DEFINITION OF GNEISS. 783 



with schists, the term gneiss was extended to banded rocks many of which 

 do not have this definite mineral groundmass. This became apparent only 

 when the microscope was applied, to thin sections of rocks. 



Since, as a matter of fact, it has been found that the rocks which have 

 been called gneiss have a very great variety of mineral constituents, and not 

 necessarily any of the constituents originally supposed to be implied by the 

 name, it is no longer possible to use the term gneiss with a dual significa- 

 tion and at the same time to have clear discrimination. I therefore propose 

 to confine the term gneiss strictly to its structural sense, including all finely 

 banded crystalline rocks, whether of ig-neous or of aqueous origin. Thus 

 defined, the term gneiss may be applied precisely as are the terms slate and 

 schist. Mineralogical qualifiers may be prefixed; the name of the rock from 

 which the gneiss is derived may be prefixed; the word gneiss may be pre- 

 fixed to the name of the original rock; and, finally, names denoting the 

 mineral composition, the name of the original rock, and the term gneiss may 

 be combined. For instance, we may say mica-quartz-feldspar-gneiss, diorite- 

 gneiss, granite-gneiss, gneissoid diorite, gneiss-diorite, gneissoid granite; 

 gneiss-granite, and finally mica- quartz-feldspar-gneiss-granite, etc. 



GENERAL STATEMENTS. 



It has been seen that all of the terms which have been discussed — 

 meta, apo, slate, schist, and gneiss — may be united with petrographical names 

 and with mineralogical qualifiers in various ways. For additional refined 

 discrimination geographical qualifiers may be used. 



Under the usages advocated the character of a metamorphosed rock 

 may be designated very loosely and broadly either in a structural or in a 

 petrographical sense, may be discriminated with a moderate degree of 

 accuracy, or may be designated very accurately. The following' usages 

 illustrate the various stages of gradation from loose to accurate designation: 

 An original unaltered rock may be granite, diorite, syenite, limestone, 

 sandstone, etc. If we wish to indicate that the rock has been altered 

 without saying anything as to the nature of the alteration, we may prefix 

 meta to these terms. If alteration has taken place, but the original texture 

 of the rock is retained, we may prefix apo to any of the names. If 

 alteration has taken place so as to produce new structures, we may prefix 

 to the names the words slate-, schist-, or gneiss-, or the adjectives slaty, 



