782 A TREATISE ON METAMORPHISM. 



The term gneiss is defined to apply to a banded rock the bands of 

 which are petrographically unlike one another and consist of interlocking 

 mineral particles. The bands in different gneisses are of variable thickness, 

 ranging from a fraction of a centimeter to many centimeters. Also there 

 is a similar range in thickness of >the different bands of the same gneiss. 

 The lithological dissimilarity of the bands of gneiss constitutes a funda- 

 mental distinction between gneisses and the slates and schists, which are 

 comparatively homogeneous. Usually the gneisses have a cleavage par- 

 allel to the banding, but this cleavage is by no means so general or 

 distinctive as in the slates and schists, and not infrequently a parallel 

 arrangement of the mineral particles resulting in cleaA-age is almost wholly 

 lacking. This is especially likely to be the case with the gneisses of 

 igneous origin. It has been explained that the slaty and schistose struc- 

 tures are mainly dependent upon recrystallization during mashing*. (See 

 pp. 688-690, 748-759.) Where a parallel arrangement of the mineral 

 particles is marked in gneiss this is likely to be largely due to the same 

 cause. But the parallel orientation of some of the mineral jmrticles of a 

 part of the original gneisses formed from magmas is due to differential stress 

 during the primary crystallization of the rocks. 



The use of the term gneiss advocated in the preceding paragraph 

 approximates closely to the practice of American and English geologists in 

 recent years. But gneiss, like schist, has been extensively used with a dual 

 meaning, comprising structural and mineralogical factors. For instance, in 

 Germany the word gneiss has been used generally to designate rocks which 

 have the structure either of schists or of gneisses, as defined in the foregoing- 

 pages, and which have a quartz-feldspar background with one or more 

 other constituents. In this sense the terms mica-gneiss and hornblende- 

 gneiss mean rocks having either a schistose or a gneissose structure, as here 

 defined, and a quartz-feldspar groundmass in which are respectively mica 

 and hornblende. The dual significance of the term gneiss has arisen natu- 

 rally from the fact that many of the gneisses, especially the abundant ones 

 first studied, have as chief constituents quartz and feldspar, and with these 

 minerals one or more other constituents. The minerals quartz and feldspar 

 common to them were taken as distinctive of the gneisses, and the names of 

 the other minerals were prefixed as qualifiers. But precisely as is the case 



