296 GEOLOGY OP THE HIGH PLATEAUS. 



extent and distribution is not accurately known. They cover a consider- 

 able area, but in a disconnected way, and their eruption appears to have 

 occurred prior to the principal epoch of faulting. The mass of conglomer- 

 ates is very great. They are composed wholly of the debris derived from 

 the destruction of the more ancient trachytes and andesites, and are well 

 stratified in layers which are nearly horizontal. 



The age of the principal eruptions of trachyte and andesite cannot be 

 ascertained, but it is very ancient, going back probably into the early Mio- 

 cene. The same indications of great antiquity are found here which have 

 been observed in the Sevier Plateau and in the Tushai- — eruptive epochs in 

 which lavas in enormous quantities were outpoured with hundreds and per- 

 haps even thousands of individual eruptions, epochs of erosion during which 

 were accumulated heavy beds of conglomerate, periods of faulting and dis- 

 location which have given a new topography to the country, periods of 

 renewed activity of volcanic forces, and a long final period of waste and 

 decay. All this conveys the impression of immense duration ; how long 

 the era may have been we do not know, even in terms of the geological 

 calendar. But the interval which separates us from the Eocene must in 

 some way be filled, and these operations are all that we have to fill it with. 



The western front of the Aquarius, from the grand gorge of Mesa Creek 

 to its southern termination, is about 17 miles in length. The lavas and 

 conglomerates are heaviest at the northwestern angle, and diminish in bulk 

 towards the south. The northwestern part of the plateau seems to have 

 been one of the great centers of trachy tic and andesitic eruption from which 

 the extravasated masses flowed outward in all directions. No cones or 

 mountain piles, however, are now visible. If any formerly existed they 

 have been leveled down nearly to a common platform, and can no longer 

 be distinguished from the rolling hills which have been sculptured by the 

 protracted erosion. There is, however, this peculiarity in the locality : 

 the lava-sheets are less stratiform and more chaotic than in localities where 

 they are collectively thinner. They are also more varied in kind and in 

 texture. As we recede from this locality the sheets become more uniform 

 and even in their bedding, as if they had spread out and become thinner. 



