706 A TREATISE ON METAMORPHISM. 



by burying without orogenic movement, Where recrystallization during 

 orogenic movements occurs the process of rejuvenation is less likely to take 

 place, but may occur in the more resistant minerals which escape destruction. 

 As is well known, feldspar is one of the most resistant minerals under deep- 

 seated conditions of deformation, and hence many of the original parti}' 

 altered particles of this mineral may be rejuvenated during the time in 

 which other minerals are recrystallized. Of course, if the deformation is 

 severe the borders of the original feldspars are likely to be granulated or 

 recrystallized, and during extreme orogenic movements there may be com- 

 plete recrystallization of the feldspar. 



The facts that under certain conditions feldspars are destroyed by 

 kaolinization, chloritization, zeolitization, etc., and under other conditions 

 are rejuvenated or newly developed, have been known for some time. We 

 can now assign reasons for the reversal of these reactions. Feldspar is 

 not adapted to the zone of katamorphism, and especially to the belt of 

 weathering, and it is 1 destroyed by the reactions demanded by the chemical 

 and physical forces there obtaining; whereas feldspar is adapted to the 

 zone of anamorphism, and the chemical and physical forces there at work 

 reproduce it. 



The nature of the process of rejuvenation has been illustrated by 

 feldspar because this is the best known case, but other minerals may be 

 rejuvenated in a similar manner." 



" Lepsius has anticipated a number of the points of the previous pages, as is shown by the following 

 quotations and summary from the Geologie von Attika (Ein Beitrag zur Lehre vom Metamorphisnms 

 der Gesteine), Dietrich Keimer, Berlin, 1893, 196 pp. With tables and atlas. 



"Die Mineralien in den Gesteinen konnen nur umkrystallisiert werden, wenn ihre Molekiile 

 fliissig werden; dies kann auf dreierlei Weise geschehen: 



1. Durch Schmelzung in der Gluhhitze und zwar fur unsern Fall bei Gegenwart von Druck, also 

 durch die eben besprochene Druckschmelze. 



2. Durch Losungsmittel auf chemischem Wege, und zwar in unsrem.Falle bei Gegenwart von Druck 

 und bei einer Temperatur, die hoher ist als die mittlere Temperatur der Erdoberfliiche, die jedoch im 

 allgemeinen nur bis zu einigen hundert Grad Celsius und wohl nicht fiber 500° steigen wird. 



3. Durch hohen Druck auf mechanischem Wege, das ist diejenige Art von Druckverfliissigung, 

 welche durch die Springschen Versuche illustriert wurde. 



Betrachten wir jetzt die zweite Moglichkeit, die Verfliissingung der Molekiile durch Losungsmittel 

 auf chemischem Wege; diese Art der Verfliissigung und Auskrystallisierung aus der chemischen Liisung 

 scheint mir am besten die von aus beobachteten Vorgange bei der Metamorphose der Gesteine 

 aufzuklaren." (P. 183.) 



Holds that the schistose Silurian and Tertiary rocks when metamorphosed were buried to a depth 

 of about 12,500 meters, which with a specific gravity of 2.5 would give a pressure of 3,125 atmospheres. 

 (P. 187.) 



