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Show 58 GEOLOGY OF THE HIGH PLATEAUS. in the deepest ravines of the Awapa, near the Aquarius, where profound excavations, near the great faults, have disclosed them beneath nearly 3,000 feet of trachytes. A question has been carefully considered, without reaching a positive conclusion, whether the tufaceous beds already spoken of may not have been derived from the waste of these propylites. The tufas are wholly water-laid beds. Their ordinary aspect is well represented in Ileliotypes V and VI. The stratification has all of the mechanical characters of ordinary arenaceous beds. In numerous places the tufas are seen to pass horizontally by gradual transition into ordinary arenaceous shales, made up wholly of materials derived from the decay of non-eruptive rocks. The propylites alone of all the massive rocks seem to have sufficient antiquity to have supplied the material for these deposits, and the only question seems to be whether these came from the visible propylites or some unknown volcanics of still greater age. The tufas have been carefully studied with the microscope in the hope of settling the question, but no solution has been reached. They contain large quantities of quartz and feldspar, which are often epigenetic, and the remaining contents are so much decayed that their original characters are obliterated. But although the antecedence of the propylites to the tufas cannot be proven, it may at least be said that there is no fact now known which forbids such a conclusion. More than that, the inference has some slight preponderance of probability in its favor. The hornblendic andesites succeeded the propylites with apparently a long interval between them. They were erupted from the same localities or from vents in the immediate vicinity. The mass of these rocks now exposed is greater than that of the propylites, and the lavas are considerably more varied in texture and appearance. Their principal locus seems to have been in the southern part of the Sevier Plateau, though the masses revealed in the northern part of the same uplift are but little inferior. The outbreaks were in massive sheets, which stretched far to the eastward and southeastward, spreading out over large areas and piling up mountainous masses. It is not, however, the quantity now exposed which gives us the real clue to the magnitude of the andesitic extravasations, but rather the great bulk of the conglomerates derived from their ruins. The andesites, |