190 GEOLOGY OF THE HIGH PLATEAUS. 



flowed northward in great volume, but since their eruption the eastern 

 Tushar fault, swinging westwardly, has uplifted full 3,000 feet the extension 

 of the sheets in that direction. The lavas which flowed eastward are all 

 trachytic, but represent two groups of trachytic rock, one being highly 

 hornblendic, the other being almost pure feldspar and granitoid in appear- 

 ance, with a very few small but well-defined crystals of biotite. The horn- 

 blendic variety is exhibited in much greater quantity than the other, is very 

 coarse-grained in texture, and lies in masses of great thickness. In sev- 

 eral places single floods are seen between 300 and 400 feet thick, as if 

 erupted in a highly viscous state, and appearing to have moved with great 

 slowness and much internal resistance. This appearance is not only com- 

 mon, but is highly characteristic of the most typical trachytes, and gives 

 rise to the exceeding coarseness and roughness which the etymology of the 

 name implies. 



Upon the western side of Dog Valley many masses of coarse dolerites 

 and some basalts are found. Being among the latest outbreaks of the 

 locality, they have suffered most from erosion, and their debris are widely 

 distributed in the form of conglomerates over the surrounding regions. 

 These conglomerates are well stratified, and when the exposures are viewed 

 at a distance great enough to render the rocky fragments no longer dis- 

 tinguishable, they reveal a lamination quite as conspicuous as a succession 

 of sedimentary strata. These conglomerates lie in the heaviest masses in 

 the northwestern portion of the valley, and turn up against the southern 

 end of the Tushar at an angle of 22°, showing a thickness exceeding 1,500 

 feet, without exposing its entire extent. No individual mass of conglomerate 

 has been observed to extend over any large area, but they seem rather to 

 have filled up depressions They increase and diminish rapidly in thick- 

 ness, and obviously represent many local accumulations, which are not 

 continuous among themselves. This arrangement is to be expected upon 

 the theory that their origin is alluvial, a theory which (if it needs any 

 special support) will appear to be abundantly sustained when we come to 

 the examination of their formation at the present time in the larger valleys 

 of the district. 



The elevation of this valley above that of the Sevier on the east is 



