280 GEOLOGY OF THE HIGH PLATEAUS. 



quite flat, giving a tabular summit to the mass about 5 miles long and 

 nearly 2 miles wide. An almost impassable talus surrounds the scarped 

 edges of this cap, and renders the ascent difficult except at a few points 

 upon the eastern side. 'J'he lavas are hornblendic trachytes and augitic 

 andesites, heavily interbedded and made up of numerous flows. These 

 rest upon a layer of Lower Tertiary, of which the thickness is not precisely 

 known, but which cannot well exceed 700 feet. Whether the diminished 

 volume of the Tertiary here is due to an originally small amount of depo- 

 sition or to an erosion of the upper members prior to the volcanic overflow 

 is not yet determined, but I incline to the former • explanation. And the 

 general indications seem to be that over the area occupied by the eastern 

 part of the Aquarius and Thousand Lake Mountain the Tertiary deposition 

 was locally much thinner than elsewhere. Immediately beneath is the 

 Jurassic white sandstone. The Cretaceous is absent from its place in the 

 stratigraphical series. Yet a few miles to the northeastward the whole 

 vast Cretaceous system is rolled up and cut off on the slopes of a grand 

 monoclinal. 



This monoclinal is the Water Pocket Fold, which is probably the grand- 

 est feature of the kind in the Plateau Country, so far as known, and per- 

 haps the most typical. Its first appearance is beneath Thousand Lake 

 Mountain (see Atlas Sheet No. 4), where the trend is east-southeast and it 

 gradually swings around towards the south-southeast, reaching to the Colo- 

 rado River, in the heart of the Glen Caflon. It crosses the river into 

 unknown regions. Upon the northwestern side of the mountain it is cov- 

 ered up by Tertiary beds and lava sheets and is wholly concealed, so that 

 neither its northern nor its southern terminations are at present known. 

 The great fault upon the west side of the mountain cuts across this mono- 

 clinal nearly at right angles, and has dropped the platform to the west 

 several thousand feet. The age of the great flexure is evidently older than 

 Tertiary time, for the Lower Eocene beds lie nearly horizontally across 

 the uptvirned edges of the whole Cretaceous system and upon the deeply- 

 eroded surface of the Jurassic sandstone. Inasmuch as the entire body of 

 Cretaceous strata, including the Laramie beds, appear in succession as Ave 

 cross the strike of the flexure and as they are all upturned upon its flanks, 



