CAUSES OF VARIATION IN STRUCTURE 403 



appearance and habit, and a common if not universal constituent of the 

 chondritic meteorites, and in particular those of the white and gray 

 chondrite groups. 



Concerning the identity of the two substances Tschermak, after a full 

 discussion,^^ says: 



. "Da die.ser isotrope Gemengtheil genau dieselbe Form uiul Vertheilung zeigt 

 wie der Plagioklas und im gewohnlichen Lichte denselben Eindruck inacht wie 

 dieser, so glaubte ich aus dieser auffallenden Gleichheit der aussereii Form 

 auf eine Gleichheit der chemischen Zusammensetzung schliessen und denselben 

 fiir Maskelynite halten zu diirfen." 



This view has been very generally accepted by subsequent workers. 

 H. Michel, in his "Die Feldspate der Meteoriten," ^'^ says : 



"Das ganze Bilder Maskelynitleisten ist, wie schon Tschermak betont, wobei 

 er sich auf die Autoritat von F. Becke stiitzt, das von Feldspaten, welch zuerst 

 auskrystallisiert sind, Hohlraume und Zwickel freigelassen haben, in denen- 

 dann der Pyroxen krystallisierte, derart, dass wohl auch die Bildungsperioden 

 ineinander iibergriffen. Es finden sich namlich bisweilen Partien mit eiuer 

 Art Eutektstruktur, bei denen sich iiber das gegenseitige Alter von Pyroxen 

 und Maskelynite nichts aussagen lasst." 



The identity of the mineral is further supported by the refractive 

 indices as given by Michel in the paper quoted and as determined by 

 myself-^ in the stones of Hartford and Ness County. 



It will be recalled that in the case of the Hartford stone (a white 

 chondrite) the questionable mineral was not merely slightly doubly re- 

 fractive, but gave by the immersion methods indices of 1.54-1.545, and 

 that in a few instances I was able to discover still remaining evidences 

 of polysynthetic twinning, indicative that the metamorphism had not 

 been sufficient to completely destroy the original structure. In the white 

 chondrite of Colby, Wisconsin (figure 3, plate 3), maskelynite is rarely 

 completely isotropic, but is unequally anisotropic throughout. 



It would seem that the question of the derivation of maskelynite from 

 a plagioclase and the identity of these minute interstitial forms with that 

 of the Shergotty stone might be considered as settled. Nevertheless, 

 authorities are not wholly in agreement on the subject. Groth, it will be 

 recalled, regarded the mineral as an independent species, related to 

 leucite, which had become anomalous in its refracting qualities through 

 pressure or a rise in temperature. This view has not been geiierally 

 accepted. • 



^^ Die Meteoriten, etc., p. 18. 



™ Tschermalj's Min. und Pet. Mitteil., vol. 31, 1912, p. 584. 



-'■ Mem. Nat. Acad. Scl., vol. 14, 1919, pp. 5-7. 



