| OCR Text |
Show 296 GEOLOGY OF THE HIGH PLATEAUS. extent and distribution is not accurately known. They cover a considerable area, but in a disconnected way, and their eruption appears to have occurred prior to the principal epoch of faulting. The mass of conglomerates 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 Miocene. The same indications of great antiquity are found here which have been observed in the Sevier Plateau and in the Tusharâ€"eruptive epochs in which lavas in enormous quantities were outpoured with hundreds and perhaps even thousands of individual eruptions, epochs of erosion during which were accumulated heavy beds of conglomerate, periods of faulting and dislocation 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 trachytic 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. |
