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Show 234 GEOLOGY OF THE HIGH PLATEAUS. Southward from the northern center of eruption of the Sevier Plateau the floods are piled up in grand succession sheet upon sheet. No narrow streams or rivers of lava were here, but great deluges, which welled up and rolled majestically over vast Phlegrsean fields, and, spreading out in broad lakes, left after their congelation an even stratification, which may be read miles away from distant summits. Standing upon the verge of the Awapa Plateau and looking across Grass Valley, these old floods are seen lying calmly and evenly with an outward resemblance to dark stratified rocks cut by ravines and terraced off into trappean ledges. Ten or fifteen miles southward they have commingled by intercalation with the coulees from the middle eruptive focus of the plateau. The eruptions from this middle locality were inferior in magnitude to those from the northern vents, though absolutely they were by no means small. Its lavas differ somewhat in character from those derived from the northern vent. Trachytes are present in considerable volume, and here as elsewhere alternate with dark doleritic lavas. They succeeded the ande-sites in the order of eruption. Here we find also the same inclination of the pseudo-strata which is observed in the Blue Mountain, the layers dipping away from the central mass in opposite directions. Around this eruptive locus we find also those great beds of conglomerate which are so conspicuous throughout the entire district and especially in its southern portions. A mighty wall of this material is presented towards Sevier Valley, just north of the middle vent, and extends for about 8 miles in that direction, where it thins out; but before being quite lost by attenuation is cut off by erosion. It is well stratified and weathers into an abrupt cliff. Here, as elsewhere, it was formed in an ancient valley, lying between the two vents, and has the alluvial-cone structure. The great Sevier fault has cut the formation, and its continuation is seen upon the eastern slopes of Sevier Valley, 3,000 feet below. Upon the southern side of the vent the conglomerate is seen in still greater mass. In truth, its magnitude here becomes astonishing. Upon the Grass Valley side of this central eruptive locality is seen what is undoubtedly a remnant of a very ancient volcanic cone, afterwards |