80 Trof. G. A. J. Cole— Phyllade, Phyllite, and Ottrelite. 



flakes of mica and granules of quartz embedded in the apparently 

 homogeneous ground. The justification for using " phy Hade" as 

 a term intermediate between mica-schist and common slate is to be 

 found in the passage (p. 97) " Le phyllade passant dn schiste- 

 micace a l'ardoise, par des nuances insensibles et diversifies sous 

 tous les rapports, le nombre de ses varietes est presque indefini." 

 D'Aubuisson believed the fissile structure to be parallel to the 

 stratification, and combated the acute views of Voigt, who had ob- 

 served a strong divergence between the two structures in Thuringia. 



Brongniart, in 1827 ("Class, et caract. min. des Roches homogenes 

 et heterogenes," p. 88), refers "phyllade" to D'Aubuisson only, 

 and states that this rock is "compose essentiellement de Schiste 



argileux comme base, et de Mica c'est le veritable schisle 



micacS." "Schiste argileux" (p. 61) is with him a common 

 clay-slate, and he swallows up " mica-schist " now altogether in 

 "phyllade." This introduces sufficient confusion, when we re- 

 member that D'Aubuisson, on the other hand, allied "phyllade" 

 more to the clay-slates than to the mica-schists. Coquand, in 1857 

 (" Traite des Roches," p. 307), grouped together as " argiloschiste " 

 all the clayey " schistes " of metamorphic origin in which the con- 

 stituents were very finely grained, and gives the term " phyllade " 

 as a synonym. Our latest, glossary (L. Lessing, " Petrographisches 

 Lexicon," 1894, p. 177) gives " Phyllit " as a translation of 

 " Phyllade," due to Naumann, and defines both rocks as micaceous 

 " Thonschiefer." It is clear, then, that these two terms, if required 

 at all, should be used in the sense in which they are already 

 generally accepted. I have shown that D'Aubuisson himself, with 

 a foresight that is not accidental, distinguishes between " phyllades " 

 and detrital rocks ; for him, phyllades were allied to granite and not 

 to clays. The minerals in them had arisen in situ, a view which the 

 study of metamorphism has since led us to support. Mr. Hunt's 

 comments on this point seem perfect]}' correct. 



We need not discuss the mineral styled "phyllite" by Thomson 

 in 1828 (Annals of the Lyceum of Nat. Hist., New York, vol. iii, 

 p. 47), but for its bearing on chloritoid and ottrelite. Only the 

 strict respect for priority entertained by the late Prof. J. D. Dana 

 caused the name to be retained as that of a mineral species. 

 Mr. Hunt surely does not find it in " popular " use, and it is given 

 as a mere synonym of ottrelite in the revised edition of Dana's 

 "System of Mineralogy," 1892. But this raises another point that 

 is well worth settling — is oxide of manganese or of magnesium 

 really present in ottrelite and in Thomson's " phyllite " ? " Manga- 

 nese " is not so obvious a clerical error in the description of 

 "phyllite" as Mr. Hunt suggests, since the relation of the mineral 

 to typical ottrelite depends on the presence of this constituent. 

 Manganese, not magnesia, is ascribed to "phyllite" in the famous 

 fifth edition of Dana's "System," and in the new edition of 1892. 

 Hintze (" Handbuch der Mineralogie ") gives the same figures for 

 magnesia. Mr. F. W. Rudler has kindly looked up Thomson's 

 original paper for me, and informs me that Hintze is correct. 



