BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 27, pp. 451-462 JULY 10, 1917 



A CLASSIFICATIOAT OF METAMORPHTC ROCKS* 



BY WILLIAM J. MILLER 



{Presented before the Society December 28, 1916) 



CONTENTS 



Page 



Introduction 451 



Discussion of important terms 453 



Metamorphism 453 



Foliation and foliates 454 



Meaning of "gneiss" 454 



Meaning of "schist" 457 



Meta-igneous rocks 458 



Foliates 458 



Primary ortho-f oliates 458 



Secondary ortlio-gneisses and schists 459 



Ortho-slate and ortho-phyllite 459 



Non-f oliates , . 459 



Meta-sedimentary rocks 461 



Foliates .• 461 



Para-gneisses and schists 461 



Para-slate and para-phyllite 461 



Non-foliates 462 



Injection foliates 462 



Foliates of unknown origin 462 



Saprolites , 462 



Introduction 



The purpose of this paper is to present a comprehensive classification of 

 metamorphic rocks, the attempt being made to work out a satisfactory 

 scheme without the introduction of new terms. In this proposed classi- 

 fication the main principles are based on (1) origin of the rocks, (2) 

 rock structure, and (3) composition. Since it is necessary to have cer- 

 tain important terms pertaining to* metamorphic rocks clearly defined 

 and differentiated, this matter is considered at some length. After the 

 terms are defined, they are, it is believed, consistently employed in the 



* Manuscript received by the Secretary of the Society .January 6, 1917. 



XXXIII— Bull. Geol. See. Am., Vol. 28, 1916 ^'^^) 



