226 GEOLOGY OF THE LTIGH PLATEAUS. 



summit, though reaching a considerably greater slope upon the eastern flank. 

 The eastern side, indeed, suggests a monoclinal flexure, but the bending of 

 the profiles is so small and their sweep is so gradual that we may forbear 

 to call it such. It is hardly pronounced enough to justify such a designa- 

 tion. 



Standing in the Sevier Valley and looking at this barrier there are 

 many stretches along its western front which appear quite like a common 

 mountain range. Profound gorges, V-shaped, heading far back in its mass, 

 have cut the table from summit to base and open through magnificent 

 gateways into the valley. The residual masses between these gorges pre- 

 sent their gable-ends to the spectator, who cannot see what is behind them, 

 and they look exactly like so many individual mountains, while in reality 

 they are merely pediments carved by erosion out of a gigantic palisade. 

 Other long stretches of the western front are unbroken and present to the 

 valley of the Sevier a wall of vast proportions. The summit of the plateau 

 is not smooth, but carved into rolling - ridges and vales, deepening eastward 

 into canons, while at several places volcanic ridges cross it transversely. 

 These last are the remnants of old volcanic piles worn down and half 

 obliterated by long ages of decay, for they belong to the middle epoch of 

 volcanic activity, which may be as old as the Middle Miocene. They 

 present from a structural point of view a peculiar relation to the table on 

 which they now stand. In almost every great mountain range of ordinary 

 type the axes of those minor ridges or superimposed features which had 

 their origin in general causes which built the entire range lie roughly 

 parallel to the main uplift in the relation of superimposed waves of displace- 

 ment. But here it is otherwise. The volcanic ridges which are planted 

 upon the Sevier Plateau run not along its major axis, but across the table 

 from side to side. The movement which hoisted the plateau en masse was 

 not sensibly embarrassed by such trifles as a few ridges of volcanic piles. 

 The features impressed by erosion, on the contrary, conform to the usual 

 law which prevails in mountain ranges. The streams pour down from the 

 summit along whatever slopes may have been generated by the details of 

 the uplift, and have carved their vales, gorges, and canons accordingly. 

 Since these run across the table or perpendicular to its major axis they 



