METAMOKPHISM OF FEAGMENTAL VOLCANIC EOCKS. 81 



or impure argillite. The obliteration of all traces of granulation in this 

 residual felsitic base is no more remarkable than it would be in an argilla- 

 ceous rock. So long as a thorough crystallization of the entire mass 

 remains impracticable for want of the requisite quantity of alkaline and 

 earthy bases, much of the groundmass must necessarily remain amorphous ; 

 and there is no difficulty in believing that this amorphous base may take 

 those forms and aspects (both microscopic and macroscopic) which are seen 

 in many forms of porphyroid eruptive rocks. 



These rocks, however, never reveal any traces of that igneous fusion 

 which is displayed by the basalts and augitic andesites on the one hand, 

 and by the true rhyolites on the other. Glass inclusions, fluidal textures, 

 fibrolites, or a spherulitic base are never found among them. This absence 

 of all evidence of igneous action at high temperature is a significant charac- 

 teristic. Hence the similarity of these metamorphic rocks does not extend 

 to all igneous or eruptive rocks, but only to limited groups of them, such 

 as porphyritic trachyte and several other trachytic varieties, to the propy- 

 lites, and to some varieties of hornblendic andesite. 



A detailed description and study of the metamorphic tufas will be found 

 in the portion of the chapter on the Sevier Plateau, in which the rocks of 

 the East Fork Canon are described. 

 6 H P 



