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



would be called upon to sustain a much greater pressure tlian others, 

 while some parts might escape almost wholl}^ With the breaking of 

 the rock under the influence of strain there would be more or less pul- 

 verization of the minerals along the sides of the crack. This would favor 

 tlie chemical action of percolating waters. With the recr3^stallizatioH of 

 the constituents taken into solution the crack would become healed. 

 This may be conceived as taking place (1) by partial solution of the 

 powder, with a secondary enlargement of the remaining portions ; or (2) 

 by complete solution and recrystallization of the pulverized material. 

 In either case the filling would be fine grained, since even if wholly re- 

 crystallized the process would prol)ably go on synchronously with the 

 movement of the rock. Evidence that this process lias taken place at a 

 later date is sometimes seen in the presence of healed cracks cutting 

 across the banding. 



With increasing pressure the rock may be reduced to a coarsely frag- 

 mental condition, and if the process were stopped at this stage the result 

 would probabU'' be a mass of irregular fragments, cemented together by 

 fine grained interstitial material. The pyroxene within reach of the 

 percolating waters would be dissolved to a greater or less extent, and on 

 subsequent recrystallization would appear in grains marking the spaces 

 occupied by the solutions. The relative solubility of pyroxene and feld- 

 spar under the conditions here postulated is unknown. The relative 

 effects of weathering, however, are well shown by the manner in which 

 the pyroxene decays on exposed surfaces of the ellipsoidal rock, leaving 

 sharp trenches surrounding the felds])athic portions. Since basic min- 

 erals melt at somewhat lower temperature than the acidic, it ma}^ be 

 supposed that the temperature of the rock became sufficiently great to 

 melt the pyroxene, but not the feldspar.* NVhile the convoluted forms 

 assumed by the ellipsoids are suggestive of plasticity, it is scarcely prob- 

 able that the heat has ever reached the point indicated. This is inferred 

 (1) from the occurrence occasionally of crystals of augite, which are ap- 

 parently original, inclosed in grains of microcline in the ellipsoidal areas; 

 and (2) from the consideration that if the heat is due to the shearing 

 movement of the rock, as generally conceived, it must be generated 

 slowly, and hence would probably be dissipated nearly, if not quite, as 

 rapidly as it is produced. 



As the ellipsoids became more and more flattened the interstitial 

 pyroxene bands would assume a parallel arrangement, as in the case of 

 other similar gneissoid structures.f 



In support of the hypothesis of dynamic origin we note : 



* J. G. Goodehild : Geol. Mag., new series, Dee. IV, vol. I, no. 1, Jan., 1891, p. 23. 

 t F. Zirkel : Lehibucli der Petrographie, band iii, p. 205. 



