GEOLOGY OF BAJA CALIFORNIA. 189 



and equivalent to the metamorpliic slates of the gold-belt of 

 the slope of the Sierra jSevada. 



THE SECOND GEOGRAPHIC BLOCK, OR THE SIERRA MADRE. 



The profile (Plate III, fig. 1) crosses the alluvial deposits 

 and small slate and granite knolls of San Rafael vallej^ and 

 continues over the second orographic block, the Sierra 

 Madre, as it is called. Bare and white granite hills rise 

 rather abruptly from the covering debris at their bases and 

 the road winds upwards for five miles at a ratljer steep 

 grade. The granite, of which enormous areas are seen 

 to the north and the south, is identical in structure and 

 composition with that of the coast range — it is a horn- 

 bleude-granitite of light color and easily disintegrating. 



Arrived at an elevation of 3,500 feet and five miles from 

 the valley, one is surprised to find oneself on a gently as- 

 cending plateau, somewhat rolling and with small knolls 

 here and there, but on the whole remarkably level; the 

 eastern sky-line is formed by the plateau; north and south 

 it extends for miles and miles covered with dense brush 

 and yucca trees. At nine miles from the valley and 4,000 

 feet elevation, a small knoll about 100 feet high lies to the 

 south of the road; it is entirely composed of brownish 

 quartzite completely enclosed by the granite, a curious re- 

 mainder probably of a formerly more extensive metamor- 

 pliic area. The plateau becomes still more level and for a 

 distance of about fifteen miles up to the summit, ascends 

 but 700 feet. Beginning at 4,000 feet and extending to the 

 divide, a belt of scattered pines relieves the monotony of 

 the landscape. Here and there are low ranges of hills or 

 small isolated peaks with the yellowish-white granite weath- 

 ering in rounded forms. Such a range, only a few hundred 

 feet high above the plateau and interrupted by passes at the 

 level of the latter, forms the peninsular divide at an ele- 

 vation of about 5,000 feet. The road from Real del Castillo 

 to Campo Nacional, a small mining camp near the summit, 



