128 ON THE FLEXURES AND 



senting striking examples of their capacity to admit 

 flexure, and indicating in a way that cannot be dis- 

 puted, any more than in the analogous cases of trap, 

 the nature of the cause which effected, both the cur- 

 vature and the displacement. 



The last argument which I need offer to prove that 

 the flexion of strata has been the result of force im- 

 pressed on a flexible body, and not a consequence of 

 any original disposition or concretionary structure, is 

 found in those rocks which, like micaceous schist, 

 present a certain parallelism in the position of one or 

 more of their integrant minerals. 



When mica occurs in flat beds of gneiss and mica- 

 ceous schist, it is found invariably to be more or less 

 accurately parallel to the planes of the strata, and, 

 consequently, to itself. There are two ways in which 

 this appearance may be explained ; both of which 

 form objects of inquiry in another part of this work. 

 The mica may have either assumed this disposition 

 from mechanical causes, as happens in the later sand- 

 stones, and even in the loose sand of sea shores, or 

 that position may be the result of a common polarity, 

 as is evinced in the parallelism of the hypersthene in 

 some varieties of hypersthene rock, or in that of the 

 mica in certain trap veins of rare occurrence. Under 

 either supposition, the present argument is equally 

 valid, so that its consequences cannot be evaded. 

 Now when gneiss or micaceous schist is contorted, it 

 is observed that the mica is no longer parallel to 

 itself or to any level plane that may be assumed in 

 the rock, although it still continues to be parallel to 

 the curvatures of the beds. Under the theory of de- 

 position, it could not have assumed the various and 

 extraordinary situations it often presents ; it could 



