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or planes marked out by textural peculiarities. This, however, is not always 

 the case. It sometimes happens that a marked parallel arrangement may be 

 observed. Thus, in certain eruptive granites the flat faces of the mica plates, 

 or of the larger felspar crystals, are arranged parallel to each other and to 

 certain larger structural features in the rock-mass. Such an arrangement, if 

 original, must be attributed to differential motion in the mass after certain 

 constituents have been formed, but before final consolidation. It is an 

 instance of fluidal structure. In the volcanic rocks this fluidal structure is 

 much more common. Inasmuch as the differential motion is caused by the 

 friction of the semi-molten matter with the boundary walls in the case of an 

 intrusive mass, and with the surface over which it flows in the case of a lava 

 current, it follows that the textural planes above referred to will stand in 

 direct relation to the external surfaces of the rock-mass. 



A parallel arrangement of the crystalline constituents is the special 

 characteristic of the crystalline schists, and the term foliation is applied to 

 this arrangement, as seen in these rocks. It has recently been shown (1) that 

 many crystalline schists have been produced by the mechanical metamorphism 

 of igneous rocks, and that the foliated texture is therefore of secondary origin. 

 The discussion of this question is deferred for the present, but the above 

 remarks are introduced for the purpose of indicating the necessity of caution 

 in determining whether the parallel arrangement in any given case is 

 original or secondary. 



(1) J. LEHMANN. Die Entstehung der alt-krystallinischen Schiefer-gesteine. BONN, 1884. 



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