122 C. H. GORDON — SYENITE-GNEISS (LEOPARD ROCk) FROM CANADA. 



All of these principles are recogaized by petrographers, though usage 

 varies greatly as to their application in the naming of the rocks. Thus, 

 according to some, the specific designation is based upon the prevailing 

 mineral characteristics, while others prefix the names of other rocks 

 with which the gneiss agrees in its textural or mineralogical characters. 

 The latter is the prevailing usage among the Canadian geologists, who 

 apply the term quartzite-gneiss, diorite-gneiss, etcetera, as specific desig- 

 nations to granitoid rocks possessing a parallel structure. Some geol- 

 ogists* restrict the terms syenite-gneiss, diorite-gneiss, gabbro-gneiss, 

 etcetera, to rocks in which there appears a gneissoid structure due to 

 differential movements acting upon the igneous mass in the later stages 

 of its original consolidation. 



The desirability of greater uniformity in the method of naming the 

 gneisses is apparent. The following slight modification of methods in 

 quite general use is suggested as a step in this direction : 



1. That the term j^neiss l)e used in its broader stnictnral sense for all rocks show- 

 ing a laminated or banded structure and in wliich the jjneissoid structure is not 

 known to be due to differential movements of the igneous mass before its final con- 

 solidation. For the latter a structural qualifying term may be used, as has already 

 been done by Geikie and others, as gneissoid or banded gabbros, et(;etera. 



2. That where the origin of the rock (whether igneous or sedimentary) is known? 

 the class designations be made to correspond with the character of the original 

 rock. Thus a gneiss known to have consolidated originally as a diorite may be 

 termed diorite-gneiss. 



3. In those cases where the character of the original rock is unknown, compris- 

 ing i)roba])ly the larger part of the group, the extent of knowledge with reference 

 to this point maybe indicated by the ending **ic." Thus a rock with gneissic 

 structure and corresponding in its mineralogic composition with the diorites, ])ut 

 whose geological relations are unknown, may be called a dioritic-gneiss. 



The following table illustrates these i^rinciples as applied to a few of 

 the more im])ortant tyi)es of rocks : f 



Gneiss. 



Analogous massive type. 



Of igneous origin. 



Origin unknown. 



Granite : X 



Biotitegranite. 

 Hornblende-granite. 



Granite-gneiss : 



Hiotite-gianite-gneiss. 

 Hornblende-graiiiie-gueiss. 



Graniiic-gneiss : 



Biotite- granitic-gneiss. 

 Hornblende-granitic-gneiss. 



Syenite : 



Hornblende-syenite. 



Mica-syenite. 



Pyroxene-syenite. 



Syenite-gneiss : 



Hornblende-syenite-gneiss. 



Miea-syenite-gneiss. 



Pyroxene-syenite-gneiss. 



Syenitic-gneiss : 



Hornblende-syeni tic-gneiss. 

 Mioa-syeni tic-gneiss. 

 A ngite-syeni tic-gneiss. 



Diorite : 



Mioa-diorite. 



Diorite-gneiss : 



M ica-diorite-gneiss. 



Dioritic-gneiss : 



Mica-dioritic-gneiss. 



Gabbro. 



Gabbro-gneiss. 



Gabbroic-gneiss, or gabbric-gneisa. 



Pyroxenite. 



Pyroxenite-gneiss. 



Pyroxenitic-gneiss. 



* J. G. Goodcbild : The Geol. Mag., new ser , Dec. IV, vol. I, number 1, p. 27 (Jan , 1894). 



fAs types of the sedimentary formations, there may be quartzite-gneisses and quartzitic- 

 gnei.«ses. 



I In the absence of a better name, the term "granite" is here used in its restricted petrographic 

 sense. 



