragments fall to the bottom of the cliff and 

 roll down the slopes everyivhere, but the character of the disinte- 

 gration of the mountain walls and the position which the detrital 

 material takes at their bases is largely determined by the amount 

 of precipitation and by the streams which are formed in conse- 

 quence. Likewise, the conditions offered by the detritus accum- 

 ulated around the bases of arid mountains are different from those 

 presented by the basal slopes of such material in places with a pre- 

 cipitation which overbalances evaporation. This may indeed be 

 a study in geology; but it is a subject which the physiologist out 

 of doors must understand if he is to attempt any adequate expla- 

 nation of the habits and action of desert vegetation. The masses 

 of detritus which are carried out of the canons of Arizona moun- 

 tains run out as rounded tongues of loose material for a distance of 

 a few to fifteen or twenty miles and, by reason of their arched or 

 curved surfaces, are known by the Mexican term of bajadas. 



The mountains of this side rise abruptly from so near the sea 

 that the detrital material which has tumbled and rolled down the 

 long, sloping bajadas has pushed out into the sea, covering its bed 

 to a very slight depth near the shore, where the loose material is 

 carried about by the waves. The vegetation of the bajadas, with 

 its characteristic spreading, flatfish tops, included xerophytic 

 shrubs, Acacia rubica var. erythraea, colocynth (Citrullus colocyn- 

 thus), and a second small melon, Cucumis prophetarium. One of 

 the most striking plants was a shrub with slender, switch-hke, 

 leafless branches six to ten feet in height with the appearance of an 

 Ephedra, which was found to be Leptadenia spartium, usually 

 occurring along the margins of dry stream ways. 



