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Show 154 GEOLOGY OF THE HIGH PLATEAUS. * older Mesozoic members does not prevail in this one, for it is highly variable not only in the mass, but also in the constitution of the beds. In some exposures it is more than a thousand feet thick; in others, it is less than two hundred. Where its volume is greatest it is more arenaceous, and where the volume is less the beds are shaly, marly, and calcareous. Usually several seams of limestone occur, and in these the fossils are found often abundantly. One notable feature is the small amount of cement in the arenaceous layers, which are, therefore, very poorly consolidated, and the rock weathers and wastes away with extreme facility. Gypsum and sele-nite occur abundantly in these beds, and especially noticeable is the latter mineral, which is seen sparkling and glittering in the sunlight in the badlands to which the decay of the strata gives rise., thp: cketaceous. Throughout the District of the High Plateaus and the broad terraces which flank it upon the south and east the Cretaceous system has the same relative magnitude and importance which distinguish it in other portions of the West. In absolute mass it is inferior only to the Carboniferous; but as the latter formation is usually covered by later ones over the greater part of the West, and especially of the Plateau Country, the Cretaceous exposures are everywhere the dominant ones and most conspicuous. The series consists of many beds of sandstone and argillaceous shale, the latter decidedly predominating. The number of beds is very great, but they show a tendency to form groups, here a series of sandstones with a few shales, there a series of shales with a few thin seams of sandstone. Two conditions, however, have combined to render the group a difficult one to study and to correlate with coeval groups in other regions. The first is the want of sharp and persistent divisional horizons; the second is the great variation of the lithological characters along the outcrops, and the changes which almost all the strata undergo as we trace them from place to place. No two sections show any close agreement in the bedding. Since the fossils are generally confined to a few of the many layers, it is frequently difficult to find a valid separation, and even when we discover one we cannot apply it to every locality. But while we are often at a loss to decide to |