34 A. JR.. Hunt — Petrological Nomenclature. 



We now turn to Webster to ascertain the popular scientific 

 meaning of "foliated." We find it to be "restricted to the variety 

 of laminated structure found in crystalline schist." Now, if in 

 Prof. Bonney's definition we substitute "crystalline schist" for 

 " foliated rocks," it would l-ead : " I apply the term schist only to 

 crystalline schist"; and this, so far as I can ascertain, is the view 

 accepted by British petrologists. If we are cai'eful to remember 

 that "foliated" does not merely mean having leaves or laminae, and 

 that schist does not merely mean being divided into leaves or lamina?, 

 the modern British technical meaning of the word may be arrivi d 

 at; but it is essential to remember that in France " schiste " and 

 "feuillete" are applied to " ardoise " (roofing-slate), and must be 

 sharply distinguished from the modern British "schist" and "folia- 

 tion." All three terms will be found in the letterpress of fig. 136 

 in Daubree's Geol. experi men tale, p. 395. 



There seems to be no laxity in the use of "schiste" by French 

 petrologists. The terra defines a fissile rock. When used for a 

 crystalline schist, the crystalline constituents are mentioned, e.g. 

 " mica-schistes, talc-schistes," etc.; and " quartzites schisteux," 

 whose "feuillete" is caused by mica, chlorite, talc, or oligiste 

 (Daubree, loc. cit. p. 399). 



The difficulty which arises from the definition of "schist" 

 adopted by British petrologists is that of uncertainty. A " schist," 

 say they, is a highly metamorphic rock where the mineral changes 

 are great ; not a rock in whicb the mineral changes are but slight. 

 Clearly, then, there exists an intermediate debatable set of rocks, to 

 whicb the term " schist " is not applicable. Who is to define the 

 borders of this petrological " No man's land " — the area in which 

 the rocks occur described by Mr. Teall as links between phyllites 

 and mica-schists ? 



The true obstacle to the selection of a consistent nomenclature 

 that will define the geologist's meaning, and not fly in the face of 

 its own etymology, lies in the fact that two words of identical 

 meaning, used indifferently by our early geologists, have assumed 

 to themselves technical meanings which place them at the opposite 

 extremes of the series of fissile rocks. Could English-speaking 

 geologists revert to the old simple meaning of " schist," we might 

 have some such series as "clay-schist," "sub-crystalline schist," and 

 "crystalline schist"; in lieu of slate, phyllite, and schist. The name 

 of the dominant, mineral being prefixed would define the ciystalline 

 peculiarity; and if advisable, to distinguish detrital minerals from 

 induced minerals, the words micaceous, talcose, hornblendic, etc., 

 might be resorted to. " Slate," unfortunately, to the world means 

 roofing-slate. 



Under the present system it is often impossible to feel sure of 

 a geologist's meaning ; and it is equally impossible to describe a 

 fissile rock with any certainty that the definition will be understood 

 by others. A most remarkable instance of this difficult}- is that of 

 the Devonshire rocks, so long known as chlorite-schists. Mr. Teall 

 has avoided giving them a specific name, and has simply called 



