DEFINITIONS OF SLATE AND SCHIST. 779 



or black, homogeneous, aphanitic rocks, which may readily be parted into 

 thin plates which are indistinguishable from one another. The microscope 

 shows that the cleavage of these rocks is due to the causes above given. 



The foregoing definition of slate is purely structural. It involves no 

 implication as to the minerals which compose the slate. It says nothing as 

 to whether the rock from which a slate is derived is sedimentary or igneous. 

 If it be desired to ignore the origin of a slate and to indicate its mineral 

 composition, this may be accomplished by prefixing mineralogical qualifiers. 

 For instance, if a slate be composed of mica and quartz as the chief con- 

 stituents, it is a mica-quartz-slate. If it be desired to emphasize the slaty 

 structure, and at the same time to indicate the original rock from which the 

 slate is derived, the name of this rock may be prefixed. For instance, if a 

 slate be derived from mud, it may be called a pelitic slate or pelite-slate; if 

 a slate be derived from dolerite, it is a doieritic slate or dolerite-slate, etc. 

 If it be desired to emphasize the original rock from which the slate is 

 derived, and only secondarily to indicate that the rock has a slaty structure, 

 this may be done by placing the word slate in the first position; as, for 

 instance, slaty pelite or slate-pelite, and slaty dolerite or slate-dolerite. 

 Finally, mineralogical qualifiers may be prefixed to the above compound 

 names, as mica-quartz-pelite-slate or mica-quartz-slate-pelite. 



Schist 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 large as to be visible to the naked eye. The, thin cleavage 

 plates of a schist which are like one another are called folia, and the rock 

 is spoken of as foliated. The most important of the cleavage-making 

 minerals is mica, and a typical example of a schist is one composed of mica 

 and quartz in which mica is the chief cleavable mineral. Such a rock may 

 therefore be called a mica-quartz -schist. There is great variation in the 

 approach to perfection of the arrangement of the minerals. For instance, 

 the plates of mica rnay be almost perfectly straight and parallel or they 

 may be curved or even crenulated. The closest approximation to the above 

 definition of schist is that given by Greikie. According to this author, "A 

 rock possessing a crystalline arrangement into separate folia is in English 

 termed a schist,"" It is to be noted that the definition of schist given is 



«Geikie, Archibald, Text-book of Geology, Macniillan & Co., London, 3d ed., 1893, p. 178. 



