NOMENCLATURE. 123 



In accordance with this view, the classification of the rock under con- 

 sideration is obvious. It is a gneissoid pyroxene microcline rock, which 

 in some places is almost or wholly free from quartz and corresponds to 

 a pyroxene-syenite, while in others not far distant the increase in the 

 amount of quartz would ally it to the pyroxene-granites. In view of the 

 generally sparing amount of quartz present in the coarse grained forms, 

 they are here referred to generally as pyroxene-syenite-gneiss or simply 

 syenite-gneiss, though it is not to be overlooked that these grade into 

 more quartzose forms, which ma}' be more fittingly regarded as pyroxene- 

 granite-gneisses. 



W. G. Ferrier* describes a gneiss from the Chateau Richer district 

 apparently identical with the " streaked gneisses " described above, to 

 which he applies the name " pyroxene-granite-gneiss." 



Lawson f describes a rock from the Rainy Lake region, which he terms 

 hornblende-syenite-gneiss and which he states is not separable geolog- 

 ically from others of the same region which he calls hornblende-granite- 

 gneiss. From his description of these rocks it is evident that, except in 

 the ellipsoidal structure and the character of the prevailing ferromagne" 

 sian constituent, these rocks show many points of resemblance to the 

 syenite-gneisses of the Du Lievre region. If the view held by Lawson as 

 to the origin of the Rainy Lake rocks is sustained, however, they should 

 be called gneissoid syenites. 



Origin of the ellipsoidal Structure. 



general hypotheses. 



Recognizing the intrusive character of the rock, in seeking an explana- 

 tion of the peculiar elhpsoidal structure two hypotheses suggest them- 

 selves : 



A. That it is primary and represents differentiation of the magma ; or, 



B. That it is secondary and due to dynamic movements subsequent to 

 solidification. 



In applying any theory to the phenomena in question we have to con- 

 sider its competency to explain — 



1. The interstitial arrangement of the augite about the ellipsoidal masses ; 



2. The gradation between this and the segregated lumps and crystals in the 

 coarsely crystallized rock on the one hand, and the parallel arrangement of the 

 streaked gneiss on the other ; 



3. The presence of the gneissic microstructure and the progressively greater 

 granulation of the constituents as we proceed from the least to the most distinctly 

 gneissoid forms ; 



* Notes on the microscopical ciiaracter of some roclvs from the counties of Quebec and Mont- 

 morency, Canada. Number 14^, p. 9. 

 t A. C. Lawson : Annual Rept. Geol. Survey of Canada, 1887-'88, vol. iii, part 1, p. 120 F. 



