6 INTEODUCTOET. 



little disarrangement, except at the fault planes which bound the several 

 blocks. These divisional lines are sometimes sharp, trenchant faults, some- 

 times that peculiar form of displacement to which Messrs. Powell and Gil- 

 bert have given the name of monoclinal flexures,* but most frequently the 

 dislocation is a combined monoclinal flexure and a fault or series of faults 

 with all shades of relative emphasis. If we look solely at the amount of 

 energy displayed in the vertical differential movements, we shall probably 

 reach the conviction that it does not fall much, if any, below that required 

 to build the most imposing mountain ranges ; yet within the limits of any 

 one of the great blocks into which this country has been divided the strata 

 have preserved their original attitudes with a singularly small amount of 

 warping, flexing, and comminution. Sometimes the blocks are slightly 

 tilted, causing a slight dip, and in the immediate neighborhood of a great 

 dislocation a single flexure of the beds is usually seen; but, on the whole, 

 the amount of bending and undulation is very small. This small amount 

 of departure from horizontality of the beds as they now lie has played its 

 part in the determination of the topographical features as they appear in 

 the landscape, and justifies the name which has been applied to it with one 

 accord by all observers — The Plateau Country. 



West of this province lies a third one — the Great Basin. Its topog- 

 raphy and structure are characterized by jagged ranges of mountains, 

 ordinarily of very moderate length, and separated by wide intervals of 

 barren plains. These ranges are usualty monoclinal ridges produced by 

 the uptilting of the strata along one side of a fault. Sometimes the faults 

 are multiple; that is, consist of a series of parallel faults, the intervening 

 blocks being careened in the same manner and direction. This repetitive 

 faulting is of frequent occurrence. Other modifications, and even different 

 types of structure, are presented; but there is throughout the Great Basin a 

 striking predominance of monoclinal ridges, in which one side of a range 

 slopes with the dip of the strata, while the other slopes lie across the 

 upturned edges. The forms impressed upon these masses by erosion are 

 rugged, bristling, and sierra-like, and their peculiarities are aggravated by 



Mr. Jukes describes a great flexure of similar nature in Ireland under the name uniclinal flexure, 

 wbicb uame is evidently defective in etymology. The nature "1' monoclinal flexures is most ably ilis- 

 eusscd by Professor Powell in Expl. of Colorado Kiver, 1669-1872. 



