456 W. J. IvriLLER CLASBTFICATION OF METAMORPITIC ROCKS 



Kemp^* defines a gneiss as "a laminated or foliated granitoid rock that 

 corresponds in mineralogical composition to some one of the plutonic 

 rocks/^ So-called "primary gneisses^^ are, of course, here included. Fur- 

 thermore, certain rocks which, by all means, should be called gneisses do 

 not possess a true granitoid texture, while still others — for example, 

 pyroxene-garnet gneiss, quartz-biotite-graphite gneiss, marble gneiss, et- 

 cetera, do not have plutonic rock mineralogical compositions. 



Finlay^s^^ definition is much like that by Kemp, namely : "Gneiss is a 

 metamorphic rock with distinct lamination and the mineralogy of a rock 

 in the igneous granitoid series." To say that a gneiss must be distinctly 

 laminated is either using the term "laminated" incorrectly or certain 

 foliated crystalline rocks must be excluded from the category of gneisses. 



Gordon^^ says the term "gneiss" should be used in its broader structural 

 sense for all (metamorphic) rocks showing a laminated or banded struc- 

 ture, and in which this structure is not known to be due to differential 

 movements of an igneous mass before its final consolidation. This is the 

 only definition thus far cited which clearly excludes the so-called "primary 

 gneisses" from the true gneisses ; also it does not require the presence of 

 feldspar in a gneiss. But, according to this definition, non-crystalline or 

 only partially crystalline foliated rocks like the. slates and phyllites would 

 be called gneisses, while foliated crystalline rocks neither truly laminated 

 nor banded, but with a lenticular structure, would not be included. 



Bearing in mind the above conceptions and criticisms regarding the 

 meaning of the term "gneiss," the writer p!-oposes the following defini- 

 tion : A gneiss is a foliated megascopically crystalline rock of secondary 

 origin produced essentially by pressure. Such a rock possesses a gneissose 

 or gneissic structure. Above all, a gneiss must possess a foliated struc- 

 ture, and, since the term "foliate" is broader in its scope than gneiss and 

 has already been defined, it is only necessary to mention the foliated 

 structure as a part of the definition of a gneiss. I^ext, a gneiss must be 

 notably crystalline in order to exclude such rocks as slates and phyllites. 

 Again, a gneiss must be decidedly secondary in origin and produced essen- 

 tially by pressure in order to exclude from the true gneisses the so-called 

 "primary gneisses," this matter being discussed below. This definition 

 . does not regard mineralogical content, and is thus in harmony with Van 

 Hise, Gordon, and others who strongly urge the use of the term "gneiss" 

 in a purely structural sense. It is impossible to consistently use the term 

 "gneiss" in a dual sense — that is, structural and mineralogical. Finally, 

 the definition here proposed does not limit gneisses to distinctly banded 



18 J. p. Kemp: Handbook of Rocks, 1911, p. 212. 



i» G. I. Finlay : Igneous rocks, 1913, p. 14. 



20 C. H. Gordon : BuU. Geol. Soe. Am., vol. 7, p. 122. 



