DEFINITION OF SCHIST. 781 



structural meaning as advocated. Illustrating the usage, if a rock which 

 has the structural characters of schists as here defined be composed of horn- 

 blende and plagioclase as chief constituents, it is a hornblende-plagioclase- 

 schist. If the plagioclase be definitely determined, as, for instance, 

 labradorite, such a rock is a hornblende-labradorite-schist; and of course 

 the classical mica-schist is mica-quartz-schist, as already stated. 



The schists in which it is not desired to direct the attention to their 

 origins, or the origins of which are not known, are thus satisfactorily dis- 

 criminated by mineralog'ical qualifiers. If the origin of a given schist be 

 known, and it is desired to indicate this, the term is used in precisely the 

 manner in which it is proposed to use slate, viz, by combining schist with 

 the names of rocks from which it has been derived. In doing this either 

 the schistose character or the origin of the rock may be emphasized. If it 

 be desired to emphasize the schistose character, and at the same time to 

 indicate the original rock from which the schist is derived, the name of this 

 rock may be prefixed. For instance, if a schist be derived from arkose, it 

 is an arkose-schist. If a schist be derived from a gabbro, it is a gabbro- 

 schist. If it be desired to emphasize the original rock from which the 

 schist is derived, and only secondarily to indicate that the rock has a 

 schistose character, this may be done by placing the word schist in the 

 first position; as, for instance, schistose arkose or schist-arkose, schistose 

 gabbro or schist-gabbro. Finally, if it be desired to combine the chief 

 minerals with the compound names which indicate the schistose character 

 and the origin of the rock, this may be done. For instance, a rock may be 

 said to be a mica-quartz-feldspar-arkose-schist. 



The usage above advocated is very advantageous in handling the 

 metamorphosed rock masses, since the mineral character of a given schist 

 may be correctly indicated without any reference to its origin, or the origin 

 of the schist may be very accurately indicated. If the term schist be used 

 with a combined structural and miiieralogical meaning, as is ordinarily 

 done, such usages are impossible. For instance, if schist means the pres- 

 ence of a certain mineral, as quartz, it can not be applied to a rock which 

 has a schistose character as here defined and which does not contain quartz. 

 For example, one could not say peridotite-schist. Considering all the fore- 

 going facts, the advantage of restricting' the term schist to a purely struc- 

 tural usage is so great that all mineral implications for the term should be 

 dropped. 



