CRYSTALLOBLASTIC ORDER AND MINERAL DEVELOP- 

 MENT IN METAMORPHISM 



FREDERIC H. LAHEE 



Massachusetts Institute of Technology 



CONTENTS 



Variations in Degree of Metamorphism 



Stages of Metamorphism in Schists of the Narragansett Basin 



Study of the Metacrysts 

 Ilmenite 

 Garnet 

 Biotite 

 Ottrelite 



Summary and Conclusions 



VARIATIONS IN DEGREE OE METAMORPHISM 



Metamorphic rocks often exhibit, within the same region, con- 

 siderable variation in the kind and especially in the amount of 

 metamorphism to which they have been subjected. This is 

 particularly true of intrusive contact zones where a more or less 

 uniform decrease in metamorphism may be observed away from 

 the intrusive body/ In regional metamorphism, too, gradations 

 may be found in the degree of alteration of the crystalline schists; 

 but the variations here are much less regular than in the former case. 



Recognizing the fact that long duration of time is an essential 

 requisite for the course of metamorphism,^ we may assume that a 

 metamorphic rock, whatever may have been the forces concerned 

 in its origin, must have passed through a sequence of stages of 



' See A. Geikie, Textbook of Geology, 4th ed. (1903), p. 773; G. Barrow, "On an 

 Intrusion of Muscovite-biotite Gneiss in the Southeastern Highlands of Scotland, and 

 Its Accompanying Metamorphism," Qiiar. Jour. Geol. Soc. Loud., XLIX (1893), 330. 



^ Grubenmann lays emphasis on the great importance of time. He writes, "Die 

 Temperatur darf wohl als der bedeutendste Faktor der Metamorphose betrachtet 

 werden." — Die Kristallinen Schiefer, 2d ed. (1910), p. 51. 



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