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 11 u via 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 

 the foot- 

 hills. 



Mountain- 

 plants. 



