VIII.—Phyllade, Phyllite, and Ottrelite

Extract

In his article in the last number of the Geological Magazine Mr. A. R. Hunt points out a number of differences in the use of the terms “schist, slate, phyllite, and foliation.” The usage of English-speaking peoples has long ago restricted the word “schist,” so that it is no more an equivalent of the French “schiste” and of the German “schiefer” than “mutton” is of “mouton.” Prof. Bonney's position is, however, one of exceptional restriction, as he himself has always carefully pointed out. The terms “phyllade” and “phyllite” are surrounded by less controversy. I regard both as unnecessary, seeing that we cannot have names for every gradation from a clay-slate to a mica-schist; but it is fairly clear that the original “phyllades” were slates containing some mica. The term was used in 1813 by Brongniart (“Essai d'une classification des Roches mélangées,” p. 35), who ascribes it to the joint authorship of himself, Brochant, and D'Aubuisson. He gives as a synonym, “Thonschiefer mélangé des minéralogistes allemands,” and defines the rock as having a ground of clay, with mica, quartz, felspar, amphibole, chiastolite, etc., in it, either together or separately; “structure feuilletée.” None of the types which he describes are free from mica. D'Aubuisson de Voisins (“Traité de Géognosie,” tome ii, p. 93) in 1819 devoted one of his elaborate disquisitions to Phyllade, giving as synonyms “Thonschiefer; Clay-slate; Ardoise, schiste de Saussure et des anciens minéralogistes; Schiste argileux de M. Brochant.” D'Aubuisson objects to the restriction of “schiste” to any one species of rock, and keeps phyllade distinct from those clays which are derived as detritus from pre-existing rocks. He says that the surface of phyllade is sometimes smooth, sometimes deeply striated, as if puckered up (“froncée”); and he shows how the true type of the rock is approached by gradations from mica-schist, until the mica and quartz are so finely divided as be indistinguishable, save for some flakes of mica and granules of quartz embedded in the apparently homogeneous ground. The justification for using “phyllade” as a term intermediate between mica-schist and common slate is to be found in the passage (p. 97) “Le phyllade passant du schistemicacé à l'ardoise, par des nuances insensibles et diversifiées sous tous les rapports, le nombre de ses variétés est presque indéfini.” D'Aubuisson believed the fissile structure to be parallel to the stratification, and combated the acute views of Voigt, who had observed a strong divergence between the two structures in Thuringia.