196 GEOLOGY OF THE HIGH PLATEAUS. 



ing pictures of shaded dells and sloping banks, with distant views of the 

 Tushar and the mighty wall of the Sevier Plateau, combine to produce a 

 medley of pleasant scenes and experiences which will always be looked 

 back to with refreshment. As a field of geological study it is in great part 

 meager. Now and then a bit of local curiosity is excited by a curious 

 result of rain sculpture, by remains of small lake deposits, by the curious 

 weathering of rocks, by some strange freak of the old lava flows, none 

 of which will find places here. Broad facts are comparatively few. 



Among the most noteworthy is the succession of eruptions. In the 

 central part of the Markagunt the oldest eruptions observed were andesitic. 

 These are displayed in a disconnected way in the deeper ravines of the cen- 

 tral and northern portions, but are elsewhere so masked by subsequent 

 floods that their extent and the circumstances of their extravasation are not 

 fully intelligible. Whether they were generally distributed over the face 

 of the plateau or represent a number of local eruptions it is not possible to 

 say with certainty. Wherever deep canons are found in the central part of 

 the area they lay open great masses of dark andesitic lava, and areas are 

 occasionally found where surface erosion has removed the later rocks and 

 laid the andesite bare. In any event, whether generally or discontinu- 

 ously distributed, the mass of this rock is very great. No propylitic erup- 

 tions have been observed in the Markagunt. 



Next in order are found great masses of trachyte. Over the greater por- 

 tion of the expanse of the Markagunt these are the surface rocks. In reality 

 their volume may not exceed that of the andesites, which they usually cover, 

 but being more frequently seen they appear to be the dominant rock, and 

 I incline to the opinion that they are so. On the whole, the varieties of 

 trachyte are less numerous in the Markagunt than in the more eastern pla- 

 teaus of the district ; but their number is still very great. The least com- 

 mon variety is the hornblendic ; but the augitic trachytes are abundant, 

 and the commonest of all is a highly porphyritic argilloid variety. The 

 latter consists of a reddish or purplish fine base, resembling a rather rough 

 argillite, holding crystals of white opaque orthoclase. One of its most per- 

 sistent characteristics is its fracture, which is very peculiar. Most volcanic 

 rocks, when broken, present a tolerably even or gently rounded though 



