NOMENCLATURE. 121 



The term pyroxene-gneiss has been applied by the last named author 

 to rocks, of much more basic composition than those under considera- 

 tion, corresponding to the pyroxenites of the apatite region. 



It requires but a brief survey of petrological literature to become aware 

 of a great difference in usage as to the term gneiss. By some authors 

 the mineralogic composition of these rocks is made the basis of defini- 

 tion, and, being regarded as having the greatest analogy with the gran- 

 ites, they are defined as characterized by the presence of feldspar and 

 quartz as essential constituents.* 



By many, however, the term is employed in a structural sense to denote 

 the coarser schists, which so often present granitoid characters — a more 

 comprehensive and preferable usage, since the gneissoid or foliated struc- 

 ture may characterize rocks of very diverse composition. The difficulty 

 attending the application of a mineralogical definition is acknowledged 

 by Zirkel when he attempts to draw the line between certain hornblende- 

 gneisses and amphibolite.f 



The mineralogical definition precludes the use of the term syenite- 

 gneiss. This name, however, has been used for a quartz-bearing horn- 

 blende-gneiss and is given as a synonym for this rock by Geikie.;]: 

 Naturally enough Zirkel does not recognize such a division and sets the 

 term aside as misleading.! On the whole, it seems to the writer that the 

 broader use of the term gneiss in the structural sense is to be preferred. 

 This usage prevails quite generally among the English and French pe- 

 trographers. 



The principles applicable to the classification of the gneisses may be 

 summarized as follows : 



1. Their mineralogic composition. 



2. Identity in composition and texture with the igneous or sedimentary rocks. 

 Origin unknown. 



3. Identity in origin, composition and texture with igneous or sedimentary rocks. 

 Under this we have to consider (a) those rocks in which the gneissoid structure 

 is due to dynamic agencies, and (6) those in which it is the result of conditions at- 

 tending their original solidification. 



*Zivkel : " Zum Wesen des eigentlichen Gneisses gehort das jedesmalige Dasein von Kalifeld- 

 spath, Qnarz und von einem triklinen K^alknatronfeldspath Oder Natron-feldspath. . . . Wesent- 

 liehen Bestandtheile der Gneisses im AUgemeinen bilden aber noch ausserdem Magnesiaglimmer, 

 Kaliglimmer and Hoi-nblende, welche indessen nicht in sammtlichen Gneissen vorkommen, 

 sondern einzein oder zii zweien auf gewisse Abtheilungen derseben besehninkf sind." (Lehr- 

 bucli der Petrographie, band iii, p. 185, 1894.) 



t Es ist schvver die Grenze gegen die letzeren (feldspath-quarz halfcigen Amphiboliten) zu Ziehen, 

 aber nicht wohlgethan, Gestein mit sehr vorwaltender Hornblende zu den Gneissen zu rechnen. 

 Namentlich ist es anch nicht zu billigen gar quarzfreie feldspathaltige Amphibolite diesea 

 Gneissen zuzuziihlen. Ferner bedingt der Name Gneiss immerliin ein gewisses planes Parallel- 

 gefiige. (Lehrbueh der Petrographie, band iii, p. 215, 1894.) 



t Text-book of Geology, 3d edition, p, 186. 



§ Lehrbueh der Petrographie, band iii, p. 215, 1894. 



