120 C. H. GORDON — SYENITE-GNEISS (LEOPARD ROCK) FROM CANADA. 



extent in the more gneissoid variety, but is overshadowed by the general 

 appearance of recrystallization. The idiomorphic character of the augite 

 inclosed in the microcline is wholly absent in all the pyroxene grains 

 appearing in tlie granular groundmass. These grains often appear pris- 

 matic, but never have regular outlines. They frequently lie transverse 

 to the augite band, giving the radiate arrangement already noted. 



The evidences of dynamic movements a})pear in the coarse rock in the 

 fracturing and breaking of the constituents and the presence of cracks 

 extending uninterruptedly across adjoining grains. Tlie last feature was 

 less apparent in the more gneissic rock. The grains of augite usually 

 show an abundance of coarse fractures and occasionally a broken grain 

 with parts slightly disarranged. The appearance of breaking occurs in 

 the augite in the granular bands only, and if these bands re})resent lines of 

 breakage, as is believed, the fracturing of the pyroxene may be correlated 

 with the agencies which initiated the development of the gneissic struc- 

 ture. No distinct evidence of the derivation of the granular pyroxene 

 from the coarse grains and crystals could be made out. 



Hornblende does not appear to have been present in the original in- 

 trusion. It sometimes appears in a very small amount in connection 

 with the augite, sometimes as an alteration of the latter. In these cases 

 it is destitute of crystallographic form. In the gneissic rock, however, 

 the hornblende becomes quite abundant and often presents well marked 

 idiomori)hic outlines. 



The quartz in the coarse granular rock, as in the case of the feldspar 

 and augite, ai)pears to be distributed in patcbes. It is a})parently more 

 abundant in the more gneissic rock, where the gniins assume elongated 

 forms and are arranged more and more in lines. 



The laminated gneissic arrangement of the constituents is a marked 

 characteristic of the granular bands. In those of the coarse grained rock 

 it is not marked. It is indicated, however, by the tendency oftliecjuartz 

 grains to become elongated parallel with the band. In the leopard rock 

 there is a distinct lamination of the constituents in the interstitial zone. 

 In cases where the feldspathic lumps are more or less granular through- 

 out, the constituents tend to assume a laminated arrangement, but this 

 is not common. 



Nomenclature. 



The characteristics of th^ streaked gneiss here described correspond to 

 what, according to Zirkel,* may be regarded as a p3''roxene-gneiss (augite- 

 gneiss). A somewhat similar rock, but apparently containing less augite, 

 has been described by Lacroix as granulitic microcline-gneiss.f 



♦ Lehrbtich der Petrographie, band iii, p. 219. 



t Bulletin de la Societe Fraa9aise de Mineralogie, April, 188'J. 



