32 A. R. Hunt — Petrological Nomenclature. 



Much perplexity has been caused by De la Beche describing 

 gneiss among tlie South Devon metamorphic rocks ; but, taking 

 his own definition of that rock, it is not uncommon. If a rock 

 composed of quartz and mica maj' become a mica-slate, the term 

 slate clearly refers to structure and not to composition, as quartz 

 is in no way argillaceous — the modern idea of a slate. If we 

 turn to recent textbooks we find the meaning attached to " slate " 

 entirely different from De la Beche's. For instance, Jukes-Browne 

 tells us that a slate is " any argillaceous rock exhibiting well- 

 marked cleavage " (Phys. Geol., p. 295) ; while Rutley describes 

 clays, shales, and slates as "impure hydrous silicates of alumina" 

 (Study of Rocks, p. 282). 



Thus the idea conveyed to De la Beche by the term slate is that 

 of the original word, the old English " sclate," a shiver or splinter, 

 whatever the composition. The modern idea is an aluminous rock 

 more or less cleaved, which De la Beche would have probably called 

 a clay-slate. 



The term "phyllite " presents even greater difficulties. According 

 to Humble, it is a petrified leaf (Diet. Geol. and Min.). Webster, 

 on the authority of Dana, defines it as " a mineral consisting chiefly 

 of the hydrous silicate of alumina, iron, and manganese [sic], and 

 occurring in thin scales or leaves." According to Bristow (Glossary 

 of Mineralogy), phyllite is a magnesian mica, and a variety of 

 ottrellite. Dana tells us that " phyllite occurs in the schist of 

 Sterling .... Massachusetts, .... and the rock in consequence 

 of it is called by Hitchcock ' spangled mica slate,' the phyllite being 

 the mica of the schist" (Syst. Min. 1881, p. 506). On examining 

 the analysis, phyllite proves to be a magnesia-potash-mica, manga- 

 nese being an obvious clerical error. 



The popular acceptation of the term phyllite founded on Dana 

 is that it is a mineral, and a species of mica. English geologists, 

 on the contrary, almost without exception, use the term phyllite 

 to define a species of rock. Sir A. Geikie tells us that " by increase 

 of its mica-flakes a clay-slate passes into a phyllite " (Class Book of 

 Geol., p. 223). According to Jukes-Browne, phyllite is " a glossy 

 slate, the metamorphism of which has probably been carried a little 

 further than usual" (Phys. Geol., p. 296). Miss Raisin defines 

 it as "a slate in which a large amount of microlithic mica is 

 developed" (Q.J.G.S., vol. xliii, p. 717). Prof. Bonney speaks of 

 a certain mineral in slate as " one of the hydrous micas which 

 seem common .... to the phyllites " (Q.J.G.S., vol. xl, p. 17). 



It is clear that the above authorities are at one in considering that 

 a phyllite is a sort of slate. Phyllites pass into schists, for we find 

 a rock described by Teall as " a schistose spotted rock intermediate 

 between a phyllite and a mica-schist " (Proc. Somerset Arch. Soc. 

 1892, p. 211). 



Phyllade is used by some geologists in lieu of phyllite. Professor 

 Cole tells us that "minute mica scales may develop along the 

 cleavage planes [of slate], and a wrinkling of the latter at the 

 same time produces the link with mica-schist, called 'phyllade' by 





