GEOLOGICAL STRUCTURE OF THE CORDILLERAS. 89 



mountain axis, but the desert basin also — yet unequally, so that while one arm of the lever at 

 the north rose 3,100 feet, that at the south was uplifted nearly 4,700 feet. 



Standing on the margin of the desert sandstones and looking at the Cordilleras, they appear 

 only small hills a few hundred feet in height, while viewed from San Bernardino valley they 

 are lofty mountains of an altitude of as many thousands ; so great is the difference of level of 

 the upper edges of the strata on the Pacific and the basin sides of the range. The south entrance 

 of the Cajon, being 1,932 feet above sea level, is the highest point of the conglomerate terrace 

 which skirts the western base of Kikal Mungo and Bernardino mountains. This conglomerate 

 terrace (which has been already described when treating of the plains of San Bernardino) has 

 been removed to the depth of two hundred feet, and the lower beds now constitute the level 

 plains. This denudation is shown in figure 3, plate 6. 



The axial rocks of the Cordilleras are granitic, varying in type from the western to the eastern 

 side^ being mostly felspathic (orthose) on the Pacific side, and passing into hornblendic and 

 magnesian species on the desert side. This is the order of appearance, whether observed at 

 Cajon pass or at Warner's pass. The breadth occupied by these rocks is very different, however, 

 in the two situations : in the northern range — the Kikal Mungo hills — the strata, if the term can 

 properly be applied to gneiss and hornblende schists, are lifted to a high angle and form the 

 lofty sharp-pointed crest of the higher hills, the porphyritic granite having been injected 

 through a comparatively narrow rent or fissure of the crust. In the pass (the Cajon) the pri- 

 mary rocks do not occupy a surface breadth of more than ten miles. In the southern range, 

 south'of San Jacinto mountain, the schists and gneissose rocks are not elevated at so high an 

 angle, while the primary rock occupies a much greater breadth of ground, at Warner's pass 

 the breadth being over thirty miles. In the passes, of course, the appearance of primary rock 

 is always of less amount than in the chain itself, and therefore these figures do not express the 

 full extent occupied by the igneous rocks in their average width — at the Kikal Mungo hills it 

 would be about sixteen miles, and in San Diego county forty miles. 



As a detailed description of the varieties of igneous rock met in passing over the axis of the 

 chain is given when describing the Cajon and Warner's passes, it is only necessary to contrast 

 the species found in both situations. 



Cajon, past. 

 West. 

 FeLspar rock, with veins of porpliyritic granite. 

 Mica slate and gneiss. 



Hornblende schist. 

 Hornblende and felspar. 

 Gneiss. 

 Talcose schists 



Felspathic granite, with wide veins of quart*. 

 Gneiss, with hornblende. 



East. 



Warner's pats. 

 West. 

 Felspathic granite. 

 Granitic porphyry. 



Granite, including gneiss in broken masses. 

 Gneiss. 

 Mica slate. 



Granite, with hornblende and mica crystals. 

 Hornblende and albito, with a paste containing carbonate 

 of lima. 



Syenite. 

 East. 



Thus, in both instances, the felspathic rocks are accumulated with micaceous granites on the 

 west ; and the porphyries — gneiss and schists — rocks on the east, all contain amphibole, either 

 as hornblende, actynolite, with tourmalines, or occasionally epidote and talc granite, (protogine.) 



Plate 6, fig. 1, gives an illustration of the strata met with in Cajon pass. 



Upon these axial rocks the sedimentary strata are disposed, unconformably, on the eastern 

 slope of the great basin, (Mohave valley,) and lapping round the southern margin of the Sierra 

 12 U 



