] 5 jl first on the mountain top 



life zone is from the window of a car as it climbs the fine 

 motor road in the Catalinas near Tucson to a height of 

 nearly eight thousand feet. 



If we begin just northeast of the city hmits we will find 

 ourselves on a flat plain composed of sandy clay. For at 

 least several hundred feet below the surface it is a hard- 

 packed "fill" from the mountains, not very pervious to 

 water and supporting little conspicuous vegetation except 

 for the omnipresent creosote bush and the occasional low 

 mesquite tree. Most birds do not very much favor it, 

 though the road runner, the irascible-looking curved bill 

 thrasher and the exuberant cactus wren are common, and 

 you may see also the crested, jet black, phainopepla 

 perched at the top of a mesquite after the manner of the 

 flycatcher kind. Jack rabbits lope off into the distance and 

 little round-tailed ground squirrels, their plumeless tails 

 high in the air, dash madly across the road. 



Suddenly, ahuost at a line which could be drawn, the 

 whole appearance changes. A solitary saguaro raises its 

 arms. Then two, then a dozen. Between them the green- 

 trunked paloverde spreads its equally green limbs and 

 twigs or, if this is early spring, is almost obscured in its 

 yellow blossoms. Prickly pears and choUa cactus are scat- 

 tered about in profusion. But what has happened? Why 

 have these things suddenly taken over? The difference in 

 altitude between the creosote desert and this succulent 

 desert is hardly perceptible. 



Most of the answer lies underfoot. Offliand one would 

 say that the soil looks less suitable for plant life than what 

 one has just left. In fact its gravelly surface appears most 

 unpromising. And for a good many feet below the smface, 

 gravelly is exactly what it is because this is the region of 



