16 THREE GEOLOGICAL PROVINCES. 



V. BASIN RANGE STRUCTURE. 



When the blocks into which a district of country Jias been broken by 

 faults are greatly tilted so that the strata dip at high angles, the uplifted 

 edges of such blocks often form long mountain ridges. Such ridges have 

 the general appearance of the monoclinal ridges already described as con- 

 comitants of other types of structure ; but in this case the ridges constitute 

 the chief mountain masses themselves, and form another general structural 

 type. The monoclinal ridges are due to the erosion of upheaved strata ; 

 these ridges are due to displacement ; they may also be eroded, but in so 

 far as erosion has progressed the ridge like structure is obscured. Many of 

 the rido-e like mountains of the Basin Province have this structure. Such a 



O 



ridge is composed of monoclinal strata, the one side presenting a bold escarped 

 front, the other a more gently sloped back conforming to a greater or less 

 degree with the dip. Sometimes the ridges themselves are faulted longitud- 

 inally, transversely or obliquely, and the faults may be slight or of great 

 magnitude ; but the more common structure is a simple ridge with slight 

 transverse or oblique faults. 



CONCOMITANT FOEMS. 



1. Monoclinal Ridges on the Sack. On the backs of these Basin ranges 

 monoclinal ridges have been observed. 



VI. ZONES OF DIVERSE DISPLACEMENT. 



In this region many zones or irregular areas of country are found to 

 be divided into small blocks by faults and flexures running in diverse direc- 

 tions, and these may be horizontal or be tipped at high or low angles, or 

 even be overturned. The total effect of this diverse displacement may be 

 to uplift the area above or depress it below the adjacent country or not to 

 change its relative altitude. These features are exhibited on a small scale 

 within a limited area, usually so elongated as to be termed a zone. 



During the past season Mr. Gr. K. Gilbert has studied an area where this 

 diverse displacement is by faulting, and the faults are of no great magnitude, 

 and the blocks into which the area has been severed are either not tilted or 

 but slightly so. This presents the simplest illustration of this type that has 



