| OCR Text |
Show THE BUILDING OF THE TUSHAR. 187 been accomplished since the close of this second activity, though some of the movement may, in the absence of evidence to the contrary, be assigned to this period. The second period of cessation in the eruptions was broken at a comparatively late epoch by a third outbreak at the northern end and at several localities on the eastern flank in the vicinity of the faults. To this third eruptive period belong the whitish rhyolite and the basalts, together with several masses in the Sevier Valley which have emanated from the foot of the range, and which will be discussed when we reach in regular order the description of that valley. The history of the Tushar, therefore, comprises five tolerably distinct periods since the commencement of the various activities which have brought it to its present stage. 1st. An older eruptive epoch, building up an ancient volcanic mass. 2d. A period of decay, in which the mass thus' built was nearly leveled down, and its fragments scattered far and wide and reconstructed in the form of conglomerates and alluvial beds. 3d. A second eruptive period, more extensive than the first, rebuilding the dilapidated mass. 4th. A second cessation of eruptions and the introduction and progress of extensive uplifting and faulting, accompanied by considerable erosion. 5th. A third series of minor outbreaks of much smaller extent than either of the others, some of which (around the bases of the range) are very recent. In this history we perceive the combination of most of the important forces and agencies of geology: eruption, displacement, erosion, and accumulation ;. all performing their parts in the general work, and yielding an intelligible result in the erection of a grand uplift. |
