462 AV. J. MILLER CLASSIFICATION OF METAMOEPHIC ROCKS 



of pervsonal judgment as to how some of these should be classified. The 

 writer believes that most of them should be classed with the slates. 



NOIS'-FOLIATES 



Some of the various rock types, like quartzite and most marble, which 

 are classifiied under this heading, are of great importance. The various 

 t37Des are produced either by chemical alteration, crystallization, cementa- 

 tion, baking, contact metamorphism, or by a combination of two or more 

 of these processes. Pressure is not essential to their origin, though much 

 marble crystallizes under notable pressure without the development of 

 foliation. 



Injection Foliates 



Practically all foliated rocks produced by either the '^^lit-par-lit" or the 

 "mosaic" type of injection belong in this category. Both the intruded 

 and the intrusive rock may possess foliation or only one may be foliated. 

 Injection foliates always consist of at least two intimately associated rock- 

 masses, and these may be wholly igneous, though they are usually foliated 

 sediments injected with magmatic material, and hence intermediate. 



Foliates of unknown Origin 



In many districts certain or all of the foliates have not yet been defi- 

 nitely determined as to their igneous or sedimentary origin. A proper 

 place should be left in the classification for such rocks, and descriptive, 

 though genetically non-committal, names should be applied to them. 

 Following the excellent suggestion of Gordon,*^ the writer would qualify 

 such foliates by the use of the terms "granitic," "dioritic," etcetera, which 

 suggest similarity to the general composition and appearance of ortho- 

 foliates, but which are really non-committal as to their igneous or sedi- 

 mentary origin. In some cases other terms not suggesting igneous rocks 

 at all might be used. 



Saprolites 



The word "saprolite" literally means "rotten rock." It is an excellent 

 term suggested by Becker*-^ to include all residual products of the decay 

 of rocks. In the broad sense of the term "metamorphism," the saprolites 

 are metamorphic products, and they should be included in a classification 

 of metamorphic rocks. 



*2 C. H. Gordon : Bull. Geol. Soc. Am., vol. 7, p. 122. 



« G. F. Becker : U. S. Geol. Survey, 16th Ann. Kept, 1895, pt. 3, p. 289. 



