48 GEOLOGY OF THE HIGH PLATEAUS. 



late A. R. Marvine, shows a series of broad platforms, uplifted with a single 

 monoclinal flexure or a fault on either side. The width of these platforms 

 varies from 20 to 45 miles, and from these masses the individual mountain- 

 piles have been carved by erosion. The restored profiles ' obtained by re- 

 placing the material removed by erosion are not indeed horizontal nor 

 straight lines, but ordinarily convex iipwards, with slight curvature, becom- 

 ing abrupt or even passing into a great fault at the margin of the uplift. 

 Inasmuch as almost anj'- configuration of the strata which is convex upwards, 

 be it never so little, is called an anticlinal, these platforms would probably 

 be so characterized by most geologists. But what a contrast to the short, 

 sharp waves of the Apalachians ! If we analyze the form carefully, it will 

 become apparent that we have to do with a structure which has nothing 

 in common with a true anticlinal except this slight convexity, and which 

 possesses characters which the true anticlinal does not. 



It has already been indicated that faults and monoclinal flexui-es are 

 homologous terms. They i-epresent varying degrees of abruptness in the 

 passage from the thrown to the lifted side of a displacement. In the case 

 of the fault the shearing is confined to a single plane ; in the case of a mo- 

 noclinal flexure the shearing is distributed through a narrow zone between 

 two planes. Both mean essentially the same thing. In the Park Movint- 

 ains we have uplifts with a fault or equivalent monoclinal on one side or on 

 both. Most frequently it is on both sides, but the shearing is almost inva- 

 riably more strongly emphasized on one side than on the other. It rarely 

 happens that the fault is clean and trenchant, but is accompanied with much 

 fracturing and shattering of the thrown edges of the strata, and there are 

 cases when the dragging of the fault has been accompanied by the over- 

 turning of a great slice of strata torn from the thrown edges. Instances are 

 abundant where the rocks in the flanks of these ranges in the vicinity of 

 the faults have been subjected to the most "heroic" treatment; but at short 

 distances from the faults in both directions the disorganization quickly 

 diminishes. Upon the summits of the platforms the traces of violence and 

 distortion attending the upward movement are much less. Where erosion 

 has laid bare the most ancient rocks they are ordinarily found to be more 



