DISCUSSION OF IMPORTANT TERMS 45S 



interlocking mineral particles." This definition clearly includes the so- 

 called "primary gneisses," which, in the writer^s opinion, should not be 

 called true gneisses (see below). Also, many rocks — for example, granite- 

 gneiss, commonly called gneisses — not only are not truly banded, but also 

 they are not distinctly cleavable in harmony with A"an Hise's definition 

 of schist as given below. Where, then, would such rocks be classified ? 



Harker^"^ says that "a, gneiss is a crystalline rock possessing a banded 

 or streaky character, due to the association or alternation of difi^erent 

 lithologic types in one rock-mass or to the occurrence of bands or lenticles 

 specially rich in some particular constituent of the rock." This defini- 

 tion, except for the fact that it also clearly includes so-called "primary 

 gneisses," is a good one, though somewhat long and involved because of so 

 much attention to the meaning of the foliated structure. 



According to Leith and Mead,^^ "gneissic structure means a banding 

 of constituents, of which feldspar is important, with or without the 

 parallel dimensional arrangement necessary for rock cleavage." Here, 

 again, the so-called "primary gneisses" would be included. Also, as in 

 many other definitions of "gneiss," the presence of feldspar is expressly 

 required; but this is open to serious criticism, because, according to this 

 conception, certain secondarily foliated rocks commonly known as pyroxe- 

 nite-gneiss, peridotite-gneiss, marble gneiss, etcetera, are really not 

 gneisses at all. If these are not gneisses, what should they be called ? 

 But why eliminate such rocks from the category of true gneisses ? 



According to Pirsson,^*^ '^:he only general definition of gneiss which 

 will cover all cases is that they are metamorphic rocks, composed of feld- 

 spar, with other minerals, which have a certain characteristic (foliated) 

 texture." Apparently, however, this definition does not cover all cases, 

 since such feldspar-free rocks as pyroxenite-gneiss, pyroxene-garnet gneiss, 

 etcetera, are strictly excluded. 



Merrill" says: "Gneissesi are holocrystalline granular rocks, as are 

 granites, but they differ in that the various constituents are arranged in 

 approximately parallel bands or layers" and "the composition of the 

 gneisses is essentially the same as that of the granites." This definition 

 is open to the same criticisms as regards the feldspar content and the 

 inclusion of the so-called "primary gneisses." Merrill gives, and speaks 

 highly of, Gordon's classification of gneisses (referred to beyond in this 

 paper) ; but, if a gneiss must contain feldspar, this certainly does not 

 harmonize with Gordon's scheme. 



1* A. Marker : Petrology for Students, 1897, p. 320. 



1^ Leith and Mead : Metamorphic geology, 1915, p. 179. 



10 L. V. Pirsson : Rocks and rock minerals, 1908, p. 351. 



" G. P. Merrill : Rocks, rock weathering, and soils, 1906, pp. 142-143. ' 



