OCR Text |
Show THE BUILDING OF THE TUSHAE. 185 ries are seen lapping around both the northern and southern extremities of the range, and it is probable that they are concealed not far from its eastern base. Such was the relation of the area to its surroundings when the earliest eruptions (so far as they have been observed) took place. They broke forth at first along the course of the present eastern front, a little east of the main divide as it now stands, and along a line nearly 30 miles in length, having a general trend north and south. They were not continuous along this line, but were massed in at least three places: one near the northern end of the Tushar, one (and this the principal one) near the central part of the front, and the other near the southern end, but a few miles southeast of it. The location of this latter center of eruption cannot be fixed at present with exactitude, and may have been more remote than I wTas at first led to suppose. The interval between the southern and middle sources is greater than that between the middle and northern, and it is not certain that this second or northern interval was well marked, though the southern interval is very distinctly so. What other vents existed, or even whether any others existed at all, it is not now possible to determine, on account of subsequent accumulations which have buried the surrounding country. This period of eruptive activity was certainly a long one; for between the outbreaks erosion went on, leaving traces of its action in the eroded surfaces of its sheets and in the many small local conglomerates formed out of their decay. But the accumulation by successive outpours was far more rapid than the waste, until there came a long period during which these vents were sealed up and degradation proceeded. At the commencement of this period of repose the eruptive masses must have been piled up to a great altitude and covered an extensive area, for the conglomerates which were formed by their dilapidation are of immense extent and thickness and sufficient in mass to build a goodly range of mountains. The southern interval was almost wholly filled up by the fragments washed into it and stratified, and the conglomerate thus formed stretches far to the southwest, always maintaining a great thickness. At least 2,000 feet of it occupy the southern interval, and it is still many hundreds of feet thick 8 or 10 miles away. In many respects the relations of the eruptive masses to the country |