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Show 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 central 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 eruptions have been observed in the Markagunt. Next in order are found great masses of trachyte. Over the greater portion 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 plateaus of the district; but their number is still very great. The least common 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 persistent characteristics is its fracture, which is very peculiar. Most volcanic rocks, when broken, present a tolerably even or gently rounded though |
