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Show 56 - GEOLOGY OF THE HIGH PLATEAUS. canic rocks, and at least three-fourths of all the important sub-groups have here their representatives. The clastic derivatives are displayed in variety and volume truly extraordinary, commanding as much attention as the massive rocks and presenting some highly interesting problems. It would be impossible, within the limits of a single chapter, to present a good synopsis of these facts with a discussion sufficiently extended (and at the same time precise) to make them intelligible. Since the greater part of the individual phenomena described in this work consists of those which belong to the volcanic category, and since no symmetrical grouping of their entire array has suggested itself to my mind, it will be practicable to set forth here only those few facts of a high degree of generality which appear to be applicable to the entire district. In those chapters of this book which are devoted to the description in detail of the component members of the High Plateaus, such facts as seem to be instructive will be adverted to, together with such of their relations as have been satisfactorily ascertained. The initial epochs and conditions of the eruptive activity of the High Plateaus are obscure. The oldest observed rocks having an eruptive origin are tufas. It is presumable, however, that tufas, especially such as are here found, are never erupted alone, nor wholly in the fragmentary or pulverulent form, but are in part the concomitants of lava floods, and in far greater part the results of the degradation of volcanic rocks. The tufas of this district are stratified water-laid rocks of arenaceous texture, sometimes marly or even shaly; their materials being derived almost entirely from the decay of lavas. Some of these tufaceous beds are metamorphosed, and the highly suggestive and interesting fact is there presented that the product of this metamorphism is a rock having the essential lithologic characters of a lava.* The rocks from which these ancient tufas were derived are not known. An abundance of old lavas lie in their vicinity, but always on top of them. There is, however, one instance in the great gorge near Monroe where a propylitic mass appears to pass under some of these tufas, but owing to the complications of faulting there may be a mistake about it. Whether the lava sheets which yielded by their decay the clastic materials of these * See Chapter XI, where this remarkable phenomenon is described and discussed. |