94 



SCIENCE. 



[X. S. Vol. ,XI. Xo. 525. 



^iKida and Allcnrolfia are set close against 

 the lava wall. This is ideal ; this we should 

 expect and here it is. 



The sands and the Java lie in the middle 

 of our desert. If we take these as a start- 

 ing-point and move toward the summit of 

 the mountains, the successive belts of vege- 

 tation gradually shape themselves so that 

 we learn presently to identify them by 

 their color. A plain below the general level 

 is gray, grass-covered, witli here and there 

 a bunch of Ephedra or nopal, no yuccas, 

 no Atriplex, 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 character and color. A little 

 further mountainward and we reach the 

 Covillca tridentata, ever in bloom, which 

 lies as a girdle of green and gold around 

 the whole base of the mountain range, vis- 

 ible for miles and marking for us the limits 

 of the talus with an exactness that is re- 

 markable. Beyond the Covillca belt come 

 the cacti as the terrene becomes more 

 rocky; Mamillaria, with its species numer- 

 ous and varied, the unique hwt widely dis- 

 tributed ocatillo, the prickly pear, often in 

 giant form — all these cover the rocky slopes 

 that lead up to the steeper walls of paleo- 

 zoic rocks. Sometimes, where a shelf oc- 

 curs, and the bare limestone forms a flat, 

 me.sa-like field, the yuccas come back, but 

 not the Eseondida form, with Agave parryi, 

 and abundant ocatillo, while in the rocky 

 defile below, locked amid gigantic ])oulders, 

 now on their tardy journey to the talus 

 plain, the creamy flowers and fruit of 

 Dasylirion lift their glorious spikes, the 

 envy and vexation of the photographer. 



The strata of the lower carboniferous 

 limestones now confront us; crystalline, 

 encrinitic and exceedingly hard, rising 

 often hundreds of feet sheer up and down. 

 But these dry walls likewise have their 



flora. Mamillaria micromeris matches with 

 its hoary spheres the weathered stone or 

 lights it up betimes with scarlet bloom, and 

 Notholcena Utnata fills with somber tufts 

 every shattered crevice. 



But the upper members of the carbon- 

 iferous are much softer and, amenable to 

 erosion, present a gentler, flowing topog- 

 raphy. These slopes are everywhere 

 clothed with oak, not trees indeed; far 

 from it; low dense shrubs, the so-called 

 shin-oak, Quercus gambcllii and Quercus 

 gunnisoni. These two species form pale 

 green belts around the mountains, and are 

 recognized easily, distinguishable for miles. 

 These species indeed form a sort of phyto- 

 graphic border land ; all below is desert ; 

 all above is forest ; for above stands, or 

 lately stood, one of the fairest bits of wood- 

 land in the United States, and that means 

 in the world. But this forest is again in 

 large measure conformable to geologic 

 structure, its distribution determined by 

 the history of what lies beneath. 



As we ascend the mountain, passing all 

 the carboniferous limestones, sands, chalk- 

 beds and shales, we presently encounter the 

 'red beds' already mentioned, the most re- 

 markable geological horizon in the country, 

 familiar to every student of our central 

 moinitains, noted even by the ordinary 

 toiu'ist, the same wherever found — in Utah, 

 Colorado, the Black Hills of South Dakota, 

 and here again in these far-off mountains 

 of the Mexican border, the same vast gyp- 

 sum-burdened deposits of clay and shale 

 and sand. The red beds yield easilj' to 

 erosion. The washings from their wasted 

 flanks have tinged the desert far below, 

 and reddened the walls of every rocky 

 canon on the way. Sloping terraces and 

 flat-topped hills afford a soil rocky but not 

 inf(>rtile, supporting once more its own pe- 

 culiar vegetation. Here are still the .shin- 

 oaks, it is true, but all overshadowed by 

 other nobler trees; here is Berberis trifolio- 



