246 GEOLOGY OF THE HIGH PLATEAUS. 



phous base holding porphyrinic crystals, which is the dominant and dis- 

 tinctive 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 nietamorphistn. 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 

 r ocks 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 con- 

 stituting 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 



