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Show 224 GEOLOGY OF THE HIGH PLATEAUS. at considerable distances from the orifices. Want of opportunities for observing such formations of unquestionable origin prevents me from having any just conception of the nature, extent, and texture of such accumulations. But it seems sufficiently clear that there could be no difficulty in distinguishing them from such as are with equal certainty attributable to sub-aqueous or alluvial deposition. I have observed but few exposures which I can attribute to such an origin. That the great mass of conglomerates of the High Plateaus were accumulated from the debris derived from the erosive destruction of volcanic beds cannot be doubted. The only question is whether they are alluvial or sub-aqueous, and of the former origin I entertain no doubt. The fragments seldom fail to reveal traces of attrition and weathering, never preserving sharp angles like those produced by fresh fracture. Bat, on the other hand, the attrition is not ordinarily extreme. In most cases there is enough of it to indicate distinctly that the fragments have really been abraded, though with no great loss of substance. The stones of sub-aqueous conglomerates, on the contrary, are almost always much worn and rounded. Again, the sizes of the stones range from a fraction of a cubic inch to several cubic feet; in rare instances to more than a cubic yard. In whatsoever manner we compare the great conglomerates now forming solid rock masses and uplifted as plateaus with the alluvial conglomerates now forming in the valleys, we cannot fail to be impressed with the evidence that both were formed by essentially the same process. The only differences of any appreciable moment which are now discoverable arise from the fact that the older conglomerates have been consolidated into rock-masses, while the later ones have not. |