172 ALBERT JOE ANN SEN 



was called Monzon-syenit by von Buch/ and later monzonit by 

 De Lapparent.'' The latter term served as a collective name for 

 some years, but in 1881 Reyer^ used it as a specific name for 

 rocks of the Monzoni type, which he thought to be augite-syenite. 

 Earlier Tschermak^ had recognized the principal rock to be an 

 orthoclase-plagioclase rock, but he applied the term monzonite 

 to the whole series. Lembergs characterized the rock as inter- 

 mediate between syenite and diorite. In those days rocks were 

 classified either as syenites or diorites, and it was not until 1895 that 

 an intermediate family was established. In that year Brogger^ 

 introduced the monzonite family. He says: "Es ist . . . . nach 

 meiner Ansicht nothwendig, zwischen den Orthoklasgesteinen und 

 den Plagioklasgesteinen .... eine Ubergangsordnung von Alkali- 

 feldspath-Kalknatronfeldspath-Gesteinen einzuschieben." The 

 plagioclase of the Monzoni rocks, according to Brogger, is generally 

 andesine to labradorite, or even anorthite.'^ Rosenbusch^ says that 

 andesine is much rarer in all monzonites than more basic plagioclase, 

 and more acid plagioclase than andesine had not been observed by 

 him in the rocks of Monzoni. Elsewhere, it would seem, orthoclase- 

 acid-plagioclase rocks are more common than those with basic 

 plagioclase. In this classification, therefore, the more acid rocks, 

 that is, those whose feldspar is oligoclase or andesine (sodi- 

 monzonites), will be considered normal monzonites, while those 

 with labradorite or bytownite will be included under the calci- 

 monzonites (2313). 



'Leopold von Buch, "Ueber geognostische Erscheinungen im Fassathale," 

 V. Leonh. Mineralogisches Taschenhuch fur das Jahr 1824, p. 347. 



= M. de Lapparent, "La constitution geologique du Tyrol, meridional," Ann. d. 

 Mines, 6 ser., VI (1864), 259. 



3 Ed. Reyer, "Predazzo," Jahrh. d. k. k. geol. Reichsanst., XXXI (1881), 36. 



'* Gustav Tschermak, Die Porphyrgesteine Osterreichs (1869), p. no. 



5 J. Lemberg, "tJber die Contactbildungen bei Predazzo," Zeitschr. d. d. geol. 

 Gesell., XXIV (1872), 188, 190. 



* W. C. Brogger, Die Eruptivgesteine des Kristianiagebietes. II: Die Eruptions- 

 folge der triadischen Eruptivgesteine bei Predazzo in Siidtyrol (Kristiania, 1895), 

 pp. 22-23. 



7 Op. cit., p. 54. 



^ H. Rosenbusch, Elemente der Gesteinslehre (Stuttgart, 1898), p. 108; also Mikro- 

 skopische Physiographie der massigen Gesteine (Stuttgart, 1907), II', 167. 



