126 C. H. GORDON — SYENITE-GNEISS (lEOPARD ROCk) FROM CANADA. 



view which does not take into consideration the movement of tlie magma 

 during or following its differentiation. 



According to the second view (b), the gneissic structure is regarded as 

 the result of the conditions attending the intrusion and consolidation of 

 the igneous magma. Professor Lawson considers * that he has abundant 

 evidence to show that the granite and S}- enite-gneisses of the Laurentian 

 were plutonic rocks which crystallized slowly from a thickly viscid, 

 tough, hydrothermal magma. Up to the time of its final solidification 

 this magma is supposed to have been subjected to differential pressures, 

 which produced a flow in the mass to which the foliation of the gneiss 

 is ascribed. Geikie and Teall have described f a gabbro in which there 

 appears a banded structure almost identical with that of the Lewisian 

 gneiss. This is regarded as inexplicable, either on the hypothesis of 

 differentiation in situ or on that of successive intrusions, but is thought 

 to have been produced by the deformation of a heterogeneous magma 

 during intrusion. 



In applying this view of the development of the gneissic structure to 

 the ellipsoidal rock, two hypotheses are suggested based on the assump- 

 tions that may be made as to the character of the differentiation. 



If in the first of these, which should be designated hypothesis III, we 

 assume, as in the second hypothesis, a differentiation of the magma by 

 liquation into lumps of feldspathic material separated by tliin layers 

 of basic material, then with a gradual movement of the magma it is evi- 

 dent that the forms would be drawn out more and more until the con- 

 stituents became arranged in [)arallel bands. It is obvious that on the 

 final solidification of the rock, a structure would result comparable in 

 many respects with that of the leopard rock. As against this explana- 

 tion, however, we have the following considerations : 



1. The uppareiitly cataclastic character of the gneissic structure and crystalliza- 

 tion appearing in places. In the coarse grained rock granular bands occur as thin 

 seams, like recemented cracks, sei)arating the rock into coarse grained patches. 

 These seams are sometimes too thin to be readily detected on the surface of the 

 rock, but under the microscope sliow as narrow, sharply defined bands of granular 

 microcline, fresh plagioclase feldspar and pyroxene inclosing the coarsely crystal- 

 lized areas of microcline. These granular bands become more pronounced and 

 regular in the ellipsoidal gneiss, where they constitute a well marked band between 

 the feldspathic lumps, with larger grains of pyroxene distributed in a line along 

 the middle of the granular groundmass. In the streaked gneiss the rock becomes 

 granular throughout, though even here quite large grains of microcline often appear 

 in the feldspathic bands. 



2. The character of the coarse grained rock and the absence of any indication in 

 it of the kind of differentiation assumed. If a regular lumpy aggregation may 



* A. C. Lawson : Ann. Rep't Geol. Survey of Canada, 1887-'88, new series, vol. iii, part 1, p. 139 F. 

 fSir A. Geikie and J.J. H. Teall: Banded Structure of some Tertiary Gabbros in the Isle of 

 Skye. Quart. Jour. Geol. Soc., Nov., 1894, p. 645. 



