HYPOTHESIS OF SECONDARY ORIGIN. 129 



this way, as claimed by Lawson* is probably true, but whether the basic 

 material may be so distributed as to crystallize out in the form shown in 

 the ellipsoidal gneiss is doubtful. 



While the argument drawn from the character of the gneissic struc- 

 ture is not beyond question, it is strongly presumptive against the primary 

 origin of the ellipsoidal structure. 



SECONDARY ORIGIN. 



In attempting to explain the ellipsoidal structure by appealing to 

 dynamic processes (hypothesis V), it is necessary to consider (1) the 

 character of the original rock and (2) the agencies and their mode of 

 operation. 



The character of the original rock, as inferred from the coarse grained 

 variety, was evidently peculiar. Its heterogeneous character is indicated 

 by the tendency of the feldspar, pyroxene and quartz to appear in large 

 grains, lumps and segregations. In some cases, however, the rock has 

 an even, granular structure and consists of feldspar and pyroxene quite 

 uniformly distributed. In places the rock is mostly coarse feldspar, with 

 scattering grains and small aggregates of pyroxene. In other places the 

 pyroxene occurs in much greater amount, both as crystals of various 

 sizes and aggregates of grains, apparently constituting from 15 to 20 per 

 cent of the rock. In general the irregular pyroxene aggregates vary from 

 the size of millet seeds up to marbles, rarely larger. They usually in- 

 close more or less feldspar in small grains ; also pyrite and titanite. In 

 areas showing a large amount of pyroxene the latter sometimes appears 

 in idiom Orphic grains, of large size, inclosed in the microcline. They also 

 appear at times in the quartz areas. In the specimens showing a uniform 

 distribution of feldspar and pyroxene, idiomorphic forms of the latter 

 were not seen. 



While the arrangement and mode of crystallization of the constituents 

 seem to indicate partial differentiation of the original magma, it is not 

 improbable that some of the pyroxene was derived from the inclosures of 

 the pyroxenites, as already suggested. 



The agencies which may be appealed to as effecting the changes ob- 

 served are those usually classed under the head of dynamic meta- 

 morphism. These may be conceived to have effected the crushing of 

 the coarse grained syenite, accompanied by solution, recrystallization 

 and rearrangement of the constituents under the influence of water, and 

 probably also of heat. The variation in the extent of the structural 

 alteration may be due to the different attitudes of the inclosing rocks- 

 It will scarcely be denied that certain portions of the intrusive mass. 



* A. C. Lawson : Ann. Rep't Geol. Survey of Canada, 1887-88, new series, vol. iii, part 1, p. 134 F. 



