164 On the Campus 



set close against the lava wall. This is ideal; this we 

 should expect and here it is. 



The sands and the lava lie in the middle of our desert. 

 If we take these as a starting point and move toward 

 the summit of the mountains, the successive belts of 

 vegetation gradually shape themselves so that we learn 

 presently to identify them by their color. A plain below 

 the general level is gray, grass-covered, with here and 

 there a bunch of ephedra or nopal, no yuccas, no atri- 

 plex, no other forms of cactus. As the terrene rises to 

 the silt-plain, thickets of cholla alternate with mesquite 

 'and the crucifixion thorn; not that other species do not 

 occur, but these are dominant, give to the belt its charac- 

 'ter and color. A little further mountainward and we 

 reach the Covittea tridentata, ever in bloom, which lies 

 as a girdle of green and gold around the whole base of 

 the mountain range, visible for miles and marking for 

 us the limits of the talus with an exactness that is re- 

 markable. Beyond the covillea belt come the cacti as 

 the terrene becomes more rocky; MamiUaria, with its 

 species numerous and varied, the unique but widely dis- 

 tributed ocatilla, the prickly pear, often in giant form 

 all these cover the rocky slopes that lead up to the steeper 

 walls of paleozoic rocks. Sometimes, where a shelf oc- 

 curs, and the bare limestone forms a flat, mesa-like field, 

 the yuccas come back, but not the Escondida form, with 

 Agave parryi, and abundant ocatilla, while in the rocky 

 defile below, locked amid gigantic boulders, now on their 

 tardy journey to the talus plain, the creamy flowers and 

 fruit of dasyliria lift their glorious spikes, the envy and 

 vexation of the photographer. 



