44 NEW YORK STATE MUSEUM 



were the result of reaction between the ferromagnesian core and the 

 feldspathic ground mass. 



The first theory made a peculiarly strong appeal to the writer 

 since, to one familiar with the geology of copper mines, garnet 

 connotes contact metamorphism. In favor of this theory is the fact 

 that the Adirondack anorthosites do cut such sediments as might 

 give inclusions that would alter to garnet rocks. Also the boundary 

 between the garnet surrounded masses of pyroxene and the ground 

 mass is at times very sharp. But the evidence against this theory is 

 too strong. The pyroxenes in those aggregates surrounded by gar- 

 net differ in no way from pyroxenes in the rest of the rock and are 

 apparently endogenous. The garnets are not limited absolutely to 

 the rims about the pyroxene ; an occasional individual is found out in 

 the feldspar, and also within the pyroxene mass. Furthermore, if 

 these rims and lenses had been formed by contact metamorphic action 

 on stoped-in fragments, they must have been produced previous to the 

 later dynamic metamorphism which granulated the mass of the rock. 

 In the hand specimen they show no effect of such metamorphism, the 

 lenticular masses lying in the rock without any tendency toward 

 parallelism. The slides show no granulation of the pyroxenes. 

 There is then the possibility that the fragments represent a stoped-in 

 pyroxenite about which the garnets have formed later. This seems 

 rather unnecessary when the same pyroxenes are found undoubtedly 

 endogenous in the normal rock. 



The next theory is that these entire lenses, both garnet and pyrox- 

 ene, represent primary segregations like those described by F. Zirkel 

 in basalts (op. cit.). This does not require that we ascribe to a 

 gabbroic magma, contact effects that are usually associated with more 

 acid intrusives. It is, moreover, an explanation which accounts for 

 the pyroxenic aggregates and the garnet at the same time. The same 

 objection as to lack of granulation in these masses formed prior to 

 the dynamic metamorphism holds against this theory as applied to 

 the garnet rims. But if it can be shown that the garnet rims are the 

 expression of that dynamic metamorphism around the pyroxenes, 

 there is no reason why the pyroxenes may not represent such 

 segregations. 



Another objection to regarding the entire lense, rim and all, as an 

 original segregation, is the comparatively large amount, of quartz 

 present. This quartz is entirely lacking in strain shadows and has 

 all the appearance of being contemporaneous with the garnet. 



