134 C. H. GORDON — SYENITE-GNEISS (lEOPARD ROCK) FROM CANADA. 



rocks as the result of the conditions attending their intrusion. In conse- 

 quence of the constitutional changes which appear to take place in rocks 

 when subjected to great orograi)hic pressure, it becomes in many cases 

 extremely difficult, if not impossible, to distinguish between the two 

 kinds of gneisses. Moreover, our knowledge of the cause of differentia- 

 tion of igneous magmas is as yet little more than a speculation, but that 

 the nature of this original differentiation conditions in ])art the character 

 of the structure resulting from subsequent processes is obvious. 



While in many respects incomi)lete, involving, as it does, much that is 

 as yet little understood in the metamorphism of rocks, on tlie whole the 

 evidence seems to favor the last hypothesis (V), namely, that of dynamic 

 metamorphism. 



Briefly summarized, this h3"pothesis supposes — 



1. That the structure characterizing the leopard rock is due to orographic agen- 

 cies and represents an intermediate stage in the development of a streaked augite- 

 syenite-gneiss out of an augite-syenite which was distinguished by a coarsely 

 crystallized structure and by a somewhat irregular aggregation of pyroxene. The 

 character of the original magma may have been modified somewhat by the absorp- 

 tion of included fragments of pyroxenite. 



2. That the distribution of the pyroxene has been effected presumably by the 

 solution of portions of the orij2;inal constituents and their recrystallization along 

 lines marking the location of the cracks. 



3. Tliat with continued pressure these lumps have been more and more drawn 

 out, the process being accompanied by recrystalUzation until the rock assumes the 

 streaked gneissoid form. 



While in general the evidence of crushing is rendered more or less 

 doubtful by the extent of recrystalUzation, in one case (number 159) it 

 is undoubted. This represents a partly developed gneiss in which, 

 however, the ellipsoidal structure is but imperfectly presented. Though 

 differing considerably from the rest of the rock in general appearance, 

 there is little doubt but that it belongs to the same mass. The ground- 

 mass is much finer than in any of the others and shows much less the 

 effects of recrystalUzation. Large grains of microcline are partly crushed 

 and drawn out into lens-like forms comparable to the augen-structure. 

 A significant feature is the arrangement of the augite in irregular bands 

 about the feldspar and quartz. In some of the augite grains the appear- 

 ance of crushing and dragging is pronounced, while the general appear- 

 ance of the rock, both in hand specimens and under the microscope, 

 admits of no other conclusion than that its present structure is due to 

 the effects of orographic pressure. 



