MESAS AND FOOT-HILLS 



205 



thrown out supporting ribs that trail away into 

 the valleys. In a granite country these foot- 

 hills are usually very smooth, and are made up 

 largely, as regards their surfaces, of the grit and 

 grind of the rocks. The rocks themselves are 

 usually wind-worn, rounded by rain and sand, 

 and sometimes fantastic in shape. Often the 

 soft granite wears through in seams and leaves 

 lozenge-like blocks linked together like beads 

 upon a string ; often the whole rock-crown of 

 the hill is honey-combed by the wind until it 

 looks as soft as a sponge. The foot-hills of 

 porphyry are more jagged and rough in every 

 way. The stone is much harder and while it 

 splits like granite and falls along the mountain- 

 side in a talus it does not readily disintegrate. 

 The last bit of it remains a hard kernel, and 

 the porphyry foot-hill is usually a keen-edged 

 mountain in miniature. 



The hills have a desert vegetation of grease- 

 wood, cactus, and sage, with occasional trees like 

 the palo verde and the lluvia d'oro ; but their 

 general appearance is not very different from 

 the mesas. Where the altitude is high say 

 five thousand feet and over there may be a 

 more radical change in vegetation ; for now the 

 oak begins to appear, and if it is open country 



Forms of 



thefoot- 



hills. 



plants. 



