292 GEOLOGY OF THE HIGH PLATEAUS. 



Eiver. The country which thus concentrates its waters into Potato Valley 

 may be regarded as a vast amphitheater, with a radius vector varying in 

 length from 12 to 18 miles, and of which the ramparts of the Aquarius and 

 Table Cliff form the upper rim. The amphitheater is the work of erosion, 

 being a westward extension of that vast denudation which has removed 

 thousands of feet of strata from the whole region spread out before our 

 gaze. 



As we study the panorama before us, the realization of the magnitude 

 of this process gradually takes form and conviction in the mind. The 

 strata which are cut off successively upon the slopes formerly reached out 

 indefinitely and covered the entire country to the remotest boundary of 

 vision. Their fading remnants are still discernible, forming buttes and 

 mesas scattered over the vast expanse. The same process of reasoning by 

 which the mind joins the edges of strata across the abyss of a narrow 

 canon enables us to join their edges across wider intervals. The restora- 

 tion of the Trias to its Pre-Tertiary condition is made almost at a glance, 

 since the vacant spaces are few. The restoration of the Jurassic and Cre- 

 taceous is precisely the same in nature and equally simple, though the 

 spaces to be covered by it are much wider. The Tertiary is wholly want- 

 ing to the eastward. There remains only a single outlier to the southward — 

 Kaiparowits Peak. But its former extension over the whole of the Plateau 

 Country admits of no serious doubt after we have once mastered the plan 

 of the drainage system and of the Post-Eocene displacements. The rivers 

 alone might not be sufficient to demonstrate the conclusion, nor would a 

 restoration of the displacements, but the two together admit of no other 

 interpretation. How far eastward and southward the lava-cap extended 

 cannot be determined. Remnants of alluvial conglomerates, with large frag- 

 ments of trachytes and augitic andesites, are found more than 20 miles 

 eastward, and they are indistinguishable from the rocks now forming the 

 summit of the plateau. But how far they have been carried is a question 

 which it is impossible to answer. 



The altitude of the eastern front of the Aquarius above the country 

 which it overlooks is upon an average about 5,500 to 6,000 feet, and the 

 thickness of the strata removed from its vicinity is probably about 4,000 to 



