186 GEOLOGY OF THE HIGH PLATEAUS. 



they occupied at the close of the earliest volcanic period presents a very 

 strong analogy to those of Central France, as described by Sir Q. Poulett 

 Scrope in his work upon that region.* In point of magnitude the earliest 

 eruptions of the Tushar were probably comparable to those of the Cantal, 

 covering perhaps a larger area but with a greater thickness. 



After a long period of comparative quiet, during which the greater 

 portion of the mass of these earlier eruptions was broken up by erosion 

 and scattered over the adjoining lowlands and intervening valleys, came 

 the second period of eruption, upon a scale grander than the first. The 

 foci of activity were in close proximity to those of the first period. The 

 outpours at the northern portion still remain in great bulk and are chiefly 

 rhyolitic. But the grandest floods of all are in the center of the range, 

 where they are laid open by several deep gorges, the largest of which is 

 Bullion Canon. The course of the streams was here to the westward 

 chiefly, where they widened out and grew thin as they receded from their 

 origin. The total thickness remaining of these rhyolitic masses probably 

 exceeds 2,000 feet, and there is good evidence that a considerable amount 

 has been lost by erosion. What floods may be hidden beneath the floor of 

 the Sevier Valley at the eastern base it is impossible to say or even to con- 

 jecture. Thus for the second time the Tushar was built up by extravasated 

 materials and to an altitude greater probably than at first. 



A second period of comparative calm now followed, during which 

 erosion was - at work cutting deep gorges, carving out pediments, and leav- 

 ing a rugged series of peaks and domes along the eastern flank. But 

 another agencj- in mountain structure also intervened. This was an exten- 

 sive vertical movement of the whole mass. At what precise epoch the 

 faults which now separate it from the platform of the Sevier Valley were 

 started it is impossible to say with precision. It is clear, however, that the 

 commencement of the displacement was subsequent to the deposition of 

 the great conglomerates which were formed by the destruction of the older 

 Tushar, and it is almost certain that the displacements had not attained any 

 great magnitude or a magnitude comparable to the present during the 

 second eruptive period. The principal part of the uplifting has apparently 



" The Geology and Extinct Volcanoes of Central France, by G. Poulett Scrope, 1858. 



