46 NEW YORK STATE MUSEUM 



might account for some of the soda. If this is admitted, and the 

 lack of a definite accounting for the soda not regarded as an insuper- 

 able obstacle, the possibility that the garnets represent a reaction 

 between feldspar and the pyroxenes has been established. 



The cause for the reaction seems to lie in the dynamic metamor- 

 phism. F. D. Adams, in the work cited above, mentions the fact 

 that the pyroxenes in the granulated anorthosites of the Morin area 

 were also granulated. The pyroxene in the specimens under dis- 

 cussion show no such granulation. It seems to the writer that this 

 indicates that the specimens come from an anorthosite that was 

 granulated under conditions differing from those that obtained in 

 the Morin area. Possibly a greater development of heat permitted a 

 fusion and recrystallization around the edges of the pyroxenic aggre- 

 gates. There may have been a greater amount of water or of other 

 mineralizers which permitted fusion at a lower temperature in these 

 pyroxenes. In fact, tests for combined water show 0.56 per cent 

 in the pyroxene and 0.34 per cent in the granulated anorthosite 

 away from the rim. Whatever the cause may be, any explanation 

 which does not admit that these garnet rims are the expression of the 

 dynamic metamorphism that caused granulation in the rest of the 

 rock, must either show the entire lenses to have been introduced 

 later than the metamorphism, or else give some other accounting for 

 the lack of granulation. If, however, these rims are admitted to be 

 the expression of the dynamic metamorphism, there is no further 

 objection to regarding the pyroxenic aggregates as original segrega- 

 tions. 



There remains the fact that certain small areas of pyroxene in the 

 specimens are not surrounded by garnet. These areas are made up 

 of very small crystals and show a great development of amphibole 

 which appears to be secondary. Plate 18A shows this. In this case 

 the amphibole formation represents the metamorphism. 



After regarding the various views suggested, the one that seems 

 most tenable is that the garnet rims represent a reaction between 

 pyroxene and feldspar induced by the same causes that brought 

 about the granulation of the ground mass. This conception has the 

 advantage of being widely applicable, as its requirements are very 

 few : a f erromagnesian mineral in contact with a feldspar, and the 

 proper degree of heat and pressure. It certainly seems to the writer 

 to fit almost all the cases that he has studied or whose description 

 he has read. 



