OCR Text |
Show THE BUILDING OF THE TUSHAE. 183 Whether the somewhat (abrupt western boundary is due to the faulting suggested above or to the termination of the old coulees it is not possible to say with confidence, but the former view seems to furnish the easiest explanation. At the western base of the Tushar, near the town of Beaver, is seen a very recent basaltic crater in a very perfect state of preservation. Farther northward are others, some of them so recent that we may easily suppose that their eruptive activity has ceased within a few hundred years. Many of the basaltic craters throughout the Plateau Country seem to be equally recent, though many others have considerable antiquity. On the whole, however, the true basalts are the most recent of all eruptions. They are seldom found in the heart of the older eruptionsâ€"indeed, I am able to recall but few such instancesâ€"but they occur around the outskirts of older volcanic districts, and often at a considerable distance from them. In respect to magnitude of eruptive mass, the basalts are here decidedly inferior to every other class of rocks. THE BUILDING OF THE TUSHAR. To go back to the commencement of the series of events and processes which have combined to rear this majestic range to its present altitude and proportions and give it its present details is no easy task. But while there is much room for conjecture, there are many facts which appear, after careful analysis, and which are sufficient, when properly arranged, to give a connected history, even though it be but a faint outline. It is necessary to find, in the first place, some initial epoch marking the beginning of the train of events which have been directly concerned in the construction of the range, and this is the same epoch which forms the starting-point probably of the processes which have built all of the High Plateaus. This is the close of the Upper Green River epoch. The direct evidence that the Tushar had its birth-throes at this period is not so clear as in the others, but the cumulative indirect evidence is very strong and will become apparent as the discussion proceeds. It may be sufficient to remark just here that this view harmonizes with all known facts and all observations, and is in conflict with none. |