A. R. Hunt — Petrohgical Nomenclature. 33 



D'Aubuisson " (Aids in Practical Geol., p. 256). In Lyell's Glossary 

 we find — " Pbyllade, D'Aubuisson'a term for clay-slate, from 

 <fiv\\a?, a beap of leaves" (El. Geol. 1865, p. 727). Tbus, according 

 to Lyell, pbyllade is a play-slate ; according to Cole, a link between 

 mica-scbist and slate, witb a peculiar wrinkled structure. 



A mineral often mentioned in connection with pbyllite (anglice), 

 is hydrous mica. Professor Judd, however, speaks slightingly of 

 these micas, stigmatizing them as "so-called hydro-micas" (Proc. 

 Roy. Soc, vol. lvii, p. 392). 



It is almost certain that by " pbyllite " geologists usually mean 

 a rock in which mica has been developed in situ, and not slaty 

 rocks in which the mica is detrital. For the latter there seems to 

 be no term but micaceous slate, witb all its uncertainties. It may 

 be observed that on one occasion I sent a rock to a friend who 

 described it as a pbyllite ; subsequently another specialist examined 

 the same specimen microscopically, and pronounced the mica detrital. 



But to return once more to the schists. We may dismiss at once 

 the original meaning of a merely cleaved rock synonymous with 

 slate, though on reading De la Beche it is essential to bear the latter 

 definition in mind. According to Dana, " slate is an argillaceous 

 rock breaking into thin laminae ; shale a similar rock, with the same 

 structure usually less perfect, and often more brittle ; schist includes 

 the same varieties of rock, but is extended also to those of a much 

 coarser laminated structure. The ordinary clay-slate has the same 



constitution as mica-slate The two pass into one another 



insensibly " (Dana, Manual of Mineralogy, 1864, p. 357). 



" Shale " has somewhat fallen into desuetude, but it is sometimes 

 used for rocks which split in the planes of bedding, and not in 

 planes of cleavage. 



Webster describes schist as a rock having a slaty structure. Slate 

 might with equal truth be described as a rock with a schistose 

 character. Lyell tells us that mica-schist "passes by insensible 

 gradations into clay-slate," which latter rock we are further informed 

 is common to the metamorpbic and fossiliferous series (El. Geol., 

 p. 726). Daubree writes of clay-schists (schistes argilleux) and 

 slate-schists (schistes ardoisiers) — (Geol. exp., p. 398). A very 

 concise definition of schists is that of Mr. Jukes-Browne, viz., 

 " foliated rocks splitting into thin layers of different mineral matter " 

 (Phys. Geol, p. 296). 



Professor Bonney defines the term schist with much precision : 



"I apply the term schist only to foliated rocks The lax use 



of the term by many Continental and some English penologists (by 

 whom it is made to include not only highly metamorpbic rocks, but 

 also some where the chemical and mineral changes are but slight) 

 causes much confusion " (Q J.G.S., vol. xl, p. 4). 



In the above definitions of " schists," a new source of trouble is 

 introduced in the word "foliated." One might suppose that the 

 typical foliated rock would be the one like a "heap of leaves," 

 i.e. a pbyllade ; but technically a pbyllade is the very type of a 

 non-foliated rock. 



DECADE IV. VOL. III. NO. I. 3 



