778 A TREATISE ON METAMORPHISM. 



SLATE AND SCHIST. 



Slate and schist are here used as general terms which are applicable 

 to all rocks having a well-defined cleavage in which the cleaved plates 

 are essentially like one another. I have explained in another connection 

 that cleavage in rocks is due to the arrangement of the mineral particles 

 with their longer diameters or readiest cleavage, or both, in a common 

 direction, and that this arrangement is caused, first and of most impor- 

 tance, by parallel development of new minerals; second, by the flattening 

 and parallel rotation of old and new mineral particles; and third, and 

 of least importance, by the rotation into approximately parallel positions 

 of random original particles." Subsequently Leith has shown that parallel 

 slicing may also be a subordinate factor. In a slate or schist the cleavage 

 may or may not be parallel to an original structure such as bedding, 

 but usually it intersects the original structures. Cleavage is fully dis- 

 cussed by Leith. 6 His work shows that cleavage ultimately rests upon a 

 parallel dimensional arrangement of the mineral particles, but for some 

 minerals this dimensional arrangement carries with it mineral cleavage, and 

 the cleavage of these minerals is usually the controlling factor in rock 

 cleavage. Moreover, he argues that the dimensional arrangement is mainly 

 caused by recrystallization. His work while advancing in an important 

 way the theory of cleavage also confirms my own view in showing that 

 rock cleavage is mainly due to the actual cleavage of mineral particles 

 produced by recrystallization, and subordinately due to the easy separation 

 between mineral particles in the direction of their greater dimensions. 



The production of slate and schist requires recrystallization during 

 mass-dynamic action. Previous textures and structures are partly or wholly 

 obliterated. In these particulars slate and schist contrast with those rocks 

 to which the term apo may be prefixed. 



SLATE. 



Slate is defined to include those cleavable rocks the cleavage pieces of 

 which are like one another and the mineral particles of which are for the 

 most part so small as to be invisible to the naked eye. The typical example 

 is furnished by the roofing slates, which, so far as the eye can see, are gray 



«Van Hise, C. K., Principles of North American pre-(?ambrian geology: Sixteenth Ann. Rept. 

 U". S. Geol. Survey, pt. 1, 1896, p. 635. 

 6 Leith, C. K., Rock cleavage. 



