266 DR. J. W. GREGORY ON THE WALDENSIAN [May 1 894, 



The differences between the elastic and the fluxion-gneisses may 

 be very conveniently expressed in diagrammatic form by M. Michel- 

 Levy's formulas. Thus, adding the letter <I> to signify foliation, the 

 following will be the formula 1 for this clastic gneiss or gneissoid 

 mica-schist : — 



The formula for the mica-schists of the same section, where 

 comparatively unaltered, will be : — 



These may be compared with that for a fluxion-gneiss : — 



The question therefore inevitably arises, what use is to be made 

 of the term ' gneiss,' as the rocks included under this name .comprise 

 those formed by three different modes of origin? That the term 

 should be used to denote a type of structure, instead of composition, 

 is shown by the general use of such names as gabbro-gneiss. The 

 question now is whether the term should be restricted to the igneous 

 or clastic varieties. Different answers have been given, according to 

 which rock was the prevalent one in the district studied. Thus 

 M. Michel-Levy, 2 working among the clastic gneisses of the Central 

 Plateau and Britanny, urges the necessity of limiting the term to 

 these, whereas Prof. Lehmann, working among the granulitic 

 gneisses, urges that all the altered sediments should be excluded. 



In the case of so old a term, it is useless to attempt to determine 

 the matter by the Morison's Pill of priority. It therefore seems 

 advisable to retain the word in a general sense, and add some prefix 

 to denote the nature of the origin of the rock. 



Consequently the writer suggests the appended classification of 

 gneisses, separating those in which foliation is an original structure 

 from those in which it has been secondarily produced ; the latter 

 series may be subdivided according to the nature of the original 

 rock. The following table summarizes the classification and terms 

 proposed : — 



A. Metamorphic. { Altered ^us . rocks or Metapyrigen 3 -gneisses. 



L [ ,, sediments or Clastic gneisses. 

 P. Igneous. Fluxion-gneisses. 



1 c=chlorite; a 2 1 =actinolite ; 0=zoisite; F 7 = sphene: F 9 = garnet. 



2 ' Note sur la Formation gneissique du Morvan et Comparaison avec quelques 

 autres regions de metne nature,' Bull. Soc. geol. France, ser. 3, vol. vii. (1879) 

 pp. 870-871. 



s From Tvpiyevrjs, formed by fire. 



