454 \V. J. MILLER CLASSIFICATION OF METAMOKPHIC ROCKS 



derivatives, because there are all sorts of gradations, "there comes a point 

 in the change of each original rock . . . where its characters and 

 relations to other rocks have become so individual that, for practical pur- 

 poses, it is best regarded as a distinct kind of rock/'^ 



FOLIATION AND FOLIATES 



By most writers the term "foliation" (or "foliated rock^') is applied 

 in a very broad sense to metamorphic rocks only, and there is pretty gen- 

 eral agreement as to the meaning of the term. A few definitions from 

 well known sources will suffice to bring out the ideas most commonly held. 



J. Geikie'^ says : "In a foliated rock (or schist) the constituent min- 

 erals . . . are arranged in more or less parallel layers." 



Kemp^*^ defines foliation as "the banding or lamination of metamorphic 

 rocks as distingaiished from the stratification of sediments." 



Scott says "foliation is the arrangement of the constituent mineral 

 particles of a rock into rudely parallel planes or undulating surfaces." 



Pirsson^^ uses the term "schistose texture" in a very broad sense as 

 one of the eminent characteristics of metamorphic rocks, and the term 

 "foliated" as one of three varieties of that texture. This is an unusually 

 restricted use of the term. 



In the attempt to make a clear distinction between foliated rocks in 

 general and true gneisses in particular (see below), the writer suggests 

 the following definition : A foliate is a metamorphic rock whicJi, due to 

 'pressure or floivage under pressure, exhibits a more or less clearly defined 

 parallelism of certain or all of its mineral constituents, giving the rock a 

 streaked, lenticular, banded, or laminated structure. Such a rock pos- 

 sesses "foliation," and, as suggested by Bastin,^^ "foliate" is a convenient 

 comprehensive term which may be applied to any rock showing a foliated 

 structure. The above definition, while essentially in agreement with most 

 definitions of "foliation," is thought to be rather more precise. As thus 

 defined, the foliates include the so-called "primary gneisses," the ordinary 

 gneisses and schists, and slate and phyllite. 



MEANING OF "GNEISS" 



According to Van Hise,^^ a gneiss is "a banded (metamorphic) rock, 

 the bands of which are petrographically unlike one another and consist of 



'^ L. V. Pirsson : Rocks and rock minei-als, 1909, p. 338. 

 ^ L. V. Pirsson : Rocks and rock minerals, 1909, p. 333. 

 » J. Geikie : Structural and field geology, 1908, p. 74. 

 1° J. F. Kemp : Handbook of Rocks, 1911, p. 209. 

 "■ L. V, Pirsson: Rocks and rock minerals, 1909, p. 341, 



12 E. S. Bastin: Jour. Geol., vol. 17, 1909, p. 449. 



13 C. R. Van Hise : U. S. Geol. Survey Mon. 47, 1904, p. 782. 



