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Show 6 INTRODUCTORY. little disarrangement, except at the fault planes which bound the several blocks. These divisional lines are sometimes sharp, trenchant faults, sometimes that peculiar form of displacement to which Messrs. Powell and Gilbert 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 observersâ€"The Plateau Country. West of this province lies a third oneâ€"the Great Basin. Its topography 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 usually 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, which name is evidently defective in etymology. The nature of monoclinal flexures is most ably discussed by Professor Powell in Expl. of Colorado River, 1869-1872. |