DISCI^SSTON OF IMPORTANT TERMS 457 



or laminated metamorphic rocks, but includes those with lenticular struc- 

 ture as well. 



MEANING OF "SCHIST" 



There is a strong tendency among American geologists to make the 

 essential difference between gneisses and schists structural rather than 

 mineralogical. Practically all American students of the subject consider 

 both gneisses and schists to be crystalline foliated rocks, and most consider 

 schists to be more perfectly or finely foliated, causing such rocks to cleave 

 more or less readily in one direction. As clearly stated by Kemp:^^ 

 "Gneisses differ from schists in the coarseness of the laminations (folia- 

 tion), but as these become finer they pass into schists by insensible 

 gradations." 



European geologists frequently use the term, "schist'^ with a much 

 broader meaning, practically synonymous with "foliated rock." Thus 

 A. Geikie^^ says : "Gneisses denote the coarser schists" ; J. Geikie^^ speaks 

 of "a foliated rock or schist"; and Grubenmann^* uses the term "Schiefer" 

 to include all foliated rocks. 



The following definitions (one from an English writer) give a fair idea 

 of the meaning of "schist" to most American geologists. According to 

 Van Hise:^^ "Schist is defined to include those cleavable (metamorphic) 

 rocks the cleavage pieces of which are like. one another and the mineral 

 particles of which are for most part so large as to be visible to the naked 

 eye." This definition differs from most others in requiring that the 

 cleavage pieces be like one another. 



Harker-^ says: "Schists are crystalline rocks which possess a parallel 

 arrangement of some or air of their elements . . . and which have 

 in consequence the property of splitting with more or less facility in a 

 definite direction (schistosity)." In short, Harker considers gneisses 

 and schists to be practically the same except that the latter are more or 

 less readily cleavable. 



Leith and Mead^^ say : "A schist always has a parallel dimensional ar- 

 rangement (of constituents necessary for rock cleavage) and may or may 

 not contain feldspar." 



Pirsson^s uses the term "schistose" in a broad sense, practically synony- 

 mous with the foliated structure of a metamorphic rock, which thus sug- 

 gests the European meaning of the term. 



21 .T. F. Kemp: Handbook of Rocks, 1911, p. 133. 



22 A. Geikie : Text-book of Geology, 1903, p. 134. 



-•'J. Geikie: Structural and field geology, 1908, p, 74. 

 2* U. Grubenmann : Die kristallinen Schiefer, vol. 2. 1907, p. 21. 

 25 C. R. Van Hise : U. S. Geol. Survey Mon. 47, 1904, p. 779. 

 2« A. Harker : Petrology for Students, 1897, p. 318. 



27 Leith and Mead: Metamorphic geology, 1915, pp. 179-180. 



28 L. V. Pirsson : Rocks and rock minerals, 1908, p. 340. 



