296 



METAMORPHIC ROCKS 



Gneiss is a term of wide significance, which includes a number 

 of rocks of different modes of origin and different mineralogical 

 composition. It is " a laminated metamorphic rock that usually 

 corresponds in mineralogy to some one of the plutonic types " 

 (Kemp). The varieties of gneiss are ordinarily named in accord- 

 ance with the most conspicuous dark silicate present, as biotite 

 gneiss, hornblende gneiss, etc. ; but this system of nomenclature 

 gives an imperfect notion of the character of the rock. A better 

 method, recently suggested (C. H. Gordon), is to name the 



, 



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Fig. 130. — Plicated gneiss. (U. S. G. S.) 



varieties in accordance with the igneous rocks to which they 

 correspond in mineralogical composition ; as granitic gneiss, 

 syenitic gneiss, dioritic gneiss, etc. The commonest variety is 

 granitic gneiss, with mica or hornblende ; the orthoclase and 

 quartz are mingled together, with conspicuous laminae and folia 

 of the dark mineral. 



Most gneisses were generated by the dynamic metamorphism of 

 granite, either before its consolidation or after it had cooled and 

 hardened. Some authorities deny that gneiss has ever been formed 

 from sedimentary rocks, but there is good reason to believe that 



