770 PRINCIPLES OF STRATIGRAPHY 



to quartzite and then to quartzite schist ; arkoses into arkose schists, 

 and arkose gneisses ; grits into graywackes, graywacke slates, gray- 

 wacke schists and graywacke gneisses, and shales or lutytes into 

 lutyte schists and lutyte gneisses (pelite schists and pelite gneisses). 

 Limestones are changed to marble and soft coals to anthracites and 

 graphites. In general, pure sediments are the least altered, while 

 mixed sediments are likely to produce the greatest variety of new 

 minerals under metamorphism. 



The Terms Slate, Schist and Gneiss. 



The term slate, though generally used as a lithological term, 

 is strictly applied only as a structural one. A slate is a metamor- 

 phic rock of a lutaceous texture and homogeneous character, split- 

 ting into parallel leaves, and whose mineral particles are for the 

 most part so small as to be invisible to the naked eye. ( See, further. 

 Chapter XX.) A schist has been defined as "a rock possessing a 

 crystalline arrangement into separate folia." (Geikie-14: iy8.) A 

 typical schist has its cleavable minerals arranged in the same way 

 and like one another and large enough to be for the most part 

 visible to the naked eye. Mica is the most important cleavage- 

 making mineral, and the most typical schist is one composed of 

 quartz and mica scales, a quartz mica schist, more commonly spoken 

 of as a mica schist. 



Van Hise has urged the use of the term schist in a purely 

 structural sense, much as the terms shales and slate should be used. 

 This is in strict accord with the definition by Geikie given above. 

 In mineral composition and in origin schists may vary greatly. 

 Thus we may have the ordinary quartz mica schist, which contains 

 quartz and mica in about equal proportions. Hornblende schists 

 are schists containing hornblende and some other minerals, generally 

 feldspar. Thus we may have a hornblende plagioclase schist. If 

 the origin of the schist is known this may be indicated in the 

 name, as arkose schist, silicirudyte schist, or if derived from an 

 igneous rock such as gabbro it becomes a gabbro schist. Both com- 

 position and origin may be indicated, as in the name mica-quartz- 

 feldspar-arkose-schist. 



In certain petrographic circles, especially the German ones, the 

 term schist was applied to rocks not only having a schistose struc- 

 ture, but also a definite composition. Thus quartz was considered 

 essential in the formation of a schist and was assumed to be pres- 

 ent. We thus had mica schists, hornblende schists, chlorite schists, 



