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Show 246 GEOLOGY OF THE HIGH PLATEAUS. phous base holding porphyritic crystals, which is the dominant and distinctive characteristic of a volcanic product. Not only are the various stages of this alteration displayed here, but they may be seen in many other localities within the district; and I infer that similar occurrences are found in many other portions of the western mountain region. Immediately beneath these tufas, in the heart of the canon, there is a very small area of common sedimentary beds. Their age is not known, since no fossils have been taken from them, but judging from their litho logical character, they resemble the Upper Bitter Creek Tertiary; and lithological correspondence here is of much more value than is elsewhere attributable to it. They show no trace of alteration, which is all the more remarkable when we find so much change in the beds which overlie them. This relation of altered volcanic clastic beds to underlying unaltered Ter-tiaries is also presented in the southern part of the Sevier Plateau. These facts appear to emphasize still more strongly the assertion that tufaceous deposits are extremely susceptible to metamorphism. Perhaps this ought not be regarded as surprising. Ordinary sediments consist of materials which have not only been comminuted, but also chemically decomposed and separated into aggregations much simpler than those constituting eruptive rocks, and their chemical correlatives among the metamorphics. Among the common sedimentaries we find chiefly siliceous, argillaceous, or calcareous deposits, with these ingredients commingled; but only now and then presenting such components as would yield by metamorphism rocks corresponding chemically to the volcanics. They are very poor in alkali. The tufas, on the other hand, consist of materials which, though thoroughly comminuted, are not so thoroughly decomposed as those constituting the common sediments, and contain the constituents which by mutual reaction are capable of yielding feldspars, hornblende, and mica. The geologist in the field is often called upon to note instances of local metamorphism for which he can discover no adequate local cause. On the other hand, he often finds occurrences where metamorphism has not operated, though the conditions seem to be identical with those which are elsewhere believed to have produced it. The phenomena of contact metamorphism have been sufficiently studied to enable us to say confidently that the |