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Show 230 GEOLOGY OF THE HIGH PLATEAUS. of conglomerate, formed of their fragments, were accumulated and are here and there brought to light where erosion has deeply excavated the still grander masses of subsequent lavas overlying them So completely were these most ancient rocks overwhelmed, that erosion has only revealed a very small portion of them and left us to conjecture what may be the extent of those portions now concealed. It is not improbable that the clastic beds, formed of the waste of volcanic rocks, and which underlie the great lava caps of the plateaus and in turn rest upon the Bitter Creek and Green River beds, may have derived their sands and clays from the decomposition of some of these propylitic masses. These ancient eruptions are succeeded by those of a middle epoch, lying across the surface of an eroded country, which they overwhelmed. These second lavas are much less chaotic in their arrangement and much less affected by erosion during the intervals between the eruption of successive floods. They are, therefore, more intelligible, and some idea of their sequences has been obtained, though less definite than is desirable, because the exposures are so partial and so much obscured by debris and soil. These outpours were upon a very large scale, the masses being often several hundred feet in thickness and spreading out over large areas. The lower masses are andesitic and show but little variety. They all belong to the hornblendic group and are characterized by triclinic feldspar, with a moderate proportion of hornblende, with some augite and magnetite, and are very compact and rather fine-grained. Higher up, these give place to coarse-grained trachytes, with both monoclinic and triclinic feldspars and abundant hornblende. These occasionally intercalate with sheets of doler-ite. Still higher, a totally distinct group of trachytes is found. They consist largely of the argilloid varietyâ€"a fine-grained, highly ferritic, reddish paste, holding porphyritic crystals of opaque monoclinic feldspar. There is probably no eruptive rock within the district more abundant. It forms the summit of the series of middle-aged eruptions in many localities. Very nearly coeval with it is a group of trachytes, having an appearance faintly resembling a fine-grained syenite, though not by any means wholly crystalline. It varies in color from iron gray to light gray. It shows a tendency to break up into slabs or tiles from an inch to four or five inches thick, the |