Prof. G. A. J. Cok—Phyllade, Phyllite, and Oth -elite. 79 



as the intensity does diminish as the distance increases, the maxi- 

 mum intensity will occur at a point C somewhat nearer the fault- 

 line. Thus the centre of the disturbed area should lie on the side 

 of the fault-line towards which the fault hades, and the greater 

 part of the disturbed area should also lie on the same side of the 

 fault-line. 



Referring to the map (Fig. 1) it will be seen that by far the 

 greater part of the area disturbed by the earthquake on July 12 

 lies on the north-west side of the fault-line, leading to the inference 

 that the hade is also in this direction. It is perhaps unnecessary 

 to accumulate evidence on this point, but it may be mentioned that 

 it is at Comrie itself, also on the same side of the fault-line, that the 

 majority of slight shocks and sounds have been in times past 

 observed. 



YIII. — Phyllade, Phyllite, and Ottrelite. 

 By Prof. Grenville A. J. Cole, M.R.I.A., F.G.S. 



IN his article in the last number of the Geological Magaztnk l 

 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 gra- 

 dation from a clay-slate to a mica-schist ; but it is fairly clear that 

 the original " phy Hades " were slates containing some mica. The 

 term was used in 1813 by Brongniart (" Essai d'une classification 

 des Roches melangees," p. 35), who ascribes it to the joint author- 

 ship of himself, Brochant, and D'Aubuisson. He gives as a 

 Bynonyin, " Thonschiefer melange des mineralogistes 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 feuilletee." None of the types which he 

 describes are free from mica. D'Aubuisson de Yoisins ("Traite 

 de Geognosie," 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 mineralogistes ; 

 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 

 (" froncee ") ; 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 

 1 Geol. Mag., 1S96, p. 31. 



