GEOLOGY OF BAJA CALIFORNIA. 195 



Younger eruptives are absent in the northern section un- 

 til the Colorado River is nearly reached, while near the 

 southern section they occupy considerable areas at the foot 

 of the great escarpment. 



It is most probable that the present metamorphic areas 

 are but small remnants compared with those that once cov- 

 ered the granite; the exposures along the section, such as 

 for instance the small quartzite knoll on the plateau, or the 

 metamorphic gravel at Campo Nacional, go far to prove 

 that the slates everywhere rest as shallow crusts on the 

 granite. 



Considering the structural relations one cannot but be- 

 lieve that the great eastern escarpment is formed by an 

 enormous fault, along which a throw of several thousand 

 feet has taken place, and the fresh character of the fracture 

 goes to prove that the dislocation is comparatively recent. 



The occurrence of auriferous river gravel, indicating 

 that a stream of no small dimensions once flowed near the 

 ver}^ summit of the range, speaks very strongly for the ex- 

 istence of such a fault; indeed it almost proves it. A part 

 of the course of this old channol must evidently have been 

 cut off by the fault. 



Eegarding the San Rafael valley, it should be noticed 

 tliat it is bordered both east and west by a rather steep 

 escarpment, and that a line carried from the summit of the 

 first range up to the beginning of the plateau of the Sierra 

 Madre, would give a nearly unbroken slope for the whole 

 peninsular range. It would, therefore, seem very plausible 

 to consider the valley as a sunken area between two north 

 and south faults (Graben Suess). The fact that in the sec- 

 tion, metamorphic rocks are found near the summit of the 

 first range, and then again at the level of the valley at the 

 foot of the steep descent, speaks strongly in favor of this 

 suggestion. At last, considering the structure of the whole 

 range, one cannot fail to be impressed with its great dif- 

 ference from the Coast Ranges of California and its 



