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Show MONEOE AMPHITHEATERâ€"PEOPYLITES. 229 tions presented. The most conspicuous rock of the oldest series is a ridge of hornblendic propylite extending across the opening of the amphitheater. The stream which drains the amphitheater has cut a cleft 20 or 30 feet wide and more than 500 feet deep through this barrier (Heliotype I), and the gorge has received the name of Gate of Monroe. The length of this chasm between propylitic walls is about half a mile. Following it downstream the massive propylite gives place suddenly to beds of conglomerate and clay, baked and altered by heat, which abut in the natural section against the propylite. They are probably younger than the volcanic rock and may have been derived from its waste. At the upper end of the gorge the propylitic mass ends suddenlyâ€"a lateral ravine parallel to its precipitous face hiding its mode of exit. On the other side of the ravine is a mass of andesite succeeded by trachyte, both apparently younger than the propylite. The propylitic mass may have been erupted at as early a period as Middle or Late Eocene, for the stratified beds which abut against its western flank have evidently been water-laid, and there is no evidence of the existence of any considerable body of water in this locality later than the epoch referred to. Moreover, beds of similar nature, sometimes altered, sometimes not, are found around the eruptive centers in many localities, and have been derived from the destruction of some unknown volcanic rocks. Fragments of similar altered rocks are brought down by the stream from some of the forks above, showing that on both sides of the propylitic mass these peculiar sediments were deposited. Very partial exposures of propylitic rock are also found elsewhere in the deepest part of the ramifying gorges, cut by the many streams that unite in the creek which cuts the cleft in the larger barrier of the amphitheater. These propylitic rocks are interesting, inasmuch as they furnish another instance of that priority in time among Tertiary eruptions which Richt-hofen has claimed for them. Here they are not only older than all other eruptives, but they appear to speak of an epoch in which they alone were erupted, and that epoch probably goes as far back as the Middle Eocene. They certainly do no appear among the later or the middle eruptions. A period of rest from volcanic disturbance succeeded their extravasation, and during that quiescent period they were much ravaged by erosion. Patches |
