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Show 72 GEOLOGY OF THE HIGH PLATEAUS. usually occurs either as the magnetic oxide or protoxide. In the protoxide forms it is always in combination in some of the mineralsâ€"the undecom-posed hornblendes and micas or such alteration products as epidote or viri-dite. These alteration compounds, particularly, are more or less thoroughly diffused throughout the mass of the rock, impregnating it with a greenish color, while the unchanged mica, hornblende, and magnetites, disseminated as black particles, give the rocks a gray color of varying shades from very dark to very light. Whenever these beds have been subject to metamor-phic action, as has often happened, the proto-compounds of iron are often converted into sesquioxide, producing a pinkish color similar to that of " Scotch granite." Thus the colors of the tufaceous beds would enable us to single them out as presumably composed of materials very different from those constituting ordinary sandstones. All of these finer beds are stratified after the manner of ordinary aqueous deposits. That they were water-laid is unquestionable. No rocks have been observed which could possibly have been accumulated by the precipitation of volcanic ashes upon the land. The agency of water in arranging them in their present form is altogether too conspicuous to admit of any doubt. The origin of these clastic materials, proximately considered, is in the break up and destruction of older massive volcanic rocks by the ordinary processes of denudation. It is, indeed, possible that some small proportion of their ingredients may have been pulverulent material blown from volcanic orifices and washed into the basins where the strata accumulated, but it seems quite certain that the great bulk of the tufas did not so reach their present positions. They differ in no other material respect from the common lacustrine beds than in the sole fact that they are the debris of volcanic rocks instead of sandstones and gneisses. In a number of instances they are seen to pass, along horizontal exposures, by a gradual transition, into common lacustrine deposits, the quantity of material derived from the break up of vocanic rocks becoming gradually less and less, while that derived from the disintegration of foliated rocks becomes greater and. greater. Instances of this transition are seen in various parts of the Sevier Plateau and in the beds beneath the lava-cap of the Markagunt. Indeed, I doubt not that those beds, which are apparently most typically " tufaceous," |