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Show CHAPTER XI. SEVIER AND PAUNSAGUNT PLATEAUS. General structure and form of the Sevier Plateau.â€"Sculpture.â€"Ravines.â€"Superposed features and details.â€"Northern portion of the plateau.â€"A gigantic cliff.â€"Monroe Amphitheater.â€"Lava beds exposed within it.â€"The Gate of Monroe.â€"Propylitic masses.â€"Clastic volcanic beds at the base of the series.â€"Hornblendic andesites.â€"Intervening period of erosion of the propylites.â€"Horn-blendic trachytes and augitic andesites.â€"Argilloid and granitoid trachytes.â€"General succession of the eruptions.â€"Comparison with the succession fouud in the Auvergne.â€"Eastern side of the Sevier Plateau and Blue Mountain.â€"Great extent of the emanations from the principal volcanic centers of the northern part of the plateau.â€"Eroded lava-capped mesas around Salina Canon.â€" The Black Cap.â€"Augitic trachytes.â€"Lava sheets south of Monroe Amphitheater.â€"Central vents of the Sevier Plateau.â€"Volcanic conglomerates.â€"An ancient cone, buried in lava and exhumed by erosion.â€"Conglomerates south of the central vents.â€"Southern focus of eruptions.â€"Andesitio conglomerates.â€"Southern termination of the Sevier Plateau.â€"General succession of eruptivo sheets.â€"Sections.â€"East Fork Canon.â€"Effect of the Sevier fault.â€"Tufaceous deposits exposed in East Fork Canon.â€"Their transitional characters.â€"Their metamorphism and the resemblance of the metamorphs to lava sheets.â€"Phonolite hill.â€"Grass Valley, its structure and origin.â€"Existence of an ancient lake in Grass Valley.â€"The causes which produced it.â€"Tufaceous deposits of Mesa Creek.â€"Their recent formation.â€"Their transitional characters.â€"Alluvial cones of Grass Valley.â€"The Paunsa"gunt.â€"Lower Eocene beds.â€"Faults.â€"The southern terraces.â€"Paria Valley. â€"A grand erosion.â€"The scenery of Paria Valley.â€"Table Cliff and KaipaYowits Peak.â€"The Pink Cliffs and architectural forms sculptured from them.â€"A recent basaltic cone.â€"Scattered basaltic craters of the southern terraces. The Sevier Plateau is next to be described. It is a long and rather narrow uplift, having a fault along its western base and inclining to the eastward; at first very gently, then with a stronger slope, which grades rapidly down into Grass Valley. The length of this table is about 70 miles, and its width varies from 10 to 20 miles. It is, therefore, long and narrow like the general ground-plan of a mountain range. But its structure has very little analogy to ordinary mountain uplifts. It has no sharply upturned strata upbn its flanks reclining against a core of meta-morphic rocksâ€"no summit ridge marking the axis along which granitoid and schistose rocks have been protruded, nor even the monoclinal ridge which characterizes the Wasatch and Basin Ranges. It is a tabular mass very like the inclined blocks of the Kaibab region to the southward. The inclination is very small, seldom exceeding three or four degrees upon the 15 h p 225 |
